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Piecing together the Ganges-Brahmaputra-Meghna River delta: Use of sediment provenance to reconstruct the history and interaction of multiple fluvial systems during Holocene delta evolution
Abstract
Three main rivers—the Ganges, Brahmaputra, and Meghna—coalesce in the Bengal basin to form the world’s largest delta system, which serves as filter and gateway between the Himalayan collision and vast Bengal fan repository. New insights into the Holocene construction of the Ganges-Brahmaputra-Meghna delta, with a focus on river sedimentation, channel migration, and avulsion history, are presented here using the Sr geochemistry of bulk sediments as a provenance tracer. The sediment load of each river transmits a distinct Sr signature owing to differences in source rocks from the Himalaya, Tibet, and local regions, allowing for effective tracking of river channels and stratigraphic development within the delta. In the early Holocene, vigorous delta aggradation occurred under rapid sea-level rise and high river discharge and supported the construction of sand-dominated stratigraphy by laterally mobile, braided-stream channels. However, the vertically (i.e., temporally) uniform, but geographically distinct, Sr signatures from these deposits indicate that the Ganges, Brahmaputra, and Meghna fluvial systems remained isolated from one another and apparently constrained within their lowstand valleys. By the mid-Holocene, though, delta stratigraphy records spatially and temporally nonuniform Sr signatures that hallmark the onset of avulsions and unconstrained channel migration, like those that characterize the modern Ganges and Brahmaputra fluvial systems. Such mobility developed in the mid-Holocene despite declining discharge and sea-level rise, suggesting that earlier channel behavior had been strongly influenced by antecedent topography of the lowstand valleys. It is only after the delta had aggraded above the valley margins that the fluvial systems were able to avulse freely, resulting in numerous channel reorganizations from mid-Holocene to present. These channel-system behaviors and their role in delta evolution remain coarsely defined based only on this initial application of Sr-based provenance tools, but the approach is promising and suggests that a more complete understanding can be achieved with continued study.
Supplementary resources (2)
... To understand the sediment provenance, the 87 Sr/ 86 Sr and εNd values were plotted on mixing curves having the end members higher Himalayas (HH), lesser Himalayas LH, APGC, and DcB (Fig. 2). Apart from these end members, mainstem averages ( 87 Sr/ 86 Sr and εNd) of Ganga, Brahmaputra rivers (Goodbred et al., 2014;Singh and France-Lanord, 2002;Singh et al., 2008), Godavari (Ahmad et al., 2009) away from the Mahanadi river bed sediments (Fig. 2). The Sr-Nd isotope ratios of the present study overlap with that of sediments recovered from the upper, middle, and lower Bengal fans (Fig. 2). ...
... The 87 Sr/ 86 Sr and εNd ratios show marked temporal variations throughout the time span of 320 ky BP (Fig. 3) which may be attributed to change in the relative contribution from the Ganga and Brahmaputra river sediments with contrasting Sr-Nd isotopic compositions (Goodbred et al., 2014;Singh and France-Lanord, 2002;Singh et al., 2008). Climatically driven changes in the relative sediment contribution by Ganga and Brahmaputra have earlier been documented from sediment cores collected from Bengal fan (Galy et al., 2010;Hein et al., 2017) and deeper waters of the east coast of India (Peketi et al., 2021). ...
... The authors declare that they have no known competing financial Complex source rocks (Mazumdar et al., 2015), Ganga and Brahmaputra river bed sediments (Goodbred et al., 2014). interests or personal relationships that could have appeared to influence the work reported in this paper. ...
... In fact, existing distinctions between these two large river basins in rock type, geography, climate, weathering intensity and runoff may causing contrasting sediment compositions. Geochemical compositions of river sediment represent the content of the rocks and/or strata and contain information about the climate, transport dynamics, weathering and diagenesis (Cox et al., 1995;Garzanti et al., 2011;Garzanti et al., 2010;Hossain et al., 2017;Maharana et al., 2018;Mclennan et al., 1993), and they have been widely analyzed in rivers around the BoB from various viewpoints (Chakrapani, 2005;Garzanti et al., 2011;Garzanti et al., 2010;Goodbred et al., 2014;Li et al., 2009;Lupker et al., 2013;Lupker et al., 2011;Lupker et al., 2012;Subramanian et al., 1985), especially the Ganges-Brahmaputra Rivers (G-B). Integrated textural, mineralogical and geochemical data of the sediments from all the major peninsular rivers of the Ganga basin (the Chambal, Sindh, Betwa, Ken, Ton and Son Rivers), Himalayan tributaries, Ganges River main channel and Brahmaputra River have been reported (Garzanti et al., 2011;Garzanti et al., 2010;Goodbred et al., 2014;Heroy et al., 2003;Lupker et al., 2013;Lupker et al., 2011;Lupker et al., 2012;Maharana et al., 2018;Singh et al., 2005a;Singh, 2009;Singh and France-Lanord, 2002;Singh et al., 2005b;Tripathy and Singh, 2010), but detailed distinctions between major rivers and their stretches in Bangladesh remain unclear. ...
... Geochemical compositions of river sediment represent the content of the rocks and/or strata and contain information about the climate, transport dynamics, weathering and diagenesis (Cox et al., 1995;Garzanti et al., 2011;Garzanti et al., 2010;Hossain et al., 2017;Maharana et al., 2018;Mclennan et al., 1993), and they have been widely analyzed in rivers around the BoB from various viewpoints (Chakrapani, 2005;Garzanti et al., 2011;Garzanti et al., 2010;Goodbred et al., 2014;Li et al., 2009;Lupker et al., 2013;Lupker et al., 2011;Lupker et al., 2012;Subramanian et al., 1985), especially the Ganges-Brahmaputra Rivers (G-B). Integrated textural, mineralogical and geochemical data of the sediments from all the major peninsular rivers of the Ganga basin (the Chambal, Sindh, Betwa, Ken, Ton and Son Rivers), Himalayan tributaries, Ganges River main channel and Brahmaputra River have been reported (Garzanti et al., 2011;Garzanti et al., 2010;Goodbred et al., 2014;Heroy et al., 2003;Lupker et al., 2013;Lupker et al., 2011;Lupker et al., 2012;Maharana et al., 2018;Singh et al., 2005a;Singh, 2009;Singh and France-Lanord, 2002;Singh et al., 2005b;Tripathy and Singh, 2010), but detailed distinctions between major rivers and their stretches in Bangladesh remain unclear. Since they are the lower reaches of the G-B Rivers and close to the river mouth, chemical compositions in these rivers must carry the catchment averaged information, such as weathering, transport processes and provenances. ...
... Three samples had the highest sand volume percentages, G-01 from the Ganges River bank (64%), M− 01 (50%) and M− 02 (49%) from the river mouth bank. Sediments from the Brahmaputra River have the Galy et al., 2010;Garzanti et al., 2010;Goodbred et al., 2014. highest silt percentages (range from 59 to 78%, average 69%), while sediments from the Ganges River and Meghna River have the highest sand (range from 36 to 64%, average 46%) and clay fractions (range from 6 to 17%, average 12%), respectively. ...
... Furthermore, the geomorphic diversity of river formation and processes in the Himalayan foreland and hinterland are demonstrated by the climatic diversity along the strike of the Himalaya (Sinha 2004). Because there is a close relationship among geomorphology, river GJC PMC Fig. 1 Map of the Ganges-Brahmaputra-Meghna river basin representing major tributaries and sediment sources including Trans-Himalayan batholiths (THB), Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Sequence (HHC), Lesser Himalayas (LH), Indus-Tsangpo suture (ITS) (after Goodbred et al. 2014). The rectangle focuses on the Bengal basin that influences the river migration and sediment dispersal across the Ganges-Brahmaputra-Meghna delta. ...
... Moreover, in the context of Bangladesh, which hosts two most diverse Ganges-Brahmaputra basins, not many studies on fluvial geomorphology and sediment dynamics have been conducted so for. Geomorphic research is still in the nascent phase in Bangladesh and previously studied by only a few researchers (Coleman 1969;Umitsu 1985Umitsu , 1987Umitsu , 1993Best and Ashworth 1997;Allison et al. 1998;Ashworth et al. 2000;Islam et al. 2001;Takagi et al. 2007;Goodbred et al. 2014;Brammer 2014;Sarker et al. 2014;Dewan et al. 2017) focusing on channel diversion, erosion and accretion, sediment deposition, and few hydro-geomorphic systems. Earlier, Takagi et al. (2005) reviewed the geomorphological and geological researches of Bangladesh and tried to reflect some of the geomorphic research gap and realities and mentioned the limited scientific researchers. ...
... The Trans-Himalayan batholiths, Tethyan Sedimentary Series, High Himalayan Crystalline Sequence, Lesser Himalayas, the Deccan Traps, the Shillong Massif, and Tripura fold belt are the source areas of GBM basin sediment, where the Indus-Tsangpo suture is connecting the sediment sources between Asia and India (Galy et al. 2010;Goodbred et al. 2014) (Fig. 1). Through the Indus-Tsangpo suture, the Brahmaputra traverse via Namcha Barwa syntaxis which encompasses only 4% of Brahmaputra's catchment but contributes about 45 ± 15% of sediment load (Singh and France-Lanord 2002;Garzanti et al. 2004;Stewart et al. 2008;Goodbred et al. 2014). ...
Geomorphological knowledge is critical in understanding watershed scale surface processes, including steep mountainous areas and flat lowlands, particularly if the mid- and downstream areas are densely populated and hazard assessments are highly required. However, our knowledge about such surface processes has relatively been limited in some areas in South Asia due likely to the lack of comprehensive studies of geomorphology and related fields. This article undertakes an overview of the geomorphological processes of the disaster-prone deltaic landscape of the Ganges–Brahmaputra–Meghna (GBM), particularly focusing on fluvial processes. The area locates in the downstream of the watershed system including Himalayan Mountains and highly connected with the upper basin morphodynamics, hydrology, and sediment flux. The previous studies are summarized at different geomorphic settings concerning hillslopes, fluvial plains, and coastal areas to provide clarity about the geomorphic processes linking erosion-prone upstream source areas to deposition-dominated downstream areas. The review found that most of the geomorphic researches in Bangladesh are exploring landslide inventory and susceptibility mapping in hilly areas; river channel or riverbank shifting, riverbank erosion and accretion in fluvial environments; watershed morphometric analysis and geomorphic unit identification in plain land; and coastline shifting or coastal erosion and accretion in coastal environments at a small scale. Then, we discuss the fluvial dynamics and sediment transport of the GBM river system to address the knowledge gap in the context of deltaic plain land in Bangladesh, where upstream fluvial sedimentation processes impact the geomorphic connectivity from Himalayan to the Bay of Bengal. Although some studies on the fluvial dynamics and sediment dispersal in the upstream GBM river basin are present, the fluvial processes in the downstream domain of Bangladesh are not fully understood with a limited number of research with field-based approaches. Some future perspectives of geomorphic research in Bangladesh are then mentioned to understand better the complex geomorphological settings in the entire GBM watershed and to strengthen the existing research capacity. This review will also develop a holistic understanding of fluvial geomorphic processes of the GBM River to the policymakers and may be helpful to improve the transboundary river basin management policies or strategies.
... For example, subsidence factors in local areas occur faster than those over a greater area, and loose material in deltas easily compacts, resulting in slow sinking of the land and RSLR to rise 3.37 mm/yr (Table 5) [49,89]. The Ganges-Brahmaputra Delta, for instance, receives a lot of water and sediment input from the Himalayas, with an input of more than 1 billion tonnes/yr [49,[90][91][92][93][94]. The subsidence levels for the lower delta showed 3 mm, the western ranged from 9 to 12.3 mm, and up 3.6 to 4 mm in the eastern, and it was also discovered that, within the first 10 years of the 21st century, immense flooding affected 85% of the largest deltas in the world, as shown in Figures 7 and 8, [93][94][95][96][97]. Further, it is projected that the risk of flooding will increase by at least 50% by 2100 [49,[93][94][95][96][97][98]. ...
... The Ganges-Brahmaputra Delta, for instance, receives a lot of water and sediment input from the Himalayas, with an input of more than 1 billion tonnes/yr [49,[90][91][92][93][94]. The subsidence levels for the lower delta showed 3 mm, the western ranged from 9 to 12.3 mm, and up 3.6 to 4 mm in the eastern, and it was also discovered that, within the first 10 years of the 21st century, immense flooding affected 85% of the largest deltas in the world, as shown in Figures 7 and 8, [93][94][95][96][97]. Further, it is projected that the risk of flooding will increase by at least 50% by 2100 [49,[93][94][95][96][97][98]. ...
... The Ganges-Brahmaputra Delta, for instance, receives a lot of water and sediment input from the Himalayas, with an input of more than 1 billion tonnes/yr [49,[90][91][92][93][94]. The subsidence levels for the lower delta showed 3 mm, the western ranged from 9 to 12.3 mm, and up 3.6 to 4 mm in the eastern, and it was also discovered that, within the first 10 years of the 21st century, immense flooding affected 85% of the largest deltas in the world, as shown in Figures 7 and 8, [93][94][95][96][97]. Further, it is projected that the risk of flooding will increase by at least 50% by 2100 [49,[93][94][95][96][97][98]. Thus, this delta is sinking so rapidly that the RSLR may reach 20 mm each year [96][97][98][99]. ...
Global greenhouse gas emissions have caused sea level rise (SLR) at a global and local level since the industrial revolution, mainly through thermal expansion and ice melting. Projections indicate that the acceleration of SLR will increase in the near future. This will affect coastal and deltaic populations worldwide, such as in Bangladesh, where almost half of the population resides in regions lower than 5 m above sea level. This study found three coastal tidal gauges and five deltaic gauge stations, which showed increases in SLR at greater rates than the regional and global averages. This research also used satellite altimetry data to analyze regional and global SLR averages in the recent past and the 21st century. There is a trend towards increasing sea level based on results from three tide gauge stations: Char Changa with 7.6 mm/yr, Hiron Point at 3.1 mm/yr from 1993 to 2019, and 14.5 mm/yr at Cox’s Bazar from 1993 to 2011. Based on the linear trend from these time frames, it is projected that SLR in Char Changa will increase by 228 mm from 2020 to 2050, and by 608 mm by 2100, at Hiron Point by 93 mm in 2050 and 248 mm by 2100, and at Cox’s Bazar by almost 435.7 mm by 2050, and more than 1162 mm by 2100. Based on an average from satellite altimeters, assuming a linear increase in SLR, the Bay of Bengal shows an increase of 0.4 mm compared to the global trend. Other river delta stations in the study area also show increasing SLR, specifically, at Kalaroa, Benarpota, Kaikhali, Tala Magura, and Elarchari. Kalaroa and Benarpota show the highest, with SLR of >40 mm/yr. It is also observed that increasing SLR trends are far higher than coastal tide gauges, indicating that physical processes in the delta region are affecting SLR, further contributing to either an increase in water volume/SLR or activating land subsidence. This is partly due to the subsidence of the delta as a result of natural and anthropomorphic effects, as well as an increase in Himalayan glacier melting due to global warming. This indicates that Bangladesh coastal areas will soon experience a far greater SLR than the rest of the Bay of Bengal or other global coastal areas.
... The balance between high sediment input from the rivers and rapidly rising sea level efficiently trapped riverine sediments and constructed a thick deltaic sequence 31 . During the mid-Holocene (after ~7,000 years BP), the rate of eustatic sea level rise slowed by an order of magnitude compared with the early Holocene [37][38][39] . The reduced rate of sea level rise facilitated the deltaic shift from an aggradational to a progradational phase, advancing the eastern portions of the subaerial delta 100 km into the sea and building a subaqueous delta plain 200 km across the shelf 9,37,39,40 . ...
... The reduced rate of sea level rise facilitated the deltaic shift from an aggradational to a progradational phase, advancing the eastern portions of the subaerial delta 100 km into the sea and building a subaqueous delta plain 200 km across the shelf 9,37,39,40 . The Ganges, Brahmaputra and Meghna rivers deposited ~8,500 km 3 of sediment within the Bengal Basin over the entire Holocene 38 , facilitating the shift to a progradational delta several thousand years earlier than otherwise expected 31 . ...
... Throughout the Holocene, the Brahmaputra river channel has periodically avulsed between two main courses on the eastern and western sides of the Madhupur Tract ( Fig. 1) approximately every 2,000-3,000 years 36,38,41 . The last avulsion into the Jamuna river valley (thereafter named the Jamuna River) occurred sometime between 1776 and 1830 (reFs 32,36,38,[41][42][43], which is when the Ganges and Brahmaputra rivers first met. ...
More than 70% of large deltas are under threat from rising sea levels, subsidence and anthropogenic interferences, including the Ganges–Brahmaputra–Meghna (GBM) delta, the Earth’s largest and most populous delta system. The dynamic geomorphology of this delta is often overlooked in assessments of its vulnerability; consequently, development plans and previous management investments have been undermined by unanticipated geomorphic responses. In this Review, we describe GBM delta dynamics, examining these changes through the Drivers–Pressures–States–Impacts–Responses framework. Since the early Holocene, the GBM delta has evolved in response to a combination of tectonics, geology, changing river discharge and sea level rise, but the dynamics observed today are driven by a complex interplay of anthropogenic interferences and natural background processes. Contemporary geomorphic processes such as shoreline change, channel migration, sedimentation and subsidence can increase flooding and erosion, impacting biodiversity, ground and water contamination and local community livelihoods. Continued human disturbances to the GBM delta, such as curtailing sediment supplies, modifying channels and changing land use, could have a more direct influence on the future geomorphic balance of the delta than anthropogenic climate change and sea level rise. In order to contribute to long-term delta sustainability, adaptation responses must therefore be informed by an understanding of geomorphic processes, requiring increased transdisciplinary research on future delta dynamics at centennial timescales and collaboration across all governing bodies and stakeholders.
... Many authors have discussed the development and classification of the GBM delta landform, timings and development phases of the Brahmaputra-Ganges river systems as well as GBM Delta (Akter et al., 2016;Chamberlain et al., 2017;Goodbred and Kuehl, 1999;Goodbred et al., 2000aGoodbred et al., , 2000bGoodbred et al., , 2003Goodbred et al., , 2014Islam and Tooley, 1999;Neidhardt et al., 2013;Pickering et al., 2018;Sarkar et al., 2009;Umitsu, 1993;Wilson and Goodbred, 2015). According to Allison et al. (2003) and Islam and Tooley (1999), the modern GBM delta has developed in five successive transgression phases during the Holocene and evolution started during the Last Glacial Maximum (~18 ka) when the sea level was around 120 m lower than today (Grall et al., 2018;Umitsu, 1993). ...
... According to Allison et al. (2003) and Islam and Tooley (1999), the modern GBM delta has developed in five successive transgression phases during the Holocene and evolution started during the Last Glacial Maximum (~18 ka) when the sea level was around 120 m lower than today (Grall et al., 2018;Umitsu, 1993). Goodbred et al. (2014), Allison et al. (2003), Goodbred and Kuehl (2000b), Allison (1998), andUmitsu (1993) reported that the main channel of the Ganges River started draining the western part of the basin from 3050 BC and that result continues to develop the present-day configuration of delta around 1550 to 1050 BC. The fluctuation of sealevel has been recorded in the last 1000 years of the Holocene (Grinsted et al., 2010;Mann et al., 2009;Nunn, 1998;Ota et al., 1990), and the depositional process of the low-lying deltaic plain is highly influenced by the relative sea-level (RSL) change. ...
... The sediment of southwestern lower delta plain has been supplied by GBM estuary, carried westward by coastal current, and deposited inland by cyclone event and monsoonal coastal setup (Figure 1a). For that, the sedimentation rate shows higher in the outer delta compare to the upper delta plain (Allison and Kepple, 2001;Goodbred et al., 2014;Willson and Goodbred, 2015). The subsidence over the area ranges from 1.5 to 2.0 mm/year (Krien et al., 2019) whereas it is 2.8 mm/year reported for the Sundarbans (Brown and Nicholls, 2015;Karpytchev et al., 2018) and subsidence reached up to 5.2 mm/year in the outer delta of the region (Hanebuth et al., 2013). ...
This study illustrates the influences of sea-level on the depositional process during the last 1000 years of the southwestern delta, Bangladesh. Sediments of eight litho-sections from landward in upper delta plain to seaward in lower delta plain along the Rupsa-Pasur river were studied. Sedimentary facies, total organic carbon, total nitrogen, δ ¹³ C value, diatom assemblages, and radiocarbon dating of deposits were carried out to determine the paleoenvironments that were influenced by the relative sea-level (RSL) change over time. During the 850–1300 AD, RSL was reached up to +80 cm higher than the present level where tidal-influenced bioturbated light yellow to gray mud deposited in the upper delta plain area. RSL was dropped up to −110 cm during 1300–1850 AD, organic-rich bluish-gray mud, mangrove peat, and terrestrial influenced yellowish-gray mud were deposited successively in the lower delta plain area, and the terrace was formed at landward due to the lowering of the base level. RSL started to rise after the period 1850 AD where tidal-influenced sediments gradually increased and deposited in the upper part at seaward and terrestrial flood sediment deposited over the erosional surface at the landward part. The estimated average sedimentation rate (1.96–2.89 mm/year) is not enough to offset the effect of subsidence and present sea-level rise over the study area. The rising trend of the sea creates inundation in the lower delta plain area, also hinders upstream water flow. For that, terrestrial flood sediments settle over the erosional surface in landward, and tidal-influenced sediment gradually onlap upon it from seaward.
... Lower Himalayas (94 µg/g, 26 µg/g, 0.85, −24.4), and Higher Himalayas (80 µg/g, 18 µg/g, 0.76, −15). In addition to the Sr-Nd isotope ratios of the present study, Sr-Nd isotope ratios of Ganga, Brahmaputra rivers sediments, Shillong plateau (Goodbred et al., 2014;Singh & France-Lanord, 2002;Singh et al., 2008) along with multiple sediment cores (France-Lanord et al., 1993;Galy et al., 2008Galy et al., , 2010Lenard et al., 2020;Lupker et al., 2013) from the Bay of Bengal were also incorporated in Figure 2. ...
... Sr-Nd isotope ratios of the Mahanadi river bed samples are tabulated in Table S1. The εNd and 87 Sr/ 86 Sr ratios of the sediments from IODP 353 Hole U1445A vary from −17.40 to −12.99 and 0.72860 to 0.76725, respectively, and fall within the average Sr-Nd isotopic compositions of Ganga-Brahmaputra river sediments (Figure 2; Goodbred et al., 2014;Singh & France-Lanord, 2002;Singh et al., 2008). The εNd and 87 Sr/ 86 Sr ratios of the Mahanadi river bed sediment vary from −22.17 to −19.22 and 0.75273 to 0.78086 and fall far away from the Sr-Nd isotope ratios of the core U1445A ( Figure 2). ...
... The εNd and 87 Sr/ 86 Sr ratios of the core U1445A fall (Figure 2) well within the average Sr-Nd isotopic compositions of Ganga and Brahmaputra river sediments (Goodbred et al., 2014;Singh & Lanord, 2002;Singh et al., 2008), overlap (Figure 2) with the Sr-Nd isotope ratios of sediments from the upper to lower Bengal fan (France-Lanord et al., 1993;Galy et al., 2010Galy et al., , 2008Lenard et al., 2020;Lupker et al., 2013) and fall far from the Mahanadi river bed sediments (Figure 2). Although the studied sediment core (U1445A) was recovered from the southern deep-water end (2,513.1 mbsl) of the Mahanadi basin (Clemens et al., 2015), the Sr-Nd isotopic compositions (Figure 2) does not show any noticeable contribution of the peninsular source rocks comprising Deccan Basalts, Archean Proterozoic gneissic complexes and Proterozoic sediments (George & Ray, 2020). ...
Strontium and Neodymium isotopic analysis of the lithogenic fractions of the hole U1445A (International Ocean Discovery Program-353) collected from the deep-water end of the Mahanadi basin (western Bay of Bengal) was carried out to evaluate the sediment sources as well as to understand the link between the temporal variations in the Sr-Nd isotope ratios and climatic and tectonic forcings during the last 6 Ma. The Sr-Nd isotopic compositions along with Fe/Al ratios and clay mineralogy show tell-tale signature of Ganga-Brahmaputra sedimentation (Bengal fan) in the study area. The significant temporal fluctuations observed in the Sr-Nd isotopic compositions suggest variation in the relative sediment contribution by the Ganga and Brahmaputra river systems, which in turn are controlled by monsoon intensity variations coupled with orographic effect, glaciation, and multiple tectonic activities. The results show the marked influence of glacial-interglacials on the relative sediment contribution by Ganga and Brahmaputra rivers during 0–1.8 Ma, while the sedimentation during the 1.8–6 Ma time window was likely influenced by both climatic and tectonic forcings such as uplift of the Shillong plateau, eastern syntaxis development of Indo-Burma wedge, reorganization of Brahmaputra river system. The present study is the first high resolution (122 samples within 6 Ma) Sr-Nd isotope ratio analysis from the Bengal Fan. The study has brought to light multiple isotope fluctuations linked to climatic and tectonic forcings. The work also establishes dominant control of Ganga-Brahmaputra sedimentation along the deep water of the Indian east coast.
... Chuadanga municipality is located in the western Bengal basin and is also a significant domain of the Ganges-Brahmaputra delta (Hossain et al., 2014). Major part of the sediments carried by the Ganges and Brahmaputra River is originated from the Himalayan orogeny (Galy and France-Lanord, 2001;Goodbred et al., 2014;Singh and France-Lanord, 2002). The primary source rocks include the trans-Himalayan batholiths, high Himalayan crystalline sequence, Tethyan sedimentary series and lesser Himalayas (Goodbred et al., 2014). ...
... Major part of the sediments carried by the Ganges and Brahmaputra River is originated from the Himalayan orogeny (Galy and France-Lanord, 2001;Goodbred et al., 2014;Singh and France-Lanord, 2002). The primary source rocks include the trans-Himalayan batholiths, high Himalayan crystalline sequence, Tethyan sedimentary series and lesser Himalayas (Goodbred et al., 2014). In that region, approximately 122,100 inhabitants have a 4% population growth each year (Biswas and Al-Muyeed, 2022). ...
This is the first study to report the groundwater (GW) quality indices for drinking and irrigation purposes in Chuadanga Municipality, Bangladesh. GW samples were collected from 32 locations across the municipality's nine administrative wards. The physical parameters, such as total dissolved solids (TDS): 383 ppm, pH: 7.4, and electrical conductivity (EC): 583 μs/cm were reported, respectively. Among the detected chemical components, arsenic (As): 0.16 mg/L, iron (Fe): 0.77 mg/L, and manganese (Mn): 0.75 mg/L were found beyond the standard values. Geospatial analysis and multivariate analyses, such as correlation coefficient analysis (CCA), cluster analysis (CA), and principal component analysis (PCA), were performed to determine the potential sources of contaminants in the GW of the selected study areas. The water quality index (WQI) and heavy metal pollution index (HPI) suggest that most samples are unsuitable for drinking and irrigation. Following the water quality indices, CCA indicates the significant impact of As and Mn on water quality. The CA and PCA suggest geogenic, anthropogenic, and natural processes are mainly responsible for releasing those contaminants in GW.
... Using bulk Sr concentration of stored sediments, which has been shown to be a reliable indicator of river source in this system [62][63][64] Esri, GEBCO, DeLorme, NaturalVue ...
... Samples were also analyzed for bulk geochemistry by X-ray fluorescence using a portable Thermoscientific Niton XL3 Analyzer (pXRF), which returns information on composition of both major and trace elements in the sediments. Strontium concentrations from the XRF results are used to document sediment provenance following published methods 62 , effectively distinguishing sediment deposited by the Ganges, Brahmaputra, mixed Ganges-Brahmaputra, or other local river sources [62][63][64] (Fig. S1). Sediments having higher bulk Sr values >130 ppm are derived from the Brahmaputra basin, which drains mafic batholiths along the Tsangpo suture zone, compared with lower bulk Sr values <110 ppm for the Ganges (in central and western areas of the delta) or local sediment sources in the north (Tista River), northeast (Shillong Massif), and east (Indo-Burman Fold Belt). ...
The principal nature-based solution for offsetting relative sea-level rise in the Ganges-Brahmaputra delta is the unabated delivery, dispersal, and deposition of the rivers’ ~1 billion-tonne annual sediment load. Recent hydrological transport modeling suggests that strengthening monsoon precipitation in the 21st century could increase this sediment delivery 34-60%; yet other studies demonstrate that sediment could decline 15-80% if planned dams and river diversions are fully implemented. We validate these modeled ranges by developing a comprehensive field-based sediment budget that quantifies the supply of Ganges-Brahmaputra river sediment under varying Holocene climate conditions. Our data reveal natural responses in sediment supply comparable to previously modeled results and suggest that increased sediment delivery may be capable of offsetting accelerated sea-level rise. This prospect for a naturally sustained Ganges-Brahmaputra delta presents possibilities beyond the dystopian future often posed for this system, but the implementation of currently proposed dams and diversions would preclude such opportunities.
... Subsequently, the final model was developed using both the training and the testing data sets. In the second step, the final model was validated against 26 lithologs that were compiled from the published literature of Umitsu (1987), Goodbred and Kuehl (2000), and Goodbred et al. (2014). ...
... Grid-based comparison of the model's predicted lithologic succession to the test lithologs showed an 81% match (step I), while the lithologs compiled from published data (Umitsu, 1987;Goodbred and Kuehl, 2000;Goodbred et al., 2014) exhibited a 67% match (step II), indicating good model performance (Figs. S5 and S6). ...
The Ganges River delta complex contains a transboundary aquifer system shared between India and Bangladesh. Although it serves as the main freshwater source for the population inhabiting the delta, the aquifer system is severely contaminated with arsenic (As). This study aimed to determine the control of the delta hydrostratigraphy on the regional-scale depth distribution of As within the aquifer system. We developed the first high-resolution, regional-scale, transboundary hydrostratigraphic model of the Ganges River delta and analyzed the patterns of As distribution as a function of the hydrostratigraphy. Model results indicate that, despite the presence of a single aquifer system across the delta, the hydrostratigraphy is spatially variable and can be architecturally divided into three distinct aquifer subsystems from northwest to southeast: a single, thick continuous aquifer (type I); a vertically segregated, semi-confined aquifer subsystem (type II); and a multilayered, nearly confined aquifer subsystem (type III). Results indicate that the spatial distribution of As is characteristically different in each subsystem. In the type I aquifer, As concentrations tend to be homogeneous at all depths, while in type II and type III aquifers, As concentrations sharply decrease with depth. The intervening aquitards in the type II and type III aquifer subsystems appear to act as natural barriers to infiltration of surficial As or organic matter–rich water to the deeper aquifer zones. This delineation of the regional-scale hydrostratigraphic architecture and resulting understanding of its plausible controls on the depth-distribution of As within the delta should significantly aid in the systematic framing of sustainable management plans for the As-safe aquifers within the Ganges River delta aquifer system.
... Changes in the monsoonal climate affect sedimentation and river base levels over various timescales (e.g., Plink-Björklund, 2015), but their effects in the proximal foreland are not well understood. While river base levels close to the coast are primarily affected by sea level (e.g., Goodbred et al., 2014), those in the frontal foreland are primarily influenced by sediment supply and the combined effects of climate and tectonics (e.g., Bookhagen et al., 2005). This impact of climate on the base level should be taken into account when interpreting tectonic deformation from geomorphology; here, we study this effect in the frontal Himalaya of central Nepal. ...
... Sinha et al., 2007;. V: Ganges-Brahmaputra fan delta (Goodbred & Kuehl, 2000a, 2000bGoodbred et al., 2014). 2. Near the tip of the Bardibas Thrust, high-angle bedding planes and deformation bands were observed in the sediment cores, ∼250 m above the fault tip. These structures characterize the shallow deformation ahead of the propagating fault tip, demonstrating that deformation by the blind thrust reaches to shallow depths via local folding and small-scale faults. ...
To investigate the subsurface structure surrounding the Main Frontal Thrust (MFT) in central Nepal, we drilled and cored sediments to depths of 45–100 m at 10 sites. Our boreholes were located along previously acquired high‐resolution seismic profiles across the MFT imaging the upper 1–2 km of the subsurface, which revealed a beveled erosional surface in the hanging wall above a broad, gentle anticline, as well as growth strata in the footwall. The boreholes exhibit interlayered clays, silts, sands, and gravels, dated with optically stimulated luminescence and radiocarbon to <72.5 ± 4.3 ka, with a transition from finer to coarser sediments at ∼13.5 ± 0.1 ka. Near the fault tip, the deposits exhibit steeper dips and deformation bands. A 25‐m‐thick section of silt and clay above the south end of the buried anticline is interpreted as a temporary lacustrine depocenter formed due to uplift near the fault tip. Based on the distribution of marker beds and sediment ages, we interpret a shortening rate of 3.1–12.1 m/ka on the MFT. Three major transitions between fluvial‐lacustrine and coarse fluvial channel facies are inferred from the boreholes, and the timings of these transitions correlate with Indian monsoonal intensity variations linked to Earth's precession. We infer that a strengthened monsoon led to increased river discharge and advance of coarse bedload‐dominant braided channels, whereas a weak monsoon formed a finer‐grained channel environment. These monsoonal climate variations have affected the depositional environment and river base levels in this region, influencing the formation and apparent relative uplift of nearby river terraces.
... 7 ka; Figure 20.3a). After this time, and as sea-level continued to rise, the rivers had essentially filled up their palaeovalleys and were then free to avulse and migrate over a wider area: this resulted in numerous channel reorganizations from the mid-Holocene to present (Pickering et al. 2014;Goodbred et al. 2014;Figure 20.3a). These provenance studies also suggest that the basin hydrology may have provided a first-order control upon sedimentation, strongly modifying the potential influence of tectonic subsidence. ...
... C: By the Late Holocene, the principal rivers interact on the central delta plain, shifting toward the present-day confluence and the estuary that receives discharge from all the rivers. Source: Modified fromGoodbred et al. (2014).(b) Palaeogeographic reconstruction of the timing and location of river channel change and associated delta lobes during the Middle-Late Holocene. ...
Bangladesh is dominated by three great rivers – the Jamuna–Brahmaputra, Ganga, and Meghna – that combine to feed sediment into one of the World's largest deltas in the Bay of Bengal. The Jamuna River has developed in a region of significant tectonic activity associated with Himalayan uplift and development of the Bengal foredeep. The bedload, although only ~10% of the total sediment load, is critical in generating a wide array of bedforms of different scale that drive channel change and migration. Within the Jamuna River, the ubiquitous occurrence of bifurcations and confluences is a key aspect of the river channel pattern and dynamics, and these features form important nodes in the braidbelt. The nature of floodplain sedimentation and inundation is vital in planning annual crop growth and may adopt great significance in the ongoing debate on the sources, causes, and accumulation of arsenic in the groundwaters of Bangladesh.
... Changes in the monsoonal climate affect sedimentation and river base levels over various timescales (e.g., Plink-Björklund, 2015), but their effects in the proximal foreland are not well understood. While river base levels close to the coast are primarily affected by sea level (e.g., Goodbred et al., 2014), those in the frontal foreland are primarily influenced by sediment supply and the combined effects of climate and tectonics (e.g., Bookhagen et al., 2005). This impact of climate on the base level should be taken into account when interpreting tectonic deformation from geomorphology; here, we study this effect in the frontal Himalaya of central Nepal. ...
... Sinha et al., 2007;. V: Ganges-Brahmaputra fan delta (Goodbred & Kuehl, 2000a, 2000bGoodbred et al., 2014). 2. Near the tip of the Bardibas Thrust, high-angle bedding planes and deformation bands were observed in the sediment cores, ∼250 m above the fault tip. These structures characterize the shallow deformation ahead of the propagating fault tip, demonstrating that deformation by the blind thrust reaches to shallow depths via local folding and small-scale faults. ...
... paucity of knowledge about ancient peoples' survival techniques and livelihoods. However, although tropical conditions such as flooding and river migration certainly take place (Dewan et al., 2017;Goodbred et al., 2014;Richardson and Thorne, 2001;Sincavage et al., 2018) as well as inadequate research methods put severe limitations on our understanding of the nature of the archaeological resource, by combining geoarchaeology (Akanda et al., 2015;Hu et al., 2020) and archaeobotany (Rahman et al., 2020) with the systematic survey of epigraphic records (Khan, 2007) it is possible to reveal vital information about ancient land use and interactions between ancient humans and this extraordinary riverine land. ...
... However, monsoonal precipitation, frequent river flooding and migration of the Ganges, Brahmaputra and Meghna, appeared to have major impacts for the continuation of settlement at a certain place (Akanda et al., 2015;Dewan et al., 2017;Hu et al., 2020;Sen, 2015a;Sincavage et al., 2018). It is notable that only the Brahmaputra River has alternated between Meghna, Old Brahmaputra and Jamuna rivers several times since about 18ka BP (Goodbred et al., 2014;Sincavage et al., 2018). Many oxbow lakes and scattered abandoned channels all over the country also indicate frequent river migrations (Richardson and Thorne, 2001). ...
Due to the exceptionally rich tropical resource, the Lower Ganges-Brahmaputra basins have attracted people of diverse
ethnic and geographical backgrounds for millennia. So far 524 protected sites in present Bangladesh indicate the busy
human occupation in the world’s largest delta at least from 5th century BCE. Although systematic archaeology began in
the 1870s there is still a paucity of knowledge about past human land use and livelihood strategies across this area, which
is especially prone to floods, cyclones, and river migrations. Here we attempt a systematic survey of human-environment
interactions in ancient deltaic Bangladesh. Revisiting the fragmentary information from archaeological records and
epigraphic references produced through over a century-long archaeological legacy, this study is the first attempt at a
synthesis of the changing relationships between ancient people and their environment elements including land, water
bodies, flora and fauna.
... The Brahmaputra basin covers five countries -Bangladesh, Bhutan, China, India and Nepal Goodbred et al. 2014). Presumably, the Brahmaputra River enters Bangladesh from Dhuburi in India through the Garo Hills. ...
... In the past, Jamalpur and Mymensingh districts were connected to the Meghna River at Bhairab Bazar. Before joining the Ganges, the river Jamuna flowed into the Baral, Atrai and Hurasagar rivers, which later formed on the sizeable Dhaleshwari river (Best et al. 2007;Goodbred et al. 2014). This section of the Brahmaputra flowing in Bangladesh is known as the Lower Brahmaputra Basin, which created sediments that rigorously influenced the livelihood (Ericksen, Ahmad and Chowdhury, 1996;De Graaf, 2003;Touchart et al. 2012). ...
Bangladesh is home to the largest delta in the world consisted of the Lower Ganges, Brahmaputra and Meghna river basins. Being located in the tropical region, rich biodiversity, rainfall and abundance of rivers attracted people of different ethnic backgrounds from the early historic period. The paper presents the results of the archaeological survey (2018-20) conducted in Khirtala village of Sirajganj district located in the Barind tract of Lower Brahmaputra Basin. Nine archaeological mounds are identified in the village. Sites are located close to the dried and present river channel of Karatoya River. Most of the archaeological localities are currently inhabited by different ethnic groups, which pose a threat to the archaeological sites.
... The Ganges, the Brahmaputra and the Meghna together form the largest delta and deep-sea fan on Earth, and this region is vulnerable to climate change (Goodbred and Kuehl 1999;Goodbred et al. 2014). The modern sediment load of the Ganges-Brahmaputra can reach c.1×1 0 9 ta -1 (Milliman and Farnsworth 2013), ranking it first among all rivers globally (Goodbred and Kuehl 2000). ...
... Geologically, Swarnadwip's evolution is tied to its location within the GBM delta, shaped by long-term sediment deposition and tectonic subsidence in the Bengal Basin. Sediments transported by the GBM river system are poorly consolidated and prone to tidal erosion (Goodbred et al. 2014), which undoubtedly played a critical role in shaping the morphology of islands in the Meghna estuary, including Swarnadwip. Historical sea-level changes have also impacted sediment dynamics and deltaic evolution of GBM , and this serves as another key factor contributing to the island's ongoing transformation. ...
A comprehensive analysis of coastline changes was conducted over a span of two decades for Swarnadwip, an offshore island of Bangladesh in the Bay of Bengal. Located near the Meghna estuary, the island is influenced by the vast sedimentary contributions of the Ganges-Brahmaputra-Meghna (GBM) river system. This makes Swarnadwip a focal point for research aimed at deepening our understanding of coastal morpho-dynamics. In this research, three different shoreline detection strategies were explored, and the suitability of each method was assessed to identify the most optimum method for this geographically complex region. Changes in the island’s area were calculated for each time interval, and further shoreline change statistics (NSM, EPR and LRR) were estimated using the Digital Shoreline Analysis System (DSAS) on the GIS platform. The study revealed that from 2003 to 2022, shoreline length extended from 50.6 to 74.15 km (1.24 km/yr), and net land accretion was estimated 10,230.20 ha (538.43 ha/yr). Between 2006 and 2010, the island witnessed a substantial land expansion. However, much of this newly acquired land mass was lost again, as erosional forces governed from 2010 to 2022, especially in the northern and western sections. These findings provide critical insights for future coastal management strategies and highlight the need for continuous monitoring to safeguard Bangladesh’s dynamic coastlines.
... Fourteen samples for OSL dating (Table S2.2) were obtained with a light-tight vertical lined sampler which collects 30-cm of undisturbed sediment within a borehole and by hammering light-tight, lined PVC tubes horizontally into exposed pond walls. All samples were analyzed for grain-size distribution and bulk strontium content (Table S2.2) which is an indicator of provenance of sand-rich deposits 34 . Luminescence dating was performed using the 4-11 or 4-20 micron silt fraction 29 (Table S2.3) which was isolated and measured using procedures of ref. 107 and described below. ...
Earthquakes present severe hazards for people and economies and can be primary drivers of landscape change yet their impact to river-channel networks remains poorly known. Here we show evidence for an abrupt earthquake-triggered avulsion of the Ganges River at ~2.5 ka leading to relocation of the mainstem channel belt in the Bengal delta. This is recorded in freshly discovered sedimentary archives of an immense relict channel and a paleo-earthquake of sufficient magnitude to cause major liquefaction and generate large, decimeter-scale sand dikes >180 km from the nearest seismogenic source region. Precise luminescence ages of channel sand, channel fill, and breached and partially liquefied floodplain deposits support coeval timing of the avulsion and earthquake. Evidence for reorganization of the river-channel network in the world’s largest delta broadens the risk posed by seismic events in the region and their recognition as geomorphic agents in this and other tectonically active lowlands. The recurrence of comparable earthquake-triggered ground liquefaction and a channel avulsion would be catastrophic for any of the heavily populated, large river basins and deltas along the Himalayan arc (e.g., Indus, Ganges, Brahmaputra, Ayeyarwady). The compounding effects of climate change and human impacts heighten and extend the vulnerability of many lowlands worldwide to such cascading hazards.
... In the Bengal Basin, accurate sedimentological data (Weinman et al., 2008) and the lithostratigraphic architecture of the subsurface hydrogeological system (Goodbred and Kuehl, 2000;Goodbred et al., 2014) have been integrated in the model by hydrostratigraphic data (Winkel et al., 2011;Chakraborty et al., 2022b). Aquifer/aquitard relations (Mukherjee et al., 2011), aquifer connectivity and surficial aquitard thickness (Chakraborty et al., 2020) have been investigated as potential controlling factors of groundwater and contaminant flow paths. ...
... While the Th/U ratios in sedimentary rocks are in the range of 0.02 to 21 (Adams and Weaver, 1958), most marine sediments have ratios much <7. The high Th/U ratio of the G-B shelf sediments ( Fig. 5; 5.5 to 9.3) may be due to monazites from the Monzonitic THB that are present in exposed outcrops of the Brahmaputra watershed (GoodbredJr et al., 2014). The Th/U ratio of the Mahanadi shelf sediments (2.2-29, except for one sample with a ratio of 33) falls along the mixing curve defined by the peninsular granitic and gneissic complex and the U-rich mineral deposits (of Proterozoic and Gondwana age) and monazites. ...
Sediments deposited on the eastern continental shelf of India are studied for texture, mineralogy and their major,
trace, and rare earth elemental composition in lithogenic fraction. The objective of this study was to identify the
sources, and evaluate the factors affecting the erosion and weathering in the catchment area and dispersal and
deposition of sediments over the shelf region. Here, we present an extensive dataset covering the entire eastern
continental shelf of India from the Ganges-Brahmaputra (G-B) basin in the north including the shelf off major
peninsular Indian river basins of Mahanadi, Godavari, and Krishna up to the Kaveri basin in the southern tip.
Textural and geochemical studies have shown that the lithology of the source rocks plays an important role in
governing the chemistry and mineralogy of the sediments. The results show that the G-B shelf sediments are
primarily derived from the Higher Himalaya Crystalline sequence (HHCS) and the Tethyan sedimentary series
(TSS) exposed along the peaks and drainage divide of the Himalayas. Mahanadi shelf sediments are mainly
derived from the felsic rocks belonging to Precambrian metamorphic formations of the Eastern Ghats, limestones,
sandstones, and shales of the Gondwanas and recent deltaic alluvium deposits, whereas the Krishna-Godavari (KG)
sediments find a part of their source in the Deccan Basalts. The shelf sediments off River Kaveri are derived
from a mixture of peninsular granitic gneisses, tonalite-trondjhemite gneisses, and charnockites. The sediment
geochemistry has also allowed us in assessing the weathering patterns in the source areas. The sediments have
undergone a moderate degree of weathering. Weathering intensities were found to have varied with the source
area lithology.
The least square regression of the geochemical data allowed us to estimate the relative contribution of each of
these sources and their geographic extent. Unlike the massive offshore deposition and formation of a major Fan
system (the Bengal fan), the fluvial sediment dispersal of the mighty Himalayan Rivers on the continental shelf is
significant only at the G-B mouth and their contribution reduces to <50% at about 18oN. The sediment dispersal
from the Godavari River is seen to extend on either side of the mouth of the river which is attributed to the
reversing East India Coastal Current (EICC), which plays an important role, in the sediment transport along the
central and south-central part of the east coast of India. The influence of the Mahanadi, Krishna, and Kaveri River
sediments is however confined to a small area near their mouths and has a limited geographic extent of influence
in the shelf region.
... The Ganga and Brahmaputra rivers joined two centuries ago to form the Meghna Estuary, together with the Meghna River sourced in the Indo-Burman Ranges and draining the Shillong Plateau and Bangladesh lowlands (Goodbred et al. 2014). Total annual water and sediment fluxes of~970 km 3 and~1.7 billion tons are delivered by the Meghna Estuary, mostly during the summer monsoon (Milliman and Meade 1983;Goodbred and Kuehl 1999). ...
High-resolution petrographic and heavy-mineral analyses of Bengal Fan turbidites from six cores drilled during IODP Expeditions 353 and 354 elucidate factors controlling their intersample compositional variability as a key to understanding sedimentary processes and erosional evolution of the Himalayan belt since the Miocene. Bengal Fan turbidites are feldspatho-quartzose to litho-feldspatho-quartzose with plagioclase > K-feldspar; slow-settling micas increase in abundance in very fine sand and coarse silt. The feldspar/quartz ratio and higher-rank metamorphic rock fragments notably increase from uppermost Miocene to Pleistocene deposits, which is ascribed to the onset of rapid exhumation of the Eastern Himalayan syntaxis since ∼ 5 Ma. The same trends are documented in Nicobar Fan turbidites, confirming that they belong to the same sedimentary system. Both Bengal and Nicobar fans record a pulse in mass accumulation rate at Tortonian times, when supply of sedimentary and very-low-grade metasedimentary detritus reflected accelerated exhumation of the Lesser Himalaya. In contrast to foreland-basin sediments, where ferromagnesian minerals have been completely dissolved in strata as young as Pliocene–Pleistocene, in both Bengal–Nicobar and Indus fans amphibole invariably represents about half of the moderately rich to rich transparent-heavy-mineral suite, demonstrating that amphibolite-facies Greater Himalaya metamorphic rocks were widely exposed in the Himalayan range well before the late Miocene and possibly since the late Oligocene, as indicated by a few sillimanite and kyanite grains in Bengal Fan sediments as old as 23 Ma and 28 Ma, respectively. Diagenetic dissolution strongly affected olivine and pyroxene in strata older than the middle and early Pleistocene, respectively, whereas amphibole decreases markedly through progressively older Miocene strata. Ferromagnesian minerals and sillimanite are almost completely dissolved in lower Miocene strata, where durable zircon, tourmaline, rutile, and apatite make up half of the strongly depleted heavy-mineral assemblage. Quaternary turbidites from the six studied cores have virtually the same compositional signatures, testifying to efficient homogenization by turbidite transport and reworking across the fan. Turbidites in western cores closer to peninsular India (U1444A and U1454B) are not different from those in eastern cores, indicating very minor supply from the subcontinent. Forward-mixing calculations based on integrated petrographic and heavy-mineral data indicate that sand supply from the Brahmaputra River to Quaternary turbidites was four times larger than supply from the Ganga River, indicating up to six times higher sediment yields and erosion rates in the Brahmaputra than in the Ganga catchment, largely reflecting superfast erosion of the Eastern Himalayan syntaxis.
... Strontium and neodymium are key trace elements because their concentrations and isotopic compositions are useful tracers for source identification (e.g., Galy et al., 1996;France-Lanord and Derry, 1997;Goodbred et al., 2014;Galy et al., 2008;Lenard et al., 2020). Strontium shows a significant loss (3%-67%) with decarbonation ( Figure F5A). ...
... normalization factor (Covelli and Fontolan, 1997;Menon et al., 1998;Liaghati et al., 2003;Goodbred et al., 2014). Element ratios that were adopted for the discrimination of ultramafic versus non-ultramafic source-rock composition in Po-Adriatic sediments and that proved very effective in reducing the textural effect are Cr/Al 2 O 3 (Amorosi et al., 2002Lucchini et al., 2003;Curzi et al., 2006;Dinelli et al., 2007Dinelli et al., , 2012Amorosi, 2012;Ilijanić et al., 2014;Buscaroli et al., 2021), Cr/V (Amorosi and Sammartino, 2007;Dinelli et al., 2012;Amorosi et al., 2014) and Ni/Al 2 O 3 (Dinelli and Lucchini, 1999;Ilijanić et al., 2014;Greggio et al., 2018). ...
The Po-Adriatic region offers an excellent case for reconstructing sediment provenance and transport pathways of a multi-sourced sediment-routing system. Through a comprehensive set of ∼1400 geochemical data, a model for provenance and sediment flux was built based on distinct compositional fingerprints of 53 fluvial systems and their comparison to coastal, shelf and deep-marine sediments. Geochemically unique catchment lithologies (mafic/ultramafic rocks, limestones and dolostones) were used as end-members to assess exclusive source-rock signatures. Following calibration with sedimentary facies, selected key elements and element ratios poorly sensitive to particle size (Ni/Cr, MgO, Ni/Al2O3, Cr/V, Ca/Al2O3 and Ce/V) were adopted as provenance indicators. The high-Ni and high-Cr source-rock signature of mafic/ultramafic rocks widely exposed in the Po River watershed and along the Albanian Dinarides contrasts markedly with the high-Ca (and locally high-Mg) geochemical composition of Eastern Alpine, Apennine, and Eastern Adriatic (Montenegro, Croatia, Slovenia) river catchments, which are, instead, carbonate-rich and virtually ophiolite-free. Relatively high Ce values from Apulian river samples serve as a key marker for a minor, but very distinct sediment provenance from southern Apennine alkaline volcanic rocks.
Despite along-shore mixing and dilution with sediment sourced from other river catchments, the geochemical signature of Adriatic shelf muds primarily reflects composition of sediment eroded from the contiguous continental areas. Chromium-rich and nickel-rich detritus generated in mafic and ultramafic complexes of the Western Alps and conveyed through the Po River into the Adriatic Sea records a geochemical signal that can be traced downstream as long as 1000 km, from the Alpine zone of sediment production to the area of final deposition, offshore Apulia.
While longitudinal dispersion linked to the general cyclonic, counter-clockwise Adriatic circulation is prevailing along the Western Adriatic Sea, conspicuous detrital input from transversal pathways to the deep sea is revealed across the Eastern Adriatic shelf using heavy metals as provenance tracers. Estimates of fluvial sediment loads and compositional fingerprinting of fluvial, coastal and shelf sediments indicate that previously neglected ophiolite-rich successions of Albania represent a major sediment-conveyor to the offshore sinks (Southern Adriatic Deep and Mid-Adriatic Deep) through significant cross-shore and NNW-directed sediment transport in the Eastern Adriatic Sea. A cut-off value of the Ni/Cr ratio targeted around 0.8 represents an effective tool for the differentiation in marine sediments of Ni-rich (serpentine-rich) ophiolite detritus of Albanian origin from mafic/ultramafic sources of Alpine affinity. High trace-metal contents found within the Adriatic deep basin are mostly of natural origin and only minimally reflect metal contamination.
... This bedload and the avulsions of river channels that its deposition drives, built up the shallow aquifer during the Holocene across the GBMD. During the Holocene, the Brahmaputra River periodically flowed through the subsiding Surma Basin (Acharyya et al., 2000) to the east of the tectonically uplifted Madhupur Tracts in central Bangladesh where the capital city of Dhaka is located (Goodbred et al., 2014) (Fig. 1a). This was the situation in 1770 when the Brahmaputra flowed through Araihazar joining up with the Meghna River (Islam, 2016;Sarker et al., 2003). ...
Shallow (<30 m) reducing groundwater commonly contains abundant dissolved arsenic (As) in Bangladesh. We hypothesize that dissolved As in iron (Fe)-rich groundwater discharging to rivers is trapped onto Fe(III)-oxyhydroxides which precipitate in shallow riverbank sediments under the influence of tidal fluctuations. Therefore, the goal of this study is to compare the calculated mass of sediment-bound As that would be sequestered from dissolved groundwater As that discharges through riverbanks of the Meghna River to the observed mass of As trapped within riverbank sediments. To calculate groundwater discharge, a Boussinesq aquifer analytical groundwater flow model was developed and constrained by cyclical seasonal fluctuations in hydraulic heads and river stage observed at three sites along a 13 km reach in central Bangladesh. At all sites, groundwater discharges to the river year-round but most of it passes through an intertidal zone created by ocean tides propagating upstream from the Bay of Bengal in the dry season. The annualized groundwater discharge per unit width at the three sites ranges from 173 to 891 m²/yr (average 540 m²/yr). Assuming that riverbanks have been stable since the Brahmaputra River avulsed far away from this area 200 years ago and dissolved As is completely trapped within riverbank sediments, the mass of accumulated sediment As can be calculated by multiplying groundwater discharge by ambient aquifer As concentrations measured in 1969 wells. Across all sites, the range of calculated sediment As concentrations in the riverbank is 78–849 mg/kg, which is higher than the observed concentrations (17–599 mg/kg). This discovery supports the hypothesis that the dissolved As in groundwater discharge to the river is sufficient to account for the observed buried deposits of As along riverbanks.
... Uncertainty as to the trajectory of regional climate and anthropogenic changes also have the potential to affect NbS strategies. Monsoonal river discharge and sediment loads of the Ganges and Brahmaputra Rivers are some of the highest in the world and drive significant river avulsion and lateral movement (Goodbred et al., 2014). However, spatially variable modifications in rivers and tidal hydrodynamics attributed to infrastructure building both upstream of and within Bangladesh since the 1960s have themselves altered natural river processes (Gazi et al., 2020). ...
The Ganges-Brahmaputra (GB) delta is one of the most disaster-prone areas in the world due to a combination of high population density and exposure to tropical cyclones, floods, salinity intrusion and other hazards. Due to the complexity of natural deltaic processes and human influence on these processes, structural solutions like embankments are inadequate on their own for effective hazard mitigation. This article examines Nature-based Solutions (NbS) as a complementary or alternative approach to managing hazards in the GB delta. We investigate the potential of NbS as a complementary and sustainable method for mitigating the impacts of coastal disaster risks, mainly cyclones and flooding. Using the emerging framework of NbS principles, we evaluate three existing approaches: tidal river management (TRM), mangrove afforestation, and oyster reef cultivation, all of which are actively being used to help reduce the impacts of coastal hazards. We also identify major challenges (socioeconomic, biophysical, governance and policy) that need to be overcome to allow broader application of the existing approaches by incorporating the NbS principles. In addition to addressing GB delta-specific challenges, our findings provide more widely applicable insights into the challenges of implementing NbS in deltaic environments globally.
... Another important consideration impacting compaction is the incision of the delta during the last glacial maximum (LGM). While lowstand deltas are found offshore near the shelf edge (Palamenghi, 2012), within the GBD, a large valley 60-90 m deep was incised into older Pleistoceneaged sediments during the LGM (Fig. 1, Pickering et al., 2014;Goodbred et al., 2014). Previously buried sediments do not significantly decompact with unloading (Chapman, 1983). ...
Deltas, the low-lying land at river mouths, are sensitive to the delicate balance between sea level rise, land subsidence and sedimentation. Bangladesh and the Ganges-Brahmaputra Delta (GBD) have been highlighted as a region at risk from sea-level rise, but reliable estimates of land subsidence have been limited. While early studies suggested high rates of relative sea-level rise, recent papers estimate more modest rates. Our objective is to better quantify the magnitude, spatial variability, and depth variation of sediment compaction and land subsidence in the lower GBD to better evaluate the processes controlling them and the pattern of relative sea level rise in this vulnerable region. We combine subsidence and compaction estimates from hand-drilled tube wells and historic sites (1–5 mm/y), GNSS and river gauges (4–8 mm/y) and RSET-MH and borehole vertical strainmeters (9–10 mm/y) in SW Bangladesh. The differences between the different types of measurements reflect the different timescales, spatial distribution and depth sensitivity of the different observations. Rates are lower for times >300y providing data on the timescale of compaction. We also observe differences related to the degree to which different devices measure shallow and deep subsidence. Higher values reflect a greater component of subsidence from young shallow deposits from soil compaction and organic matter degradation. Thus, we observe different rates for different environments and physical settings. These differences indicate that in planning adaptation for rising sea level, hard construction with a solid foundation may experience different subsidence rates than open fields or reclaimed land with recent natural or anthropogenic sedimentation.
Significance statement
Land subsidence increases the impact of sea level rise. We combine six different types of measurements that examine land subsidence in coastal Bangladesh. The results show that causes of subsidence, including compaction of the sediments varies both spatially and with depth, and that compaction and organic matter degradation from young shallow deposits is a significant contribution to subsidence. This suggests that hard construction with a solid foundation, such as buildings and embankments, may experience a lower subsidence rates than open fields or reclaimed land with recent natural or anthropogenic sedimentation.
... Examination of the Bengal Basin sedimentary archives in recent years has yielded detailed insights into allogenic and autogenic controls on the transfer and storage of detritus through the Ganges-Brahmaputra delta-fan system across spatial and temporal scales (Alam et al., 2003;Bergmann et al., 2020;Blum et al., 2018;Bracciali et al., 2015;Goodbred et al., 2014;Govin, Najman, Copley, et al., 2018;Govin, Najman, Dupont-Nivet, et al., 2018;Lang & Huntington, 2014;Pickering, Carter, et al., 2020;Pickering et al., 2014;Sincavage et al., 2018Sincavage et al., , 2020. Above (Section 5.1), we postulate that ∼18-15 Ma integration of the ancestral Brahmaputra and Yarlung watersheds occurred within or shortly after early Miocene (∼20-17 Ma) tectonic uplift and subsequent reversal of the paleo-Yarlung river (Carrapa et al., 2014;DeCelles et al., 2011;Leary et al., 2016), delivering the first Trans-Himalayan Arc sediment to the Bengal Basin (Bracciali et al., 2015). ...
This article is open access and now available online at G-cubed via the above DOI link.
Abstract: The Bengal Basin preserves the erosional signals of coupled tectonic-climatic change during late Cenozoic development of the Himalayan orogen, yet regional correlation and interpretation of these signals remains incomplete. We present a new geologic map of fluvial-deltaic deposits of the Indo-Burman Ranges (IBR), five detrital zircon fission track (dzFT) analyses, and twelve high-n detrital zircon U-Pb age distributions (dzUPb) from the Barail (late Eocene–early Miocene), Surma (early–late Miocene), and Tipam (late Miocene–Pliocene) Groups of the ancestral Brahmaputra Delta. We use dzUPb statistical tests to correlate the IBR units with equivalent age strata throughout the Bengal Basin. An influx of trans-Himalayan sediment and the first appearance of ~50 Ma grains of the Gangdese Batholith in the lower Surma Group (~18-15 Ma) records the early Miocene arrival of the ancestral Brahmaputra delta to the Bengal Basin. Contributions from Himalayan sources systematically decrease up section through the late Miocene as the contribution of Trans-Himalayan Arc sources increases. The Miocene (~18-8 Ma) deposition of the Surma Group records upstream expansion of the ancestral Brahmaputra river into southeastern Tibet. Late Miocene (< 8 Ma) progradation of the fluvial part of the delta (Tipam Group) routed trans-Himalayan sediment over the shelf edge to the Nicobar Fan. We propose that Miocene progradation of the ancestral Brahmaputra delta reflects increasing rates of erosion and sea level fall during intensification of the South Asian Monsoon after the Miocene Climate Optimum, contemporaneous with a pulse of tectonic uplift of the Himalayan hinterland and Tibet.
... Examination of the Bengal Basin sedimentary archives in recent years has yielded detailed insights into allogenic and autogenic controls on the transfer and storage of detritus through the Ganges-Brahmaputra delta-fan system across spatial and temporal scales (Alam et al., 2003;Bergmann et al., 2020;Blum et al., 2018;Bracciali et al., 2015;Goodbred et al., 2014;Govin, Najman, Copley, et al., 2018;Govin, Najman, Dupont-Nivet, et al., 2018;Lang & Huntington, 2014;Pickering, Carter, et al., 2020;Pickering et al., 2014;Sincavage et al., 2018Sincavage et al., , 2020. Above (Section 5.1), we postulate that ∼18-15 Ma integration of the ancestral Brahmaputra and Yarlung watersheds occurred within or shortly after early Miocene (∼20-17 Ma) tectonic uplift and subsequent reversal of the paleo-Yarlung river (Carrapa et al., 2014;DeCelles et al., 2011;Leary et al., 2016), delivering the first Trans-Himalayan Arc sediment to the Bengal Basin (Bracciali et al., 2015). ...
This article is open access and now available online at G-cubed.
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GC010026
Abstract: The Bengal Basin preserves the erosional signals of coupled tectonic-climatic change during late Cenozoic development of the Himalayan orogen, yet regional correlation and interpretation of these signals remains incomplete. We present a new geologic map of fluvial-deltaic deposits of the Indo-Burman Ranges (IBR), five detrital zircon fission track (dzFT) analyses, and twelve high-n detrital zircon U-Pb age distributions (dzUPb) from the Barail (late Eocene–early Miocene), Surma (early–late Miocene), and Tipam (late Miocene–Pliocene) Groups of the ancestral Brahmaputra Delta. We use dzUPb statistical tests to correlate the IBR units with equivalent age strata throughout the Bengal Basin. An influx of trans-Himalayan sediment and the first appearance of ~50 Ma grains of the Gangdese Batholith in the lower Surma Group (~18-15 Ma) records the early Miocene arrival of the ancestral Brahmaputra delta to the Bengal Basin. Contributions from Himalayan sources systematically decrease up section through the late Miocene as the contribution of Trans-Himalayan Arc sources increases. The Miocene (~18-8 Ma) deposition of the Surma Group records upstream expansion of the ancestral Brahmaputra river into southeastern Tibet. Late Miocene (< 8 Ma) progradation of the fluvial part of the delta (Tipam Group) routed trans-Himalayan sediment over the shelf edge to the Nicobar Fan. We propose that Miocene progradation of the ancestral Brahmaputra delta reflects increasing rates of erosion and sea level fall during intensification of the South Asian Monsoon after the Miocene Climate Optimum, contemporaneous with a pulse of tectonic uplift of the Himalayan hinterland and Tibet.
... Initially, the deltas comprise various marshes that can be used for aquaculture (Alam, Sasaki & Datta, 2017). With sediment depositing, they can be gradually lifted, and eventually transform into saline-alkaline lands, as observed in the Yellow River delta, China (Yang, 2012;Goodbred et al., 2014;Liu et al., 2014). The Yellow River delta is the largest river delta in northern China, and its size is still growing (Fan et al., 2012;Mao et al., 2016). ...
Background
Crude flats in delta areas are often saline–alkaline and unsuitable for agricultural cropping. In the 1990s, people residing in the Yellow River delta constructed terraces on the flats for agricultural development. Herein, we investigated environmental changes resulting from this agricultural development and evaluated whether the current land use is effective and sustainable.
Methods
We sampled soil and weeds from croplands, terrace slopes, and crude flats within the delta terrace landscape. The measured soil properties included soil salinity, pH, total N, total P, and organic matter in different lands and soil layers: 0–10, 10–20, and 20–30 cm. The surveyed weed characteristics were the biomass of roots and rhizomes, species composition, life form, cover and height. These indices were statistically verified by different land types and soil layers.
Results
Soil salinity in the terrace crop lands was found to have reduced to <4 g·kg ⁻¹ , whereas in the crude flats, remained >6 g·kg ⁻¹ . Soil pH in the terrace croplands was surprisingly increased to >9 ; meanwhile, organic matter content decreased drastically, which is significantly different from that observed in the case of terrace slopes and crude flats. Total N and P content in the terrace crop lands were seemingly unchanged on averages but at the depths >20 cm, they reduced unavoidably. Plant underground organs were relatively scarcer in the croplands than in the other lands. Weeds grew well on the terrace slopes but were insufficient in the croplands. Overall, terrace construction is effective for developing coastal saline flats for agricultural use, but the traditional land use in the Yellow River delta has caused chronic soil degradation that is against a sustainable productive industry.
... During the second half of the Holocene, i.e., since eustatic sea level started stabilizing, the delta has prograded despite rising sea level due to the enormous annual fluvial water and sediment discharges of about 1300 km 3 and more than 1 Gt, respectively (Goodbred et al. , 2014Pate et al. 2009;Milliman and Farnsworth 2011). Presently, the submarine portion of the delta progrades across the shelf at an average lateral rate of 15 m/yr, for instance (Michels et al. 1998). ...
This study reconstructs the coastal subsidence over the past 1300 years in a mangrove region along the coast of the Ganges–Brahmaputra Delta, an area not affected by anthropogenic interference. The relative sea level (RSL) history is based on radiocarbon and luminescence ages measured on 108 submerged kilns and in situ mangrove stumps. While the regional, long-term average subsidence rate is calculated to be 2.7 ± 0.3 mm/yr, modern RSL (including eustacy, isostasy, ocean level, and subsidence) rises by 8.7 ± 0.4 mm/yr. This rate has been balanced by natural sediment accretion so far. A reduction in sediment supply by engineering projects along rivers and coasts may, however, accelerate coastal inundation and retreat. Subsidence has accelerated during specific episodes since the eighth century. A major land-sinking event happened in the eighteenth century, with a downward displacement of 1–2 m, depending on geographic area. We propose that the subduction-related 1762 Arakan earthquake caused this sudden lowering. Prior to this event, RSL was nearly stable for 900 years. An earlier major subsidence event occurred around 900 CE, when the land suddenly sank by about 1 m, which also coincided with a major earthquake along the Arakan coast. Event-driven, sudden, significant subsidence, thus, needs to be considered a potential major hazard for coastal Bangladesh.
... The Ganges (Padma), the Brahmaputra (Jamuna), and the Meghna-the three large river systems of the Indian sub-continent-have coalesced in the Bengal basin to form the world's largest delta (Goodbred et al. 2014). The Bengal delta is also one of the youngest and most dynamic alluvial environments in the world, as the preceding geological studies indicate that current morphological instabilities of its river systems may be traced back to at least 5000 years (mid-Holocene). ...
This paper reviews multiple issues of concern to women living in chars. The majority of chars women live with extreme poverty, social marginalization, low literacy rates, plus the need to cope with annual floods and displacement caused by periodic erosion events. Despite their social and economic disadvantages, women contribute significantly to the population’s resilience in the face of crises. The impacts of large-scale interventions (economic, educational, health, domestic, and political participation) are discussed in terms of their impacts on women’s lives. Challenges of service provision to island chars are daunting, but they can be overcome. Short-term, limited project interventions cannot solve problems at the required scale. Recommendations include putting women’s issues at the center of any new human development strategies and developing approaches that respect women’s dignity and strengths, rather than viewing them merely as “victims.” Governmental institutions need to do more than they have thus far, giving priority to chars women’s human rights andHuman rights and womens rights gender impacts of any future efforts must be carefully monitored, so that all can continue to learn from experience.
... The Ganges (Padma), the Brahmaputra (Jamuna), and the Meghna-the three large river systems of the Indian sub-continent-have coalesced in the Bengal basin to form the world's largest delta (Goodbred et al. 2014). The Bengal delta is also one of the youngest and most dynamic alluvial environments in the world, as the preceding geological studies indicate that current morphological instabilities of its river systems may be traced back to at least 5000 years (mid-Holocene). ...
Riverbank erosion is common as a disaster event in riverine Bangladesh. It not only affects people living in inland char areas and along the major braided rivers, but also the coastal areas. Charlands are characterized by both erosion and deposition. When the rate of deposition exceeds the rate of erosion, over time, some chars become attached to the mainland, particularly, in the coastal areas. While coping strategies of displaced people in inland areas have been widely studied, relatively little is known about such strategies for people displaced by riverbank erosion in coastal Bangladesh. This chapter addresses this gap via a questionnaire survey conducted among 413 households in 15 villages of Ramgati Upazila in the Lakshmipur District. The Survey was followed by Focus Group Meetings and Key Informant Interviews. All study villages are located along the eastern bank of the lower Meghna River, around the main outlet of the Ganges–Brahmaputra–Meghna drainage basin. The survey revealed that people of the study area mitigated impacts of riverbank erosion by adopting various coping strategies, some similar to those adopted by erosion affected people of inland areas and others different. Finally, a number of recommendations are made to assist those adversely affected by riverbank erosion in the lower Meghna River estuary in coastal Bangladesh.
... This delta has been considered as one of the most dynamic ones which attribute the highest sediment discharge and one of the three highest water discharges in the world. It was estimated that the Ganges-Brahmaputra-Meghna (GBM) systems had been produced the combined mean annual sediment load of 1.0 to 2.4 BT/year, in which the Brahmaputra contributed the highest (390 to 1160 MT/year) (Goodbred et al., 2014;Rahman et al., 2018) which is around 40% to 50% of the total incoming sediment in GBM delta. The Brahmaputra river originated from the southern glacier of the Himalaya and was named Jamuna River when it enters Bangladesh (Islam et al., 1999). ...
Jamuna, a major braided river in Bangladesh, has an enormous hydrological impact on the surrounding areas and streams. Erosion and sedimentation in the Jamuna river cause a large flow fluctuation and floods round the year. Bangladesh Water Development Board has initiated a pilot capital dredging project in the Jamuna river in 2011-2012, aiming to guide the flow to reduce the risk of failure of the city area and right guide bundh of the Jamuna Bridge. This study explores the long-term role of dredging on river morphology us-ing erosion-sedimentation numerical modeling approaches. Primary data were employed in numerical models to estimate the erosion-sedimentation and compared outputs with the real-time cross-sectional variation at selected sections along the reach during 2012-2013. The analysis suggested that the rate of sedimentation is higher (60% to 80%), where the dredging alignment crosses through the existing sandbar/char. Moreover, a cross-section com-parison revealed that the channel near Sirajganj Hardpoint shifted towards the left (east) bank, and the channel within the study area developed very fast along the right (west) bank. However, satellite image analysis revealed that the major bankline shifting occurred from 2000 to 2010 and the channel shifting was observed from 2014 to 2018 along the reach, mostly, after the construction of some river training works. The variation of the channel per-sistence (40% - 100%) selected part of the study area in the channel inci-dence map, indicating the rapid dynamic behavior of the river morphology. This study showed a good agreement of measured data and simplified em-pirical relationships to predict the long-term morphodynamic behavior of the braided Jamuna river.
... The canyon that deeply incises the Bengal shelf near the Ganges-Brahmaputra river mouth, cuts the foreset beds of the subaqueous river delta and acts as temporary depocenter between river mouth and Bengal fan (Sarma et al., 2000). It is estimated that a total of 20-29% (Goodbred et al., 2014) of total 1 billion tons of suspended sediments carried by the GBM river system finally drifted to the SNG before they disappear into the deep sea through. This sub-marine canyon thus has a significant role in sediment dynamics, including sediment reworking, suspension, and drifting. ...
Multi-dynamic nature of the Bay of Bengal is characterized by its numerous natural resources and their uses, although during recent years the concern of pollution, toxicity, sediment dynamics and shelf ecology of the Bay have become a vital concern. Particularly, in the south-western shelf zone of Bangladesh, due to its existence in between two dynamic ecosystems- the Swatch of No Ground in the oceanic side and Sundarbans forest in the landward side- can be treated as an ecologically critical zone. The marine environment of that particular zone is being polluted due to Mongla port functions, transportation of different toxic materials through the forest rivers, pollution due to oil and other toxic chemical spillages and also other anthropogenic actions. Therefore, the understanding of shelf ecology and physico-chemical properties of bottom sediment of this part of the Bay of Bengal deserves special attention. In this study, it has been attempted to study the physico-chemical properties of three bottom sediment samples collected from the south-west shelf zone of Bangladesh. Samples were collected through a marine cruise, using the commercial fishing vessel of a private company. In situ reading of some parameters such as, EC, PH, and salinity, were measured on board. Samples were treated for particle size analysis and loss-on-ignition using convention techniques. However, heavy metals such as Cd, Pb, Zn, Fe, Mn, Mg and Cu have been measured using AAS (Atomic Absorption Spectroscopy) technique. The study shows that the bottom sediment and water column contain higher amount of heavy-metals than that of the standard level almost in all cases. For instance, the heavy metal concentrations of bottom sediment at site-1 are 1.95 μg/g for Cd, 42.4 μg/g for Pb, 229.5 μg/g for Zn,42818.7 μg/g for Fe, 979.75 μg/g for Mn, 5309 μg/g for Mg and 51 μg/g for Cu, although their standard levels are 0.115 μg/g, 22.8 μg/g, 119 μg/g, 27000 μg/g, 229 μg/g and 1290μg/g, 33μg/g, respectively. Such high level of sub-aquatic toxicity might have long-lasting impact on shelf ecology and its biodiversity, including fish resources and could be a threat to human food chain. It is thus necessary to take immediate measures to conserve and protect the sub-aquatic environment of the shelf zone of Bangladesh.
Bangladesh's chars are primarily generated by silt deposits generate from river erosion from three rivers Padma, Meghna and Jamuna. The overall Charlands area in Bangladesh is 1722 km 2 , which makes up 1.7% of the country's total land area. Every year a new charland of 45 km 2 is raised from sediment deposition in the Meghna river estuary (lower Meghna floodplain and coastal plain), whereas 25 km 2 of occupied charland is lost to the river bed. As a result, despite a net charland gain of 20 km 2 each year in the Meghna estuary, there is a major loss of settlement and agricultural land. The charlands are the home to the poor people of Bangladesh. Charland is home to around 5% of Bangladesh's total population. This amounts to approximately 6.5 million. The char people rely primarily on agriculture and related activities. Charlands consists of very good fertile soil, which makes a wide variety of plants and vegetables grow there. The fertile soil on charland also supports agriculture, making it an important source of food for the local communities. The charlands of Bangladesh are being affected by natural disasters such as floods, river erosion and cyclones. Natural calamities such as floods, river erosion, and cyclones are wreaking frequently on Bangladesh's char regions. The biodiversity, vegetation, flora and fauna of the island char area have been extremely damaged as a result of this natural disaster. Many vegetation (flora) that was formerly common on the island char has been extinct as a result of river erosion. Many fruit plants and animals (fauna) are disappearing and being washed away in waterways. The catkin is useful vegetation that is the most abundant species in charland ecosystem. Its dense root system helps to stabilize the soil and prevent erosion, while its ability to accelerate silt deposition improves the fertility of the charland. Catkin has multipurpose uses, the Chars people make major use of it as thatching material for their homes. It is also a source of income for those who sell the catkin as thatching material in nearby mainland marketplaces. Catkin also provides shelter and nesting sites for various bird species. It plays a crucial role in protecting biodiversity from natural disasters.
Coeval input systems in rift basins may interact with each other to form a segmented multi-source sediment routing system. Importantly, its division into proximal zones, where a single source dominates, and interaction zones, where multiple sources mix, enables the interactions between input systems to be characterised. Here, we exploit this conceptual framework to revisit the middle Eocene–early Oligocene hangingwall dipslope of the Xihu depression in the East China Sea Shelf Basin, where extensive 3D seismic data, detrital zircon U–Pb ages and heavy mineral compositions are available. We first combined palaeogeomorphological and sedimentological features with age signatures to distinguish three areas: the northwestern area was identified for its proximity to the Haijiao uplift and invariably high proportions of Palaeoproterozoic ages (41%–54%); the southwestern area adjacent to the Yushan uplift was distinct for enriched Cretaceous-aged zircons (36%) and the transition area in between was characterised by its remoteness to both uplifts, an embayed geometry and mixed age signatures that are not identical to any individual input. These spatial variations support the segmented framework for the multi-source system, with the northwestern and southwestern areas representing two palaeo-input systems and the transition area as their interaction zone. In this context, we then used mixture models to determine spatio-temporal variations in the mixing proportions of the two palaeo-input systems. The zircon-based results indicate that the mixing proportion sustained from the middle to the late Eocene, during which the basin was in the late syn-rift stage and marine environments. This is corroborated by heavy mineral composition that shows only minor changes. We interpret the roughly sustained mixing proportions as reflecting both the spatially uniform nature of broad subsidence and the strong tidal processes that ‘erased’ the effects of avulsions. In contrast, a clear provenance shift in both zircon ages and heavy minerals occurred from the late Eocene to the early Oligocene, coinciding with a transition to the tectonic inversion stage and a shift towards non-marine environments. The provenance shift, together with the southward expansion of the axial drainage, likely represents the sedimentary response to the southward decreasing inversion magnitude of the Yuquan Event. In addition, we hypothesize that in the absence of strong tides, avulsions might have controlled the mixing proportion, particularly over short timescales. Ultimately, this study demonstrates the segmented multi-source framework, if properly incorporated, can provide key insights into dipslope sedimentation.
The Bay of Bengal (BoB) is crucial in understanding sediment dynamics in the Indian Ocean. Analysis of sediment cores from the western BoB, focusing on the Middle and Lower Bengal Fan since 16,000 years ago, reveals rapid deposition within the submarine canyon. This deposition was driven by the Ganges-Brahmaputra (G-B) river system during lower sea levels, leading to the formation of turbidity currents. Increased glacial activity during cold phases enhanced physical weathering in the Himalayan highlands, affecting sediment transport to the BoB until the early Holocene. Following the early Holocene, the lower fan displayed a mixture of sources, while the deeper fan predominantly reflected Indian sources throughout the Holocene. Sediment contributions from the Indian subcontinental commenced after 9,100 years ago, becoming dominant since 7,000 years ago, particularly in arid-semiarid conditions in the middle to lower fan, primarily driven by monsoonal water currents. Cores located near active channels exhibited consistent sourcing patterns influenced by regional factors such as river systems and active channels since the mid-Holocene. Sediments with coarser-grain, higher smectite content, and primarily sourced from Indian rivers indicated an intensification of monsoonal currents since 9,100 years ago. Fluctuations in smectite/(illite + chlorite) ratios were correlated with changes in summer monsoon rainfall, impacting erosion and sediment dynamics. Active channels played a crucial role in transporting Himalayan sediments via the G-B river system to the deep ocean during periods of low sea levels, but their influence has diminished with rising sea levels. Therefore, the proximity of sediments to active channels denotes the dominance of Himalayan sources; however, sediment from the lower fan transitioned to Indian sources since 9,100 years ago, influenced by the combined effects of monsoonal currents, active channels, and underwater currents shaping sediment transport dynamics. During the Holocene Climatic Optimum (HCO), intensified summer monsoons led to increased sediment contributions from Indian sources, driven by water currents during the winter monsoon, with notable impacts after 7,000 years ago. This study highlights the complexity of sedimentary processes and provides insights into past environmental conditions, contributing to our comprehension of sediment deposition patterns in the northeastern Indian Ocean.
The GBM Delta in Bangladesh is home to a large and densely populated ecological region that relies on its natural resources for survival. However, the delta is facing severe environmental and socio-economic threats, such as climate change, river erosion, salinity, sea-level rise, and extreme weather events. These threats pose serious challenges to agriculture, infrastructure, and human settlements, increasing the risk of poverty and social injustice. Bangladesh has implemented various adaptation measures to deal with these challenges, such as embankments, flood protection, agricultural resilience, sustainable aquaculture, water management, disaster risk reduction, and preparedness. The Bangladesh Delta Plan 2100 is a long-term strategic plan for the delta that aims to achieve economic growth while ensuring water, ecology, environment, and land sustainability and resilience to natural disasters and climate change. To attain sustainable development in the delta, the country has adopted integrated land-use planning and community-based resource management approaches that enable local communities to adjust to the changing environment while maintaining their cultural values and traditional knowledge. Moreover, the country has diversified its livelihood options to reduce dependence on vulnerable sectors. The country has also engaged in international cooperation and research collaboration to learn from and share its experiences with other delta regions facing similar challenges. This study underscores the importance of continued efforts to address the specific issues of the GBM Delta and offers a framework for sustainable development in other delta regions around the world.
The Chittagong Tripura Fold Belt (CTFB), the folded eastern part of the Bengal Basin is critical for understanding sediments origin and tectonic evolution linked to interactions among the Indian, Eurasian, and Burma plates. The upper Bhuban and Bokabil sediments (Surma Group) from Atharamura anticline, Tripura, India were analyzed using whole-rock geochemistry and petrography to assess their provenance, tectonics, weathering, and depositional milieu in this least explored area. Petrographic study indicates sub-lithic arenite to sub-arkosic composition, with moderate-high quartz, feldspar poor containing metamorphic and sedimentary lithic fragments (Bh Q88.3F3.0L8.6; Bk Q83.5F7.6L8.8), indicating significant recycled orogen source. Geochemical characteristics such as Eu/Eu* (0.6, 0.7), La/Sc (3.2, 2.4), Th/Sc (1.7, 1.4), Th/Co (1.9, 1.8), CIA/WIP (∼1–2), high Hf, Zr/Sc, low K2O/Al2O3 and enriched LREEs, depleted HREEs, and negative Eu anomalies, indicate a recycled provenance of felsic origin that underwent moderate to intense chemical weathering in active continental margin settings. Radiolarian chert, volcanic lithic fragments with Barail-like recycled sediments strongly suggests an additional detritus from uplifted Indo-Burman Ranges from the east. Elevated Th/U and Th/Rb ratios, sediment immaturity and poor sorting suggest first-cycled detritus. This study indicate that Tripura Neogene sediments are chiefly Himalayan-derived, with minor inputs from IBR and Meghalaya Plateau. The sediments were deposited under brackish near shore to shallow marine oxic basins to the west of the uplifted IBR during the Miocene. The study offers insights into regional geological processes, facilitating correlation of foreland Neogene basins and enhances understanding of the geological evolution of the CTFB.
Groundwater containing high concentrations of dissolved arsenic (As) and iron (Fe(II)) discharges to rivers across the Ganges-Brahmaputra-Meghna delta. Observed Fe(III)-oxyhydroxide (FeOOH)-As deposits lining the riverbanks of the Meghna River may have been created by bidirectional mixing in the hyporheic zone (HZ) from ocean tides. This process has been named the Natural Reactive Barrier (NRB). Sedimentary organic carbon (SOC) is deposited annually on floodplains. Floodwaters that infiltrate through this layer may chemically transform the groundwater prior to discharging through the HZ in ways that influence the capture and retention of As in the NRB. The goal of this study is to understand how the interaction of these two scales of river-groundwater mixing influence the fate of As trapped within an NRB. Monitoring wells were installed to 1-17 m depth, up to 100 m distance from the river’s edge during the dry season on the East (Site 1) and West (Site 2) sides of the river. They were sampled during the dry season (January) under gaining river conditions. The physical properties and elemental composition of the sediment was described by hand observation and hand-held X-Ray Fluorescence (XRF), respectively. Mixing with river water was quantified using the sum of charge of major cations (TC). Site 1 has a sloping bank that is only partially inundated during the wet season. The aquifer is composed of homogeneous sand. Site 2 is flat and therefore fully inundated in the wet season. The aquifer is composed of sand with thin (1-20 cm thick) clay layers. Both sites generate the dissolved products of FeOOH-reduction coupled to organic carbon oxidation, and silicate weathering beneath the floodplain. These products are dissolved Fe, As, silica, bicarbonate, calcium and phosphate. This chemistry is conducive to the formation of crystalline iron oxide minerals such as goethite which may co-precipitate with As, trapping it long-term.
Globally increasing demand for construction sand needs to be met with transparent and responsible supply-networks. Currently, there are few scalable methods for tracing construction sand distribution without direct observation. He wee examined sand “fingerprinting” as a tool to trace construction sand supply-networks from “source to sink” in a case study from Texas, USA. Both natural bulk major element and optical petrography fingerprints are preserved through construction sand processing and transport such that sand can be tied back to its original mining source even at the final point of distribution. Additionally, we developed an image analysis model called sandID that is ~90% effective at determining the original mining source of sand in the study area. Our results demonstrate that sand fingerprinting, has untapped potential to support traceability and certification schemes and to support monitoring and enforcement in areas where there are concerns about illegal, illicit or simply unknown construction sand sourcing.
Groundwater is the primary source of water in the Bengal Delta but contamination threatens this vital resource. In deltaic environments, heterogeneous sedimentary architecture controls groundwater flow; therefore, characterizing subsurface structure is a critical step in predicting groundwater contamination. Here, we show that surface information can improve the characterization of the nature and geometry of subsurface features, thus improving the predictions of groundwater flow. We selected three locations in the Bengal Delta with distinct surface river network characteristics—the lower delta with straighter tidal channels, the mid‐delta with meandering and braided channels, and the inactive delta with transitional sinuous channels. We used surface information, including channel widths, depths, and sinuosity, to create models of the subsurface with object‐based geostatistical simulations. We collected an extensive set of lithologic data and filled in gaps with newly drilled boreholes. Our results show that densely distributed lithologic data from active lower and mid‐delta are consistent with the object‐based models generated from surface information. In the inactive delta, metrics from object‐based models derived from surface geometries are not consistent with subsurface data. We further simulated groundwater flow and solute transport through the object‐based models and compared these with simulated flow through lithologic models based only on variograms. Substantial differences in flow and transport through the different geologic models show that geometric structure derived from surface information strongly influences groundwater flow and solute transport. Land surface features in active deltas are therefore a valuable source of information for improving the evaluation of groundwater vulnerability to contamination.
Packages of fluvial systems in the Lower Ganga plains (LGP) archive the changes in stratigraphic architecture induced by perturbations in the climatic system. In some parts of this landscape, where the fluvial sequences predominate, the role of climate in landsculpting is strongly indicated by the presence of sediment architecture. The present study is focused on the formation of alluvial plains of the Ajay River in the Rarh region that represent a region's environmental templates for understanding the landscape of LGP. The sedimentological, geomorphological, and optically stimulated luminescence (OSL) chronological investigations were attempted to document sedimentation patterns and the timing of deposition. The stratigraphic data from two exposed cliff sections and three boreholes are compared with proxy records to understand the Indian Summer Monsoon (ISM) controls on alluvial sedimentation for the last ~2.4 ka. The fluvial packages in the study area correspond to two significant channel phases: Period-I (2.4-1.3 ka) and Period-II (800-200 years ago). Period-I is characterized by the episodes of floodplain development and lateral migration of the trunk river during Marine Isotope Stage (MIS) I, especially around 2 ka corresponding to the Roman Warm (RW) period in the Indian subcontinent. The sediment filling was high during ~2.4 ka due to ISM intensification, but in the later stage, the sedimentation was at a fairly steady rate. Conversely, Period-II is characterized by flooding phenomena and aggradation of channel bodies across much of the present valley area during the last 800 years. The distinct flood events occurred during the times of significant shifts in ISM, from fluvial dormancy to sudden outbursts of monsoons (end of Little Ice Age [LIA] ~19th century), indicating that climatic patterns can be associated with the occurrences of abrupt flood events. The alluvial records fit well with the historical, instrumental, and published proxy data on the ISM validating the chronology and the potential of sedimentary archives for further studies related to the palaeoenvironment and palaeogeography.
Ganga-Brahmaputra-Meghna (GBM) Delta is the world’s one of the most populated regions and has the highest concentration of population affected by floods. Millions of people become displaced and tens to hundreds die each year of the flood incident. Floods of the GBM delta are primarily because by heavy rains and the cyclonic storm is in second. Although the GBM system covers a vast area of varying topography, diversified lithology, heterogeneous soils, varying vegetations, the delta formed by the rivers is one and unique geomorphic unit. In this chapter, we focused broadly on the geology, relief, and river systems of this geomorphic unit. We also discussed the nature of floods in the region and their causes- natural and anthropogenic. The seasonality of floods in the GBM delta is also addressed in the chapter. This is not the age of combating floods, but rather the age of coexistence of man with floods. We illuminated this issue of living with floods. Finally, we outlined the prime theme of all other chapters.
Globally increasing demand for construction sand needs to be met with transparent and responsible supply-networks. Currently, there are no scalable methods for tracing construction sand distribution without direct observation. We examined sand “fingerprinting” as a potential tool to trace construction sand supply-networks from “source to sink” in a case study from Texas, USA. Both natural bulk major element and optical petrography fingerprints are preserved through construction sand processing and transport such that sand can be tied back to its original mining source even at the final point of distribution. Additionally, we developed an image analysis model called sandID that is ~90% effective at determining the original mining source of sand in the study area. Our results demonstrate that sand fingerprinting, has untapped potential to support traceability and certification schemes and to support monitoring and enforcement in areas where there are concerns about illegal, illicit or simply unknown construction sand sourcing.
Based on two-dimensional planform data on channel area and distance, interpreted from non-sequential snapshots of the Google Earth satellite imageryGoogle Earth imagery, the main objective of this study is to analyze spatial variations in morphological instabilities in major rivers of Bangladesh. The results indicate that while riverbank erosion is a pervasive problem in all of the major rivers of Bangladesh, there are significant spatial variations in densities of channel bars from one river section to another. It further relates such channel instabilities to longer-term lateral channel migration responsible for successive bank erosionBank erosion and accumulation of charlands and floodplainsFloodplains.
Is erosion important to the structural and petrological evolution of mountain belts? The nature of active metamorphic massifs co-located with deep gorges in the syntaxes at each end of the Himalayan range, together with the magnitude of erosional fluxes that occur in these regions, leads us to concur with suggestions that erosion plays an integral role in collisional dynamics. At multiple scales, erosion exerts an influence on a par with such fundamental phenomena as crustal thickening and extensional collapse. Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.
Background
Channel Scale Morphology and Historical Changes in the Course of the Brahmaputra-Jamuna RiverBedform Types and DynamicsBifurcations, Offtakes and ConfluencesFloodplain SedimentationSedimentology of the Jamuna RiverApplied Geomorphology and Engineering in the Jamuna RiverSummaryAcknowledgementsReferences
The physical geography of Bangladesh’s coastal area is more diverse and dynamic than is generally recognised. Failure to recognise this has led to serious misconceptions about the potential impacts of a rising sea-level on Bangladesh with global warming. This situation has been aggravated by accounts giving incorrect information on current rates of coastal erosion and land subsidence. This paper describes physical conditions within individual physiographic regions in Bangladesh’s coastal area based on ground-surveyed information, and it reviews possible area-specific mitigation measures to counter predicted rates of sea-level rise in the 21st century. Two important conclusions are drawn: the adoption of appropriate measures based on knowledge of the physical geography of potentially-affected areas could significantly reduce the currently-predicted displacement of many millions of people; and the impacts of a slowly-rising sea-level are currently much less than those generated by rapidly increasing population pressure on Bangladesh’s available land and water resources and by exposure to existing environmental hazards, and the latter problems need priority attention.
The surfaces of alluvial fans and river deltas (collectively fans) are
often dissected by a small number of channels radiating from the fan
apex. On long timescales, channels migrate via avulsion, the process of
channel bed deposition and abandonment that often results in
catastrophic flooding and loss of life on densely populated fans. We
present results of an experimental fan that creates realistic channel
patterns by avulsion. The avulsion cycle occurs with a period that is
predictable from conservation of mass. Selection of a new flow path is
inherently stochastic; however, once a network of 4-5 channels is
established, flow oscillates among these channels indefinitely. We
demonstrate that a directed random walk model with memory quantitatively
reproduces these dynamics and limiting behavior, and is consistent with
natural fans.
The densely populated, low-lying Ganges-Brahmaputra Delta is highly vulnerable to global
sea-level rise. In order to estimate the rate of subsidence of the delta, we examined submerged
salt-producing kiln sites in the coastal Sundarbans (a huge UNESCO-protected mangrove
forest). These kilns were built just above the winterly spring high-tide level of the time, but
their bases are currently located ~155 cm below the corresponding modern level. According
to optically stimulated luminescence (OSL) dating, the kilns were last fi red ~300 yr ago, and
salt production was terminated by a catastrophic event that affected the kiln sites at different
levels and locations. 14C ages of charcoal at the kilns’ bases and associated mangrove stump
horizons support the OSL dates. Based on the elevations and ages, the 300 yr average rate of
sinking of the outer delta is 5.2 ± 1.2 mm/yr, which includes 0.8 mm/yr of eustatic sea-level rise.
With the expectation of further acceleration of sea-level rise, the already-present problematic
situation will be aggravated, and only prudent control of sediment accretion will keep southern
Bangladesh above sea level.
Any understanding of sediment routing from mountain belts to their forelands and offshore sinks remains
incomplete without estimates of intermediate storage that decisively buffers sediment yields from erosion
rates, attenuates water and sediment fluxes, and protects underlying bedrock from incision. We quantify
for the first time the sediment stored in > 38 000 mainly postglacial Himalayan valley fills, based on
an empirical volume-area scaling of valley-fill outlines automatically extracted from digital topographic
data. The estimated total volume of 690(+ 452/− 242) km³ is mostly contained in few large valley fills > 1 km³, while catastrophic mass wasting adds another 177( ± 31) km³. Sediment storage volumes are
highly disparate along the strike of the orogen. Much of the Himalaya’s stock of sediment is sequestered
in glacially scoured valleys that provide accommodation space for ~ 44% of the total volume upstream of
the rapidly exhuming and incising syntaxes. Conversely, the step-like long-wave topography of the central
Himalayas limits glacier extent, and thus any significant glacier-derived storage of sediment away from
tectonic basins. We show that exclusive removal of Himalayan valley fills could nourish contemporary
sediment flux from the Indus and Brahmaputra basins for > 1 kyr, though individual fills may attain
residence times of > 100 kyr. These millennial lag times in the Himalayan sediment routing system may
sufficiently buffer signals of short-term seismic as well as climatic disturbances, thus complicating simple
correlation and interpretation of sedimentary archives from the Himalayan orogen, its foreland, and its
submarine fan systems.
The Ganga River is one of the main conveyors of sediments produced by Himalayan erosion. Determining the flux of elements transported through the system is essential to understand the dynamics of the basin. This is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions. Using Acoustic Doppler Current Profiler (ADCP) acquisitions with sediment depth profile sampling of the Ganga in Bangladesh we build a simple model to derive the annual flux
and grain size distributions of the sediments. The model shows that ca. 390 (±30) Mt of sediments are transported on average each year through the Ganga at Haring Bridge (Bangladesh). Modeled average sediment grain size parameters D50 and D84 are 27 (±4) and 123 (±9) mm, respectively. Grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation. The integrated sediment flux is characterized by low Al/Si and Fe/Si ratios that are close to those inferred for the Himalayan crust. This implies that only limited sequestration occurs in the Gangetic floodplain. The stored sediment flux is estimated to c.a. 10% of the initial Himalayan sediment flux by geochemical mass balance. The associated, globally averaged sedimentation rates in the floodplain are found to be ca. 0.08 mm/yr and yield average Himalayan erosion rate of ca. 0.9 mm/yr. This study stresses the need to carefully address the average composition of river sediments before solving large‐scale geochemical budgets.
Sedimentology, carbon isotope and sequence stratigraphic analysis of subsurface sediments from western part of Ganges–Brahmaputra (GB) delta plain shows that a Late Quaternary marine clay and fluvial channel-overbank sediments of MIS 5 and 3 highstands are traceable below the Holocene strata. During the Last Glacial Maximum (LGM) sea-level lowering of >100 m produced a regional unconformity (type 1), represented by palaeosols and incised valley. C4 vegetation expanded on exposed lowstand surface in an ambient dry glacial climate. At ∼9 ka transgression inundated the lowstand surface pushing the coastline and mangrove front ∼100 km inland. Simultaneous intensification of monsoon and very high sediment discharge (∼4–8 times than modern) caused a rapid aggradation of both floodplain and estuarine valley fill deposits between 8 and 7 ka. The Hoogli River remaining along its present drainage possibly acted as the main conduit for transgression and sediment discharge that was subsequently abandoned. C3 vegetation dominated the delta plain during this time. From 7 ka onward progradation of delta plain started and continued till recent. This period experienced a mixed C3–C4 vegetation with localized mangroves in the mid-Holocene to dominant return of C4 vegetation in the late Holocene period. The study indicates that while the initiation of western part of GB delta occurred at least 1 ka earlier than the global mean delta formation age, the progradation started at ∼7 ka, at least 2 ka earlier than thought before. The terrestrial vegetation change was modulated by changes in depositional environment, specific ecological niches and climate rather than pCO2.
Sedimentology, carbon isotope and sequence stratigraphic analysis of subsurface sediments from western part of Ganges–Brahmaputra (GB) delta plain shows that a Late Quaternary marine clay and fluvial channel-overbank sediments of MIS 5 and 3 highstands are traceable below the Holocene strata. During the Last Glacial Maximum (LGM) sea-level lowering of >100 m produced a regional unconformity (type 1), represented by palaeosols and incised valley. C4 vegetation expanded on exposed lowstand surface in an ambient dry glacial climate. At ∼9 ka transgression inundated the lowstand surface pushing the coastline and mangrove front ∼100 km inland. Simultaneous intensification of monsoon and very high sediment discharge (∼4–8 times than modern) caused a rapid aggradation of both floodplain and estuarine valley fill deposits between 8 and 7 ka. The Hoogli River remaining along its present drainage possibly acted as the main conduit for transgression and sediment discharge that was subsequently abandoned. C3 vegetation dominated the delta plain during this time. From 7 ka onward progradation of delta plain started and continued till recent. This period experienced a mixed C3–C4 vegetation with localized mangroves in the mid-Holocene to dominant return of C4 vegetation in the late Holocene period. The study indicates that while the initiation of western part of GB delta occurred at least 1 ka earlier than the global mean delta formation age, the progradation started at ∼7 ka, at least 2 ka earlier than thought before. The terrestrial vegetation change was modulated by changes in depositional environment, specific ecological niches and climate rather than pCO2.
The bulk chemical compositions, mineralogy, and mineral proportions of sands and muds of the Mallacoota Basin in southeastern Australia reflect the composition of weathering profiles mantling source rocks, rather than bedrock. Muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles. Sands are strongly enriched in quartz relative to source rocks, even in the headwaters of the fluvial system, demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system. K-feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or K-feldspar in weathering profiles. Subsequent erosion and sorting produce sands enriched in quartz, with high K-feldspar:plagioclase ratios relative to source rocks. The composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat...
Published data are used to construct an approximate sediment budget for the 1.656 x 106 km2 catchment of the Ganga and Brahmaputra rivers. From these data, it has not been possible to identify the major sources of sediment. Recently published Nd/Sr tracer results suggest that the High Himalaya is the main source of sediment, providing for the first time a focus for more detailed research on the role of land use and other factors in the generation of sediment. Much is yet to be learned about this globally important catchment, and the budget provides a framework for further research.
The Indo-Burman Ranges in western Myanmar extend along the Sunda Arc subduction zone and may be divided into a western portion of Neogene sedimentary rocks and an eastern portion of Palaeogene sedimentary rocks separated by the Kaladan Fault. Both Himalayan and Burman sources have been proposed for these sediments. Our thermochronological analyses on detrital grains, isotopic analyses on bulk rock, and petrographic and heavy mineral data indicate that the Palaeogene Indo-Burman Ranges contain a significant component of arc-derived material interpreted as derived from the Burmese portion of the Mesozoic-Tertiary arc to the cast. And older crustal component is also identifiable, which may have been sourced from the Himalaya or the Burmese margin. By contrast, the Neogene Indo-Burman Ranges show dominant derivation from the Himalaya. A minor arc-derived component may have been sourced form the Trans-Himalaya or recycled from the arc-derived Paleogene Indo-Burman Ranges.
Editor: M.L. Delaney Keywords: sedimentary geochemistry sedimentary petrology settling equivalence selective entrainment placer sands opaque minerals REE-bearing minerals Eu anomaly Himalaya This study investigates the natural processes that control concentration of detrital minerals and consequently chemical elements in river sand. The novelty of our approach consists in the systematic integration of detailed textural, petrographical, mineralogical and chemical data, and in the quantitative description and modeling of relationships among mineralogical and chemical variables for each sample and each grain-size class in each sample. Bed sediment in transit in the largest sedimentary system on Earth chiefly consists of fine-grained lithofeldspathoquartzose sand including rich amphibole–epidote–garnet suites, mixed with minor very-fine-grained-sand to silt subpopulations containing less heavy minerals and representing intermittent suspension. Mineralogical and particularly chemical differences between Ganga and Brahmaputra bedload are orders of magnitude less than both intersample variability associated with selective-entrainment effects and intrasample variability associated with settling-equivalence effects. Any provenance interpretation of mineralogical, chemical, or detrital-geochronology datasets therefore requires quantitative understanding of hydraulically controlled compositional variability. Mineralogical and chemical, intrasample and intersample variability can be deduced with simple equations and numerical solutions. The underlying assumptions on the chemical composition of detrital minerals, as well as the possible pitfalls, uncertainties and approximations involved are discussed. Principal results include calibration of rare REE-bearing ultradense minerals, ill-determined by optical analyses but crucial in both detrital-geochronology and settling-equivalence studies, and assessment of progressively changing concentration for any detrital component with increasing intensity of selective-entrainment effects. Contributions by each mineral group to the chemical budget were inferred with sufficient precision and accuracy. Although complex because of diverse controlling factors including provenance, weathering and anthropogenic pollution, mineralogical and consequently chemical variability of fluvial sediments can be quantitatively predicted. This path, difficult because of insufficient information but far from hopeless, shall eventually lead to more accurate calculation of sediment fluxes and chemical budgets, as well as to a deeper understanding of sedimentary geochemistry and fluvial sedimentology. © 2010 Elsevier B.V. All rights reserved. "The sands of the river Ganga are violently trampled on by fishes, tortoises, porpoises, crocodiles, buffaloes, lions, elephants, but the sands are not troubled, have no ill feelings, nor are they unconscious of being trampled on; they are without imagination, beautifully clear and devoid of impurities. The sands of the river Ganga flow along the stream and are permanent, and so is the essence of Buddahood." Mahayana Lankavatara Sutra, The parable of the sands of the Ganga
Analysis of data from 280 rivers discharging to the ocean indicates that sediment loads/yields are a log-linear function of basin area and maximum elevation of the river basin. Other factors controlling sediment discharge (e.g., climate, runoff) appear to have secondary importance. A notable exception is the influence of human activity, climate, and geology on the rivers draining southern Asia and Oceania. Sediment fluxes from small mountainous rivers, many of which discharge directly onto active margins (e.g., western South and North America and most high-standing oceanic islands), have been greatly underestimated in previous global sediment budgets, perhaps by as much as a factor of three. In contrast, sediment fluxes to the ocean from large rivers (nearly all of which discharge onto passive margins or marginal seas) have been overestimated, as some of the sediment load is subaerially sequestered in subsiding deltas. Before the proliferation of dam construction in the latter half of this century, riv
The Earth is an unusual planet in our solar system in having a bimodal
topography that reflects the two distinct types of crust found on our
planet. The low-lying oceanic crust is thin (˜7 km on average),
composed of relatively dense rock types such as basalt and is young
(≤200 Ma old) (see Chapter 3.13). In contrast, the high-standing
continental crust is thick (˜40 km on average), is composed of
highly diverse lithologies (virtually every rock type known on Earth)
that yield an average intermediate or "andesitic" bulk composition
(Taylor and McLennan (1985) and references therein), and contains the
oldest rocks and minerals yet observed on Earth (currently the 4.0 Ga
Acasta gneisses (Bowring and Williams, 1999) and 4.4 Ga detrital zircons
from the Yilgarn Block, Western Australia (Wilde et al., 2001)),
respectively. Thus, the continents preserve a rich geological history of
our planet's evolution and understanding their origin is critical for
understanding the origin and differentiation of the Earth.The origin of
the continents has received wide attention within the geological
community, with hundreds of papers and several books devoted to the
topic (the reader is referred to the following general references for
further reading: Taylor and McLennan (1985), Windley (1995), and Condie
(1997). Knowledge of the age and composition of the continental crust is
essential for understanding its origin. Patchett and Samson (Chapter
3.10) review the present-day age distribution of the continental crust
and Kemp and Hawkesworth (Chapter 3.11) review secular evolution of
crust composition. Moreover, to understand fully the origin and
evolution of continents requires an understanding of not only the crust,
but also the mantle lithosphere that formed more-or-less
contemporaneously with the crust and translates with it as the
continents move across the Earth's surface. The latter topic is reviewed
in Chapter 2.05.This chapter reviews the present-day composition of the
continental crust, the methods employed to derive these estimates, and
the implications of the continental crust composition for the formation
of the continents, Earth differentiation, and its geochemical
inventories.
Lutites are commonly metasomatized during diagenesis, but the analysis presented here accounts for most postdepositional change. Potassium metasomatism is particularly common, and typically involves the conversion of kaolin (residual weathering product) to illite by reaction with K+-bearing pore waters. Sandstones also undergo K metasomatism, which involves the replacement of plagioclase by potassium feldspar. These changes can be identified petrographically and are quantitatively accounted for by techniques discussed herein. Bulk chemical analyses and ternary diagrams are used to determine the amount of K addition, premetasomatized sediment composition, and composition of provenance areas. The premetasomatized mineralogy of paleosols can be compared with the mineralogy of recent soil profiles and thus, climate and topographic conditions determined for past weathering events. Some weathering indices lead to erroneous conclusions because, by excluding K2O from consideration, correction cannot be made for metasomatic effects.
Book synopsis: Understanding of rivers and their sediments, both as modern systems and as ancient counterparts in the geological record, has progressed steadily but markedly over the past several decades, with contributions by practitioners in diverse fields of geosciences and engineering. This book contains 31 papers, with authors from 13 countries, who participated in the Sixth International Conference on Fluvial Sedimentology held in Cape Town, South Africa, in 1977. True to the nature of these quadrennial conferences, the papers in this book discuss a broad range of fluvial subjects that include the character of bedforms and sediment transport in river channels, morphological and sedimentological features of modern fluvial environments, modern and ancient avulsions, internal and external controls on the behaviour of river systems, and the facies and architectural organization of alluvial deposits.
Recent years witnessed a vast advent of stalagmites as palaeoclimate archives. The multitude of geochemical and physical proxies and a promise of a precise and accurate age model greatly appeal to palaeoclimatologists. Although substantial progress was made in speleothem-based palaeoclimate research and despite high-resolution records from low-latitudinal regions, proving that palaeo-environmental changes can be archived on sub-annual to millennial time scales our comprehension of climate dynamics is still fragmentary. This is in particular true for the summer monsoon system on the Indian subcontinent. The Indian summer monsoon (ISM) is an integral part of the intertropical convergence zone (ITCZ). As this rainfall belt migrates northward during boreal summer, it brings monsoonal rainfall. ISM strength depends however on a variety of factors, including snow cover in Central Asia and oceanic conditions in the Indic and Pacific. Presently, many of the factors influencing the ISM are known, though their exact forcing mechanism and mutual relations remain ambiguous. Attempts to make an accurate prediction of rainfall intensity and frequency and drought recurrence, which is extremely important for South Asian countries, resemble a puzzle game; all interaction need to fall into the right place to obtain a complete picture. My thesis aims to create a faithful picture of climate change in India, covering the last 11,000 ka. NE India represents a key region for the Bay of Bengal (BoB) branch of the ISM, as it is here where the monsoon splits into a northwestward and a northeastward directed arm. The Meghalaya Plateau is the first barrier for northward moving air masses and receives excessive summer rainfall, while the winter season is very dry. The proximity of Meghalaya to the Tibetan Plateau on the one hand and the BoB on the other hand make the study area a key location for investigating the interaction between different forcings that governs the ISM. A basis for the interpretation of palaeoclimate records, and a first important outcome of my thesis is a conceptual model which explains the observed pattern of seasonal changes in stable isotopes (d18O and d2H) in rainfall. I show that although in tropical and subtropical regions the amount effect is commonly called to explain strongly depleted isotope values during enhanced rainfall, alone it cannot account for observed rainwater isotope variability in Meghalaya. Monitoring of rainwater isotopes shows no expected negative correlation between precipitation amount and d18O of rainfall. In turn I find evidence that the runoff from high elevations carries an inherited isotopic signature into the BoB, where during the ISM season the freshwater builds a strongly depleted plume on top of the marine water. The vapor originating from this plume is likely to memorize' and transmit further very negative d18O values. The lack of data does not allow for quantication of this plume effect' on isotopes in rainfall over Meghalaya but I suggest that it varies on seasonal to millennial timescales, depending on the runoff amount and source characteristics. The focal point of my thesis is the extraction of climatic signals archived in stalagmites from NE India. High uranium concentration in the stalagmites ensured excellent age control required for successful high-resolution climate reconstructions. Stable isotope (d18O and d13C) and grey-scale data allow unprecedented insights into millennial to seasonal dynamics of the summer and winter monsoon in NE India. ISM strength (i. e. rainfall amount) is recorded in changes in d18Ostalagmites. The d13C signal, reflecting drip rate changes, renders a powerful proxy for dry season conditions, and shows similarities to temperature-related changes on the Tibetan Plateau. A sub-annual grey-scale profile supports a concept of lower drip rate and slower stalagmite growth during dry conditions. During the Holocene, ISM followed a millennial-scale decrease of insolation, with decadal to centennial failures resulting from atmospheric changes. The period of maximum rainfall and enhanced seasonality corresponds to the Holocene Thermal Optimum observed in Europe. After a phase of rather stable conditions, 4.5 kyr ago, the strengthening ENSO system dominated the ISM. Strong El Nino events weakened the ISM, especially when in concert with positive Indian Ocean dipole events. The strongest droughts of the last 11 kyr are recorded during the past 2 kyr. Using the advantage of a well-dated stalagmite record at hand I tested the application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to detect sub-annual to sub-decadal changes in element concentrations in stalagmites. The development of a large ablation cell allows for ablating sample slabs of up to 22 cm total length. Each analyzed element is a potential proxy for different climatic parameters. Combining my previous results with the LAICP- MS-generated data shows that element concentration depends not only on rainfall amount and associated leaching from the soil. Additional factors, like biological activity and hydrogeochemical conditions in the soil and vadose zone can eventually affect the element content in drip water and in stalagmites. I present a theoretical conceptual model for my study site to explain how climatic signals can be transmitted and archived in stalagmite carbonate. Further, I establish a first 1500 year long element record, reconstructing rainfall variability. Additionally, I hypothesize that volcanic eruptions, producing large amounts of sulfuric acid, can influence soil acidity and hence element mobilization.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Continental margins are valuable for many reasons, including the rich record of Earth history that they contain. A comprehensive understanding about the fate of fluvial sediment requires knowledge that transcends time-scales ranging from particle transport to deep burial. Insights are presented for margins in general, with a focus on a tectonically active margin (northern California) and a passive margin (New Jersey). Formation of continental-margin strata begins with sediment delivery to the seabed. Physical and biological reworking alters this sediment before it is preserved by burial, and has an impact upon its dispersal to more distal locations. The seabed develops strength as it consolidates, but failure can occur and lead to sediment redistribution through high-concentration gravity flows. Processes ranging from sediment delivery to gravity flows create morphological features that give shape to continental-margin surfaces. With burial, these surfaces may become seismic reflectors, which are observed in the subsurface as stratigraphy and are used to interpret the history of formative processes. Observations document sedimentary processes and strata on a particular margin, but numerical models and laboratory experimentation are necessary to provide a quantitative basis for extrapolation of these processes and strata in time and space.
This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.
The RbSr isotope systematics of bedrock, soil digests, and the cation exchange fraction of soils from a granitic glacial soil chronosequence in the Wind River Mountains, Wyoming, USA, were investigated. Six soil profiles ranging in age from 0.4 to ∼300 kyr were studied and revealed that the ratio of exchangeable strontium in the B-horizons decreased from 0.7947 to 0.7114 with increasing soil age. Soil digests of the same samples showed much smaller variation in from 0.7272 to 0.7103 and also generally decreased with increasing soil age. Elevation of the ratios of Sr released by weathering over the soil digest and bedrock values results from the rapid weathering of biotite to form hydrobiotite and vermiculite in the younger soils. Biotite is estimated to weather at aaproximately eight times the rate of plagioclase (per gram of mineral) in the youngest soil profile and decreases to a rate of only ∼20% of that of plagioclase in the oldest soil. ratios of the soil cation exchange fraction are estimated to be depleted by factors of up to 11 over the ratios released by weathering, due to ion exchange partitioning. This study demonstrates that the ratio released by weathering of crystalline rocks can deviate significantly from bedrock values, and that in soils less than ∼20 kyr in age which contain biotite in the soil parent material, weathering-derived values can be elevated so dramatically that this factor must be considered in estimations of weathering rates based on strontium isotopes.
Clastic sediments in the Bengal Fan contain a Neogene history of erosion
and weathering of the Himalaya. We present data on clay mineralogy,
major element, stable and radiogenic isotope abundances from Lower
Miocene-Pleistocene sediments from ODP Leg 116. Nd and Sr isotope data
show that the Himalayan provenance for the eroded material has varied
little since > 17 Ma. However, from 7 to 1 Ma smectite replaces
illite as the dominant clay, while sediment accumulation decreased,
implying an interval of high chemical weathering intensity but lower
physical erosion rates in the Ganges-Brahmaputra (GB) basin. O and H
isotopes in clays are correlated with mineralogy and chemistry, and
indicate that weathering took place in the paleo-Gangetic flood plain.
The 87Sr 86Sr ratios of pedogenic clays
(vermiculite, smectite) record the isotopic composition of Sr in the
weathering environment, and can be used as a proxy for 87Sr
86Sr in the paleo-GB basin. The Sr data from pedogenic clays
shows that river 87Sr 86Sr values were near 0.72
prior to 7 Ma, rose rapidly to ≥ 0.74 in the Pliocene, and returned
to ≤ 0.72 in the middle Pleistocene. These are the first direct
constraints available on the temporal variability of 87Sr
86Sr in a major river system. The high 87Sr
86Sr values resulted from intensified chemical weathering of
radiogenic silicates and a shift in the carbonate-silicate weathering
ratio. Modeling of the seawater Sr isotopic budget shows that the high
river 87Sr 86Sr values require a ca. 50% decrease
in the Sr flux from the GB system in the Pliocene. The relationship
between weathering intensity, 87Sr 86Sr and Sr
flux is similar to that observed in modern rivers, and implies that
fluxes of other elements such as Ca, Na and Si were also reduced.
Increased weathering intensity but reduced Sr flux appears to require a
late Miocene-Pliocene decrease in Himalayan erosion rates, followed by a
return to physically dominated and rapid erosion in the Pleistocene. In
contrast to the view that increasing seawater 87Sr
86Sr results from increased erosion, Mio-Pliocene to
mid-Pleistocene changes in the seawater Sr budget were the result of
reduced erosion rates and Sr fluxes from the Himalaya.
The age and origin of the Transhimalaya (Gangdese) plutonic belt in the
Ladakh area has been studied by high-resolution U sbnd Pb analyses of
accessory minerals (zircon and monazite/allanite) and Rb sbnd Sr
measurements on whole rock samples. The ages determined of 101 ±
2m.y. for a granodiorite and of 60.7 ± 0.4m.y. for a granite
substantiate that the Transhimalaya plutonism was active at least in
mid-Cretaceous (Albian) and earliest Tertiary (Palaeocene) times. A Rb
sbnd Sr isochron of 73.4 ± 2.4m.y. might be fortuitous because
the Rb sbnd Sr systematics shows important heterogeneities on the whole
rock scale. The inherited radiogenic lead in zircon demonstrates that
anatexis of continental crust was involved in magma genesis. The thus
recycled continental material was heterogeneous with respect to its
primary ages and/or metamorphic history; the approximate minimum ages of
the continental sources range from 350 to 590 m.y. The 101 ±
2m.y. old Transhimalaya granodiorite, which intrudes a series of mainly
basaltic island arc rocks (Dras Series) shows that this island arc was
attached to the continental margin at that time. This is consistent with
the hypothesis that the "Ladakh Tethys" closed through two subduction
regimes: (1) an early Cretaceous subduction, which formed the Dras
island arc, and (2) a late Cretaceous to Palaeocene subduction at the
continental margin, which caused the emplacement of the Transhimalaya
plutonic belt.
We present an extensive river sediment dataset covering the Ganga basin from the Himalayan front downstream to the Ganga mainstream in Bangladesh. These sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column. Mineral sorting is the first order control on the chemical composition of river sediments. Taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain. By comparing sediments sampled at the Himalayan front with sediments from the Ganga mainstream in Bangladesh it is possible to budget weathering in the floodplain. Assuming a steady state weathering regime in the floodplain, the weathering of Himalayan sediments in the Gangetic floodplain releases ca. (189 ± 92) × 109 and (69 ± 22) × 109 mol/yr of carbonate bound Ca and Mg to the dissolved load, respectively. Silicate weathering releases (53 ± 18) × 109 and (42 ± 13) × 109 mol/yr of Na and K while the release of silicate Mg and Ca is substantially lower, between ca. 0 and 20 × 109 mol/yr. Additionally, we show that sediment hydration, [H2O+], is a sensitive tracer of silicate weathering that can be used in continental detrital environments, such as the Ganga basin. Both [H2O+] content and the D/H isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions. By comparing the chemical composition of river sediments across the floodplain with the composition of the eroded Himalayan source rocks, we suggest that the floodplain is the dominant location of silicate weathering for Na, K and [H2O+]. Overall this work emphasizes the role of the Gangetic floodplain in weathering Himalayan sediments. It also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into account.
A negative correlation between sediment yield and weathering history, as measured by the chemical alteration (CIA) of the suspended sediment, is observed for many of the world's major rivers and other regions of denudation. The weathering history is a first-order control on the sediment yield of such areas, termed equilibrium denudation regions. For other areas, data scatter with either apparent increases or decreases of sediment yield for given CIA values. These areas are termed nonequilibrium denudation regions. Low sediment yeilds can be attributed to moderated erosion (either natural or human induced) and/or the incorporation of unweathered glacial debris. Accelerated erosion, resulting in high sediment yield, is primarily human-induced and results from cultivation and other land use. Each of these effects has a profound influence on global sediment discharge from the continents. Pre-human suspended sediment discharge from the continents is estimated to be 12.6×1015 g/yr or about 0.6 the present discharge. -Author
I. General Considerations .
1. Introduction —It may seem presumptuous in one who is neither a geologist nor has any pretension to geological knowledge to venture to address this Society on a subject so nearly akin to their special science. My excuse must be that, having resided for five years on the banks of one of the most active of the Bengal rivers, I have had opportunities which are not vouchsafed to every one of observing their phenomena, and have been a witness of the changes I am about to describe. I may also, perhaps, be allowed to state that, when I first became aware of the disturbance that was taking place around me, I set myself carefully to measure and observe what was passing; and , in 1835, made a sketch-survey of the lower Ganges and Brahmapootra, from Jaffiergunge to the sea. This was published by Mr. Tassin a few years afterwards, and is, so far as I know, the only survey that was made—certainly the only one published—between that made by Major Rennell and the survey now in progress, but which has not yet been given to the world. I may also mention, in extenuation, that I have waited for more than a quarter of a century in order that some one more worthy might undertake the task; but, as no one has come forward, I may perhaps be now excused for venturing upon it.
In order, however, to obviate the reproach of presumption, my intention is to confined myself
Geodynamic modeling demonstrates the strong potential for erosion to influence the pattern and style of deformation in active mountain belts, but field studies yield conflicting views on the importance of erosion in influencing orogenesis. Here we compare patterns in river power, inferred excess fluvial-transport capacity, topographic relief, precipitation, and mineral-cooling ages to assess the coupling between surface erosion and rock uplift within the vicinity of the Namche Barwa-Gyala Peri massif, an active antiformal structure within the eastern Himalayan syntaxis. Our rich and dense data set reveals a tight spatial correspondence of fluvial incision potential, high relief, and young cooling ages. The spatial coincidence is most easily explained by a sustained balance between rock uplift and denudation driven by river incision over at least the last ∼1 m.y. The Yarlung Tsangpo-Brahmaputra River is the largest and most powerful river in the Mmalaya, and two lines of evidence point to its active role in the dynamic interaction of local erosion, rock uplift, thermal weakening of the lithosphere, and deformation: (1) Whereas along the rest of the Himalayan front, high relief and high rock uplift rates are essentially continuous, the high relief and rapid exhumation in the syntaxis is restricted to a "bull's-eye" pattern exactly where the largest river in the Himalaya, the Yarlung Tsangpo-Brahmaputra, has the most energy per unit area avaflable to erode its channel and transport sediment. (2) The location of rapid incision on the Yarlung Tsangpo-Brahmaputra has been pinned for at least 1 m.y., and without compensatory uplift of the Namche Barwa-Gyala Peri massif during this time the river would have eroded headward rapidly, incising deeply into Tibet.
Reconnaissance of the Bengal provinces of East Pakistan and India indicates that structural activity, primarily faulting, has significantly influenced Quaternary geology. Two areas of Pleistocene terrace border the Bengal basin on the east and west and flank Tertiary and older hills of India. Two large inliers of Pleistocene sediments within the basin are surrounded by Recent flood-plain deposits of the Ganges and Brahmaputra rivers and their combined deltaic plain. Block faulting and echelon faulting have so disturbed the topography of the Pleistocene terrace that the reconnaissance was insufficient to permit determination of whether multiple terraces are present within the basin. Faulting and structural uplift have continued into the Recent epoch, necessitating a physiographic subdivision into an early and a late phase. Changes in the courses of the Ganges and Brahmaputra rivers through Bengal during the last few hundred years can be attributed to faulting and resultant tilting of fault blocks. These changes have caused the Ganges to abandon numerous western distributaries in favor of joining the Brahmaputra-Meghna system to the southeast. At present about 12,000 square miles of former Ganges deltaic plain in southwest Bengal has been abandoned. A series of surface echelon faults plus evidence of structural control of stream courses suggest the presence of a subsiding structural trough or major fault at depth. This active structural zone apparently has controlled both the Brahmaputra and Ganges rivers in their lower reaches. Subsurface information is lacking, but this subsiding trough may possibly be related to the arcuate chain of mountains in adjacent Burma.
A geological and geophysical survey in 1968 has shown that the Bengal Deep-Sea Fan is almost 3000 km long, and 1000 km wide. We estimate that it may exceed 12 km in thickness. The sediments of the fan have been transported by turbidity currents from the Ganges-Brahmaputra River delta, through the "Swatch of No Ground" submarine canyon and into an extensive, complex, meandering, and braided net of fan valleys. Present rate of sediment influx suggests a regional rate of denudation in the Himalayan source area of over 70 cm/103 years. The sediment section in reflection profiles of the fan has been subdivided into three units separated by prominent unconformities. Volumes of the upper two units compared with the sediment influx rate extrapolated into the past suggest that the unconformities may be late Miocene and earliest Pleistocene. These times correspond to periods of orogeny in the Himalayas and suggest contemporaneity between plate-edge orogeny and mid-plate tectonic activity.
The Brahmaputra River slices an exceptionally deep canyon through the eastern Himalaya. Fission-track and laser-ablation U-Pb ages of detrital zircon grains from the river document very rapid erosion from this region and its impact on sediment fluxes downstream in the Brahmaputra. Downstream from the canyon, 47% of the detrital zircons in the river's modern sediment load comprise a fission-track age population averaging only 0.6 Ma. Equally young cooling ages are reported from bedrock in the canyon through the Namche Barwa-Gyala Peri massif but are absent from riverbank sands of major tributaries upstream. Simple mixing models of U-Pb ages on detrital zircons from samples taken above and below this massif independently suggest that 45% of the downstream detrital zircons are derived from the basement gneisses extensively exposed in the massif. Constraints on the extent of the source area provided by bedrock cooling ages together with sediment-flux estimates at Pasighat, India, suggest exhumation rates averaging 7-21 mm yr(-1) in an area of similar to 3300 km(2) centered on the massif. This rapid exhumation, which is consistent with the very young cooling ages of the detrital zircons from this area, produces so much sediment that similar to 50% of the vast accumulation in the Brahmaputra system at the front of the Himalaya comes from only similar to 2% of its drainage. This extreme localization of rapid erosion, sediment evacuation, and bedrock cooling bear on (1) common assumptions in geodynamic and geochemical studies of the Himalaya about sources of sediment, and (2) plans for hydroelectric development and flood management in southeastern Tibet and the heavily populated areas of eastern India.
We present a comprehensive summary of the available palaeoclimate records from India and compare the results from different proxies. The results indicate (i) fluctuating lake levels during the early Holocene. The period of relatively higher lake levels from increased precipitation efficiency was reached only ∼7.2–6.0 cal kyr BP, possibly due to increased contribution from winter rainfall; (ii) the onset of aridity in NW India could have begun as early as ∼5.3 cal kyr BP. Subsequently, there were multiple wet events but of shorter duration and smaller magnitude than during the mid Holocene; (iii) there is evidence of several short term climate events in the proxy record. However, in the absence of a rigorous chronological framework a detailed regional correlation is not possible at this stage. Finally, a comparison between marine and terrestrial records indicates that episodes of strongest and weakest monsoon winds were not always associated with wettest and driest episodes respectively in the NW Indian lakes.
The high-relief and tectonically active Himalayan range, characterized by markedly varying climate but relatively homogeneous geology along strike, is a unique natural laboratory in which to investigate several of the factors controlling the composition of orogenic sediments. Coupling of surface and tectonic processes is most evident in the eastern Namche Barwa syntaxis, where the Tsangpo–Siang–Brahmaputra River, draining a large elevated area in south Tibet, plunges down the deepest gorge on Earth. Here composition of river sands changes drastically from lithic to quartzofeldspathic. After confluence with the Lohit River, draining the Transhimalayan-equivalent Mishmi arc batholiths, sediment composition remains remarkably constant across Assam, indicating subordinate contributions from Himalayan tributaries. Independent calculations based on petrographical, mineralogical, and geochemical data indicate that the syntaxis, representing only ∼4% of total basin area, contributes 35±6% to the total Brahmaputra sediment flux, and ∼20% of total detritus reaching the Bay of Bengal. Such huge anomalies in erosion patterns have major effects on composition of orogenic sediments, which are recorded as far as the Bengal Fan. In the Brahmaputra basin, in spite of very fast erosion and detrital evacuation, chemical weathering is not negligible. Sand-sized carbonate grains are dissolved partially in mountain reaches and completely in monsoon-drenched Assam plains, where clinopyroxenes are selectively altered. Plagioclase, instead, is preferentially weathered only in detritus from the Shillong Plateau, which is markedly enriched in microcline. Most difficult to assess is the effect of hydraulic sorting in Bangladesh, where quartz, garnet and epidote tend to be sequestered in the bedload and trapped on the coastal plain, whereas cleavable feldspars and amphiboles are concentrated in the suspended load and eventually deposited in the deep sea. High-resolution petrographic and dense-mineral studies of fluvial sands provide a basis for calculating sediment budgets, for tracing patterns of erosion in mountain belts, and for better understanding the complex dynamic feedback between surface processes and crustal-scale tectonics.
The Brahmaputra is a major river system draining the Himalaya. The concentration of Sr and its 87Sr/86Sr have been measured in dissolved and particulate phases of the Brahmaputra and its tributaries in India to trace the sources of dissolved Sr. Dissolved Sr ranges from 250 to 1050 nM with 87Sr/86Sr from 0.71298 to 0.75975. The Sr data along with the available concentrations of major ions in the samples show that major contributors of Sr in the Brahmaputra system are the silicates and carbonates of the Himalaya and the Transhimalaya and carbonates and evaporites of the Tibetan basin. Silicate Sr in the Brahmaputra river system ranges from 20% to 80% with an average of ∼45%. The silicate component of Sr in the Brahmaputra system is similar or marginally higher than that in the Ganga system due to contribution from the Transhimalayan calc-alkaline plutonic rocks which have higher concentration of Sr. Evaporites of the Tibetan sub-basin and hot springs along the Indus Tsangpo Suture could be the other significant contributors to the dissolved Sr budget of the Brahmaputra. Contribution from evaporite dissolution decouples the total dissolved Sr flux from the Tibetan basin from atmospheric CO2 consumption. Radiogenic Sr of the Brahmaputra system is derived from the Himalayan silicates whereas other Sr sources tend to dilute its radiogenic signature. The 87Sr/86Sr of the dissolved phase shows significant correlation with indices of silicate weathering, indicating that 87Sr/86Sr can serve as a proxy of silicate weathering. At their outflow, the Brahmaputra is less radiogenic (∼0.72) compared to the Ganga (∼0.73), however, the flux of 87Sr from the Brahmaputra is similar to that of the Ganga.
The forcing of sediment dispersal systems by climate is widely accepted, but there remains disagreement on the general responses to climate change such as, how resulting signals propagate downstream and, ultimately, how they are preserved in the stratigraphic record. Towards a better understanding of these issues, this paper presents a comprehensive overview of Ganges dispersal system behavior since the last interstade, which reveals major, coeval responses to changes in the South Asian monsoon. At >3000 km from its mountain headwaters to deep-sea fan, the Ganges dispersal system is immense and yet shows tight coupling between source area, catchment basin and coastal and marine depocenters. Furthermore, system-wide responses and downstream transfer of the signal appear to occur at least as fast as current temporal resolution of 1–2 ky. This tight linkage of source-to-sink components is considered a function of the southwest (summer) monsoon's overwhelming control on regional hydrology. About 80% of the Ganges discharge and 95% of its sediment load are delivered to the margin during only 4 months, making the system extremely sensitive to this seasonal forcing. In addition, the regional scale and distribution of the monsoon weather system means that changes in atmospheric circulation affect the entire drainage basin rather than local subcomponents. Finally, despite its present intensity, strength of the summer monsoon has varied significantly over the past 150 ky under orbitally driven changes in insolation and global (glacial) boundary conditions. The resulting changes in precipitation, both well above and below modern values, have forced system-wide responses ranging from glacier advance/retreat to sediment fluxes to the margin and deep-sea fan. The overall conclusions are (1) that this immense dispersal system responds to multimillennial-scale (<104 years) climate change in a system-wide and largely contemporaneous manner and (2) that major sedimentary signals can be transferred rapidly from source to sink with little apparent attenuation. Furthermore, these acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing.
Sediments carried in suspension represent a fundamental part of fluvial transport. Nonetheless, largely because of technical problems, they have been hitherto widely neglected in provenance studies. In order to determine with maximum possible precision the mineralogy of suspended load collected in vertical profiles from water surface to channel bottom of Rivers Ganga and Brahmaputra, we combined Raman spectroscopy with traditional heavy-mineral and X-ray diffraction analyses, carried out separately on low-density and dense fractions of all significant size classes in each sample (multiple-window approach). Suspended load resulted to be a ternary mixture of dominant silt enriched in phyllosilicates, subordinate clay largely derived from weathered floodplains, and sand mainly produced by physical erosion and mechanical grinding during transport in Himalayan streams. Sediment concentration and grain size increase steadily with water depth. Whereas absolute concentration of clay associated with Fe-oxyhydroxides and organic matter is almost depth-invariant, regular mineralogical and consequently chemical changes from shallow to deep load result from marked increase of faster-settling, coarser, denser, or more spherical grains toward the bed. Such steady intersample compositional variability can be modeled as a mixture of clay, silt and sand modes with distinct mineralogical and chemical composition. With classical formulas describing sediment transport by turbulent diffusion, absolute and relative concentrations can be predicted at any depth for each textural mode and each detrital component. Based on assumptions on average chemistry of detrital minerals and empirical formulas to calculate their settling velocities, the suspension-sorting model successfully reproduces mineralogy and chemistry of suspended load at different depths. Principal outputs include assessment of contributions by each detrital mineral to the chemical budget, and calibration of dense minerals too rare to be precisely estimated by optical or Raman analysis but crucial in both detrital-geochronology and settling-equivalence studies. Hydrodynamic conditions during monsoonal discharge could also be evaluated. Understanding compositional variability of suspended load is a fundamental pre-requisite to correctly interpret mineralogical and geochemical data in provenance analysis of modern and ancient sedimentary deposits, to accurately assess weathering processes, sediment fluxes and erosion patterns, and to unambiguously evaluate the effects of anthropogenic modifications on the natural environment.
Understanding the future evolution of the Ganges-Brahmaputra delta is fundamental to planning and management of this heavily-populated low-lying delta plain. Through the Holocene it was shaped by the interacting effects of sediment supply, sea level and marine reworking. Unlike traditional models, tectonics and climate have also played important roles in the delta's evolution. Superimposed upon the active, but relatively unknown, natural system, human alterations to natural processes have become increasingly important, both (1) indirectly by changing land use and water management within the catchment, and (2) directly within the delta itself due to groundwater withdrawal, flood mitigation efforts, infrastructure development, etc. This human influence will intensify in the future as coastal and catchment populations grow, urbanise and develop their economies. Hence, the Ganges-Brahmaputra delta, like all deltas, is evolving in response to multiple drivers of change. To analyse future changes for management and planning purposes, this paper argues that an integrated assessment framework of the entire delta plain is required. This would take account of the interacting natural and human drivers of change within the delta plain, as well as the external drivers, such as global-mean sea-level rise. Hence it would emphasise the multiple factors and stresses that are influencing the delta evolution. Such a framework also allows the full range of management responses to be considered. In addition to traditional engineering approaches in deltas, there are opportunities to work with nature. The deltaic system has the capacity to be highly resilient in the face of these multiple pressures with important ecological and social benefits. This resilience is largely due to the delivery of sediment to the delta from the catchment. A challenge for future management is to understand and use this capacity to the maximum, including consideration of possible catchment changes such as dam construction.
Analogs – ancient and modern – are key to our understanding and interpretation of the stratigraphic record, which is too often incomplete and sparingly exposed. Here we describe an upward-coarsening Holocene delta sequence that sits unconformably on another, remarkably comparable, delta sequence of Pleistocene age. Such a complete and well-preserved Pleistocene example is rare given extended periods of sea-level lowstand and fluvial incision during the past 200 ka.These stacked delta sequences allow us to consider how analogous our well-studied Holocene analogs are.The comparison reveals a nearly identical facies succession, with modest differences only in the relative timing of delta response to rising sea level. One key difference, though, is a unique facies in the Pleistocene sequence suggesting that major floods from the Himalayas impact the Bengal margin, perhaps periodically, during glacial-interglacial climate transitions.
Extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fluviodeltaic environments. Here we examine patterns of arsenic heterogeneity in the shallow (<23 m) groundwaters of a well-studied floodplain setting in Araihazar, Bangladesh. Ninety-five augers and 200 shallow wells sampled at spacings of 10(1)-10(3) m in the 25 km(2) area indicate that the concentration of arsenic in shallow groundwater largely varies with the grain size, thickness, and distribution of fine-grained (<63 mu m) sediments that overlie buried aquifer sands. The overall pattern shows that lower arsenic concentrations are typically found where aquifer sands outcrop at or near the surface, whereas higher arsenic levels typically underlie, or are adjacent to thicker, fine-grained deposits. Furthermore, chronostratigraphic reconstructions of aquifer sediments indicate that sediment distribution, and consequently the patterning of dissolved arsenic, is readily explained in the context of local river history and floodplain development within the past 1000 yr. An important implication is that complex patterns of groundwater arsenic in afflicted fluviodeltaic settings can be better understood through reconstructions of local aquifer history. This finding is especially relevant because the village and tube-well locations are closely linked with surface landforms such as former levees and bars. An additional and worrisome finding is that the artificial filling of villages to protect from flooding can mimic the natural fine-grained stratigraphy commonly associated with high concentrations of arsenic.
Rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control on coastal evolution and sequence development. However, little is known about past changes in fluvial sediment loads despite the recognition of significant varia- tion under changing climatic regimes. In this study we present the first quantified estimate of sediment discharge for a major river system under conditions of an intensified early Holocene monsoon. Development of the Ganges-Brahmaputra River delta began ca. 11 000 yr B.P., when rising sea level flooded the Bengal basin, thereby trapping most of the river’s discharge on the inner margin. Chronostratigraphic data from these deltaic deposits are used to calculate the rates of sediment storage on the margin, which provide a minimum estimate of the river’s past sediment load. Results reveal that ~5 × 1012 m3 of sediment was stored in the Bengal basin from ca. 11 000 to 7000 yr B.P., which corresponds to a mean load of 2.3 × 109 t/yr. In comparison, modern sediment load of the Ganges-Brahmaputra is ~1 × 109 t/yr, ranking it first among the world’s rivers and underscoring the significance of a two-fold increase sustained over 4 k.y. Furthermore, the timing of immense discharge in the early Holocene strongly suggests its relation to a stronger than present southwest monsoon in South Asia. Similar patterns of high monsoon-related sediment discharge have been noted throughout the tropics and subtropics, suggesting a widespread fluviosedimentary response, the potential magnitude of which is show- cased by the Ganges-Brahmaputra system.
Late Quaternary sediments of the Bengal basin contain a history of river switching and climate change as revealed from sand- and clay-size mineralogy of boreholes and modern riverbed grabs. Epidote to garnet ratios (E/G) in sand fraction sediments are diagnostic of source, with high (>1) E/G indicating Brahmaputra provenance and low (<1) E/G indicating Ganges provenance. In the clay fraction of surficial sediments smectite is diagnostic, with high values (f39%) in the Ganges and low values (f3%) in the Brahmaputra. In contrast, the Brahmaputra contains more kaolinite (29% vs. 18%), more illite (63% vs. 41%), and more chlorite (3% vs. 1%) than the Ganges. Analysis of mineralogic and stratigraphic data indicates that the two rivers have changed position several times during the Holocene. Extended periods of mixed river inputs appear to be isolated to the Early Holocene, suggesting rapidly migrating braided channels during sea level lowstand. Tectonically driven accommodation in the Sylhet Basin may have contributed to the favored easterly course of the Brahmaputra during much of the Holocene. Relative abundances of illite and chlorite (IC) vs. smectite and kaolinite (SK) record varying degrees of physical and chemical weathering, respectively. High IC values in early post-glacial deposits suggest a dominance of physical weathering at that time. However, a general increase in SK concentrations throughout the Holocene appears to reflect enhanced chemical weathering under increasingly warmer and more humid conditions. Notably, a peak in SK concentrations that corresponds to an Early Holocene warm period (f10 – 7 ka) suggests that weathering patterns in the catchment respond quickly to climatic shifts.
The partitioning of fluvial sediment load across continental margins is an important con-
trol on strata formation and sequence development; however, few quantitative sediment budgets that encompass entire dispersal systems exist. For the Ganges-Brahmaputra river system, sedi- ment discharge is estimated to be 10^9 t/yr at gauging stations ~300 km inland of the coast, but little has been known of the downstream fate of this material. Geochronological, geophysical, and stratigraphic investigations of the lowland flood plain, delta plain, and shelf help to delineate the extent of Holocene fill and allow calculation of a first-order sediment budget. Results reveal that 1500 × 109 m3 of sediment fill has been sequestered within the flood plain and delta plain since ca. 7000 yr B.P., or about one-third of the annual discharge. The remaining load appears to be apportioned between the prograding subaqueous delta (1970 × 109 m3 ) and transport to the deep-sea Bengal fan via a nearshore canyon. Modern (<100 yr) budget estimates based on short-term accretion rates indicate a similar dispersal pattern and show that contemporaneous deposition continues within these disparate depocenters. The roughly equal partitioning of sedi- ment among flood-plain, shelf, and deep-sea settings reflects the respective influence of an inland tectonic basin, a wide shelf, and a deeply incised canyon system. The findings also support new sequence stratigraphic models for these settings and indicate the important insight that modern river deltas can provide for ancient margin systems. Furthermore, results affirm that values of riverine sediment flux to the oceans may be considerably overestimated by not accounting for loss to the flood plains downstream of the gauging stations.
Two main sand provinces comprising nearly all the sediments of East Pakistan can be recognized on the basis of their sand mineralogy. One province contains most of the folded Tertiary sedimentary formations bordering East Pakistan in the north, east and southeast as well as old clays, “Madhupur clay”, which occupy extensive terrace-like areas in the centre and west. These sediments are characterized by low contents of feldspars and micas and by high contents of epidote, zircon, tourmaline, kyanite, staurolite and sillimanite in the heavy sand fraction. Except perhaps for the youngest sedimentary formations of this group which are of Pliocene-Early Pleistocene age, the sediments of this main province probably mainly originated from the crystalline rocks of the old Indian peninsula.
Understanding the dynamics of sediment fluxes is a key issue to constrain modern erosion rates in mountain belts and determine the still debated level of control exerted by precipitation, topography and tectonics. The well defined monsoon seasonality in the Himalayas, together with active tectonics and strong relief provide an ideal environment to assess these possible interactions. For this purpose, we present a new compilation of daily suspended sediment data for 12 stations of the major rivers of the Nepal Himalayas. We analyze the relationships of sediment transport with daily river discharge and precipitation data as well as with morphometric parameters. We show that suspended sediment concentrations vary systematically through the seasons and asynchronously to river discharge displaying a hysteresis effect. This clockwise hysteresis effect disappears when suspended sediment fluxes are directly compared with direct storm discharge. Therefore we attribute the hysteresis effect to groundwater dilution rather than a sediment supply limitation. We infer a rating model to calculate erosion rates directly from long river discharge chronicles. We show that, when normalized by drainage area and mean sediment flux, all rivers exhibit the same trend. This similarity implies that all river basins have the same erosion behavior, independent of location, size and catchment characteristics. Erosion rates calculated from suspended sediment fluxes range between 0.1 and 2.8mm/yr. The erosion rates of the three main basins of Nepal are in the range 0.9–1.5 mm/yr, Erosion rates in the Higher Himalayas are relatively low (o0.5 mm/yr, except for Kali Gandaki), while in the Lesser Himalayas they range from 0.2 to 2 mm/yr. We propose that material transport in the rivers depends on hillslope sediment supply, which is, in turn, controlled by those rainfalls producing direct runoff. In other words, the rivers in the Nepal Himalayas are supply-limited and the hillsopes as a contributing source are transport-limited. We also show that erosion processes are not as much controlled by infrequently occurring extreme precipitation events, than by moderate ones with a high recurrence interval.