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Erosion of Deccan Traps determined by river geochemistry: Impact on the global climate and the 87Sr/86Sr ratio of seawater

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Abstract

The impact of the Deccan Traps on chemical weathering and atmospheric CO2 consumption on Earth is evaluated based on the study of major elements, strontium and 87Sr/86Sr isotopic ratios of the main rivers flowing through the traps, using a numerical model which describes the coupled evolution of the chemical cycles of carbon, alkalinity and strontium and allows one to compute the variations in atmospheric pCO2, mean global temperature and the 87Sr/86Sr isotopic ratio of seawater, in response to Deccan trap emplacement. The results suggest that the rate of chemical weathering of Deccan Traps (21–63 t/km2/yr) and associated atmospheric CO2 consumption (0.58–2.54×106 mol C/km2/yr) are relatively high compared to those linked to other basaltic regions. Our results on the Deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of CO2 consumption during weathering of basalts, according to the relationship:

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... Chemical weathering of silicate rocks in the Earth's epigenetic environment plays a crucial role in continental weathering fluxes and the chemical composition of rivers and oceans. Moreover, weathering of Ca-Mg silicate rocks takes up CO 2 and transforms it to dissolved HCO 3 in continental waters, which is a key mechanism in the sequestration of atmospheric carbon (Beerling et al., 2020;Dessert et al., 2001Dessert et al., , 2003Gaillardet et al., 1999;Li & Elderfield, 2013;Raza et al., 2022). Such a process is a major contributor to the global carbon cycle and influences climate variability on geological timescales (Kump et al., 2000). ...
... The sensitivity of CO 2 consumption by weathering Ca-Mg silicate rocks to climate variables such as temperature and hydrology is unclear, despite the acknowledged importance of global fluxes. Our understanding of the patterns and mechanisms controlling CO 2 consumption by weathering primarily derives from the analysis of geochemical tracers in river waters (Battin et al., 2023;Berner et al., 1983;Dessert et al., 2001Dessert et al., , 2003Gaillardet et al., 1999;Harris et al., 1998;Li & Elderfield, 2013;Pogge von Strandmann et al., 2019;Tipper et al., 2006). These studies estimate weathering fluxes by examining the dissolved load of large rivers. ...
... These studies estimate weathering fluxes by examining the dissolved load of large rivers. Estimates of CO 2 consumption have been derived by determining the current weathering rates of silicates and carbonates and their respective contributions to the average dissolved load of global rivers, and multiplying these values by the river water discharge to the oceans (Dessert et al., 2001(Dessert et al., , 2003Gaillardet et al., 1999;Li & Elderfield, 2013). However, these indirect estimates are averaged over catchments that integrate processes operating under variable hydrological and temperature conditions. ...
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The weathering of silicate rocks exerts a significant control on the weathering fluxes of metals and atmospheric CO2 consumption. In this study, we present new magnesium (Mg) isotope data from a basalt weathering profile in Hainan Island, South China, to investigate Mg isotope fractionation and calculate weathering fluxes and CO2 consumption. The Mg mobility (τMg,Ti) in saprolites decreases from −34.1% to −95.7%. The δ²⁶Mg values in saprolites vary from −0.25 ± 0.07‰ to 0.43 ± 0.07‰, higher than those of the parent rock (−0.25 ± 0.07 ‰). The significant Mg loss during the formation and decomposition of clay minerals influences Mg isotope fractionation, particularly with changes in kaolinite structure under different pH conditions, which prefer heavy Mg isotopes. By applying a mass balance model, we have developed a novel method to calculate weathering fluxes based on the weathering profile, yielding Mg elemental fluxes (MgFlux) of 2.45–5.85 mol/cm²/Myr, Mg isotopic fluxes (δ²⁶MgFlux) of −0.44 to −0.04‰/mol/cm²/Myr, and CO2 consumption of 2.3 × 10¹² mol/yr for the weathering outputs of basaltic rocks. This highlights the crucial role of basalt weathering in global carbon sequestration. Our findings improve the understanding of Mg cycling and isotope fractionation in epigenetic environments and facilitate the quantification of weathering fluxes and atmospheric CO2 consumption during basalt weathering.
... Models of the climate response to the meteorite impact include an "impact winter" lasting months to millennia due to atmospheric loading of dust, soot, and sulfate aerosols (13)(14)(15)(16)(17), and longer-term warming caused by CO 2 released by wildfires and/or impact-volatilized carbonates (18). Two principal climate models are associated with Deccan volcanism: first, global warming, caused by eruption-, venting-, and contact-metamorphism-derived CO 2 (19) and sustained over thousands to hundreds of thousands of years [e.g., (20)]; and second, global cooling driven by the conversion of SO 2 into sulfate aerosols, but lasting only for the duration of the eruption (21)(22)(23). ...
... with the notion of quasicontinuous eruption (and CO 2 release) [e.g., (34)]. Models based on the understood volumes and rates of total Deccan lava eruption, and assumptions about associated CO 2 emissions imply that they resulted in net global warming of less than 1°C (47) to 4°C (19,20,48). Recent geochemical analyses of accompanying intruded (i.e., unerupted) magmas increase total CO 2 emission estimates and imply that up to 6°C warming may have been possible (49) but diminish the temporal connection between lava eruptions and the tempo and rates of CO 2 outgassing. ...
... As an explanation, Hernandez Nava et al. (49) found that late Deccan magmas became depleted in CO 2 , meaning that the earlier phases of outgassing were more efficient in driving global warming. Furthermore, combined biological and mineral reaction sequestration rates could not fully offset the volumes of CO 2 injected into the atmosphere at the timescales between individual Deccan events (20,48), resulting in diminishing greenhouse effects of each new mole of CO 2 emitted (50). The 5°C of LMWE warming that has been ascribed to the first Deccan eruptive event 300 to 200 kyr before the K-Pg boundary is greater than the 3°C observed in our record, consistent with this hypothesis (45). ...
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Alongside the Chicxulub meteorite impact, Deccan volcanism is considered a primary trigger for the Cretaceous–Paleogene (K–Pg) mass extinction. Models suggest that volcanic outgassing of carbon and sulfur—potent environmental stressors—drove global temperature change, but the relative timing, duration, and magnitude of such change remains uncertain. Here, we use the organic paleothermometer MBT′ 5me and the carbon-isotope composition of two K–Pg-spanning lignites from the western Unites States, to test models of volcanogenic air temperature change in the ~100 kyr before the mass extinction. Our records show long-term warming of ~3°C, probably driven by Deccan CO 2 emissions, and reveal a transient (<10 kyr) ~5°C cooling event, coinciding with the peak of the Poladpur “pulse” of Deccan eruption ~30 kyr before the K–Pg boundary. This cooling was likely caused by the aerosolization of volcanogenic sulfur. Temperatures returned to pre-event values before the mass extinction, suggesting that, from the terrestrial perspective, volcanogenic climate change was not the primary cause of K–Pg extinction.
... It is likely that the Os flux from juvenile Deccan basalt weathering is greater than that from erosion of the same area of Indian cratonic rocks, given that juvenile basalts are thought to be 5-10 times more easily eroded than typical silicate rocks (Dessert et al., 2001(Dessert et al., , 2003, and that the basalts may also have had a higher Os concentration than the Indian cratonic rocks. ...
... x = >5. Juvenile basalts are thought to be 5-10 times more readily weathered than cratonic lithologies (Dessert et al., 2001(Dessert et al., , 2003. However, the relative increase in Os flux from Deccan basalts versus Indian Craton rocks also depends on the respective average Os concentrations of the two lithologies, which are less well constrained. ...
... However, unless the average Os content of Deccan rocks was very high, the minimum volume of rapidly eroded juvenile basalts that erupted during the formation of that large igneous province is still huge. Such extensive basalt erosion at low latitudes likely would have functioned as an efficient sink of CO 2 (see also Dessert et al., 2001Dessert et al., , 2003, as has been previously proposed following the eruption of basalts associated with the Central Atlantic Magmatic Province and Paraná-Etendeka large igneous province (see Schaller et al., 2012;Percival et al., 2023). ...
Article
The Cretaceous−Paleogene boundary is marked by a large impact and coeval mass extinction event that occurred 66 m.y. ago. Contemporaneous emplacement of the volcanic Deccan Traps also affected global climate before, during, and after the mass extinction. Many questions remain about the timing and eruption rates of Deccan volcanism, its precise forcing of climatic changes, and its signature in the marine geochemical sedimentary proxy record. Here, we compile new and existing mercury (Hg) concentration and osmium isotope (187Os/188Os) records for various stratigraphic sections worldwide. Both geochemical proxies have been suggested to reflect past variations in Deccan volcanic activity. New data from deep marine pelagic carbonate records are compared to contemporaneous records from shallower marine sites correlated through high-resolution cyclostratigraphic age models. The robustness of the proxy records is evaluated on a common timeline and compared to two different Deccan eruption history scenarios. Results show that the global 187Os/188Os signal is clearly reproducible, while the global Hg record does not form a consistent pattern. Moreover, the deep marine sections investigated do not record clear variations in the Hg cycle, particularly in the latest Cretaceous, prior to the extinction event. A detailed reevaluation of the precise depth of the redistribution of impactor-sourced platinum group elements does not exclude the possibility of a minor drop in 187Os/188Os corresponding with a pulse of Deccan volcanism ∼50,000 years before the Cretaceous−Paleogene boundary. Simple Os isotope mass balance modeling indicates that the latest Cretaceous was marked by significant levels of basalt weathering. CO2 sequestration during this weathering likely overwhelmed the emission of Deccan volatiles, thereby contributing to the end of the late Maastrichtian warming.
... Studies of chemical and physical denudation rates, involving a mass balance in large or small watersheds, have focused mainly on fluvial geochemistry, especially in granitic and basaltic watersheds under different climates, relief, vegetation, and land use conditions [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. These studies conclude that temperature and runoff are the main parameters controlling the water/rock interactions and soil removal worldwide. ...
... Despite the small size of the Amoras Stream, it is useful to compare the F w values obtained in this study with values measured in alkaline watersheds under different climatic and tectonic settings in Brazil or elsewhere (Table 6). Temperature and runoff are the most important parameters controlling the F w values in watersheds composed of alkaline rocks [17]. The F w value for PC is slightly lower than those obtained for the Paraná CBF province and Tapira and Catalão I alkalinecarbonatite rocks, areas located under tropical climate in Brazil or in São Miguel, Deccan Traps, and Iceland. ...
... Considering the tropical climate in southeastern Brazil, the F CO 2 in the PC is lower than those calculated for the Paraná CBF province (0.4 × 10 6 mol/km 2 /a) [35] and igneous and metamorphic rocks belonging to Ribeira Belt (0.2 × 10 6 mol/km 2 /a) [37]. In addition, this rate is lower than in carbonate areas (0.8 × 10 6 mol/km 2 /a) [69] and basaltic watersheds, such as São Miguel (0.6 × 10 6 mol/km 2 /a) [14], Deccan Traps (1.3 × 10 6 mol/km 2 /a) [17], Iceland (0.7 × 10 6 mol/km 2 /a) [23], and Réunion island (2.3 × 10 6 mol/km 2 /a) [13]. ...
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Chemical and physical denudation rates have been assessed in areas with different litholo-gies. Surprisingly, there are no studies that attempt to document these rates in the Poços de Caldas Alkaline Massif (PC), the largest alkaline magmatism in South America and an important Al super-gene deposit in Brazil. Therefore, the chemical and physical denudation rates were assessed and explained in the PC. Surface water and rainwater samples were collected at the Amoras Stream basin, covering one complete hydrological cycle (2016). All samples were analyzed for dissolved cations, silica, anions, total dissolved solids (TDS), and total suspended solids (TSS). The results reflected the seasonal variation on discharge, water temperature, and electrical conductivity in the Amoras Stream, with most of the cations, anions, silica TDS, and TSS being carried in the wet season. Partial hydrolysis and silicate incongruent dissolution are the main water/rock interactions in the PC, with an atmospheric/soil CO 2 consumption rate of 1.6 × 10 5 mol/km 2 /a. The annual fluxes of Cl − , PO 4 3− , NO 3 − , and Al 3+ were significantly influenced from rainwater. Chemical and physical weathering rates were 4 ± 0.8 and 3.0 ± 0.6 m/Ma in the PC, respectively, indicating that under the current climatic condition, the weathering profile is in dynamic equilibrium.
... Therefore, the importance of MNT river systems in controlling the biogeochemistry of the Arabian Sea is expected to be significant (Ramaswamy and Nair 1994;Goswami et al. 2012). The Narmada and the Tapti rivers are well-studied for their hydrogeochemistry (Dessert et al. 2001;Sharma and Subramanian 2008;Gupta et al. 2011Gupta et al. , 2020; however, that of the Mahi River has been restricted to a few studies (Srivastava et al. 2011;Sharma et al. 2012Sharma et al. , 2013Sharma and Kumar 2018). These studies were confined to socio-economic dimensions investigating the quality and suitability of the river water for domestic and agricultural uses and lacked geochemical insights on the major sources, their contributions to dissolved loads, and the rates of chemical weathering and associated CO 2 drawdown occurring at the catchment scale. ...
... Table 1 gives the mean concentration of physicochemical parameters of Mahi River water. For comparative analysis data from the other west-flowing rivers (e.g., the Narmada and the Tapti) draining into the Arabian Sea are taken from previously published studies (Dessert et al. 2001;Sharma and Subramanian 2008;Gupta et al. 2011). ...
... CCR values in the MRB (~ 3 × 10 5 mol km −2 yr −1 ) are similar to values obtained for the basins of Narmada (3.3 × 10 5 mol km −2 yr −1 ; Gupta et al. 2011); the Krishna (3.6 × 10 5 mol km −2 yr −1 ; Das et al. 2005); Nethravati (2.8 × 10 5 mol km −2 yr −1 , Gurumurthy et al. 2012) and Sharavati (3.9 × 10 5 mol km −2 yr −1 ; Amrish et al. 2022). Our values are somewhat lower than those calculated for the Godavari (5.8 × 10 5 mol km −2 yr −1 ; Jha et al. 2009); Muthapuza (5.5 × 10 5 mol km −2 yr −1 ; Trivikramaji and Joseph 2001); Payaswini-Chandragiri (9.5 × 10 5 mol km −2 yr −1 ; Nisha et al. 2021); while much lower than values of those of Narmada (~ 21 × 10 5 mol km −2 yr −1 ; Sharma and Subramanian 2008); Tapti (~ 18 × 10 5 mol km −2 yr −1 ; Sharma and Subramanian 2008), and the Narmada-Tapti-Wainganga (NTW) (12.6 × 10 5 mol km −2 yr −1 ; Dessert et al. 2001) rivers. ...
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The geochemistry of rivers draining into the Arabian Sea has received lesser attention than their eastern counterparts draining into the Bay of Bengal. The major ion concentrations in the third-major river (Mahi) draining into the Arabian Sea have been reported. Hydrochemistry of seasonally collected samples (n = 67) during two consecutive years reveals that the average solute export and the cationic charges are ~ 1.2 and ~ 4 times their corresponding global average values. Generally, lower solute levels (except Ca) in the rainy season reflect the dilution effect. In contrast, the higher intensity of carbonate weathering out-weighing the dilution effect, aided by CaCO3 precipitation in summer, dominates the Ca dynamics. Chemical weathering in the catchment is mediated predominantly by carbonic acid, as evident from the alkalinity/(Ca + Mg) and SO4/(Ca + Mg) molar ratios. Using a forward model, the estimated average contributions from various sources to the cationic composition are atmospheric (5 ± 1%), evaporites and soil salts (27 ± 9%), silicates (39 ± 12%), and carbonates (29 ± 13%). Runoff exerts the dominant control on the weathering rates, with the best estimated average silicate (11 t km⁻² yr⁻¹), carbonate (22 t km⁻² yr⁻¹), and evaporite and soil-salt (8 t km⁻² yr⁻¹) weathering rates, which sums up to a dissolved chemical flux of ~ 41 t km⁻² yr⁻¹. The lower-bound estimate of CO2 consumption by silicate weathering of the Mahi basin is ~ 2.8 × 10⁵ mol km⁻² yr⁻¹, translating into CO2 consumption of ~ 9.7 × 10⁹ mol yr⁻¹. This is ~ 0.08% of the global average value (by silicate weathering) and is disproportionately higher than its fractional continental drainage area (~ 0.03%). Integrating the data of Mahi with those available for the other two major rivers (Narmada & Tapti), this study underscores the prominent role of these rivers in supplying solutes to the Arabian Sea and controlling the biogeochemistry of the Arabian Sea.
... Climate variability, over Myr timescales, is influenced by a greenhouse forcing that is modulated by a balance between the rates at which CO 2 is expelled from volcanoes and drawn down from the atmosphere via chemical weathering processes (Walker et al. 1981;Staudigel et al. 1989;Dessert et al. 2001;Coogan and Dosso 2015). The global rate of CO 2 outgassing is governed by the character and pace of a planet's volcanic activity. ...
... Surface velocity can be used to track creation of mountain topography associated with tectonic deformation (Whipple and Meade 2004;Roe et al. 2008). Topography generation can be linked with a weathering parameterization to track greenhouse gas withdrawal from the atmosphere (Walker et al. 1981;Dessert et al. 2001;Riebe et al. 2004). The volcanic-tectonic and thermal history model can then be linked to a radiative-convective climate model to track surface temperature over the thermal history of a model planet (Foley 2015;Foley and Driscoll 2016;Lenardic et al. 2016b). ...
... sink for CO 2 , thereby increasing weatherability. Due to the relatively high concentrations of Ca and Mg in these rocks, they can sequester more carbon through carbonate precipitation in the ocean than typical crustal compositions (Dessert et al. 2001;Park et al. 2020b). Key to evaluating these potential mechanisms for cooling past a critical threshold are high-precision geochronological and paleomagnetic constraints, as both the albedo and weathering hypotheses make predictions for the timing of the Franklin LIP relative to onset of the Sturtian glaciation, and both albedo and chemical weathering are strongly dependent on latitude. ...
... Models and proxy data for pCO 2 following LIP emplacement show that warming from the input of volcanic CO 2 into the atmosphere will be counteracted by silicate weathering within ~1-2 My of emplacement, turning the LIP into a net CO 2 sink (Dessert et al. 2001;Schaller et al. 2012). Subsequently, the development of regolith and soil shielding will mute the effects of a LIP on weatherability (Park et al. 2020b;Fig. ...
Article
Large igneous provinces (LIPs) can potentially cause cooling on tens- to thousand-year timescales via injection of sulfur aerosols to the tropo-sphere, and on million-year timescales due to the increase of global weatherability. The ca. 719-Ma Franklin LIP preceded onset of the Sturtian Snowball Earth glaciation by less than two million years, consistent with CO2 drawdown due to weathering of Ca- and Mg-rich LIP basalts, which may have contributed to cooling past a critical runaway ice-albedo threshold. A relatively cool background climate state and Franklin LIP emplacement near a continental margin in the warm wet tropics may have been critical factors for pushing the Earth’s climate past the threshold of runaway glaciation.
... This implies that more weathered sediment is preferentially derived from younger more primitive bedrock terrains, although these could be represented by either the Deccan Plateau flood basalts or the magmatic arc units in the Indus Suture Zone, including Kohistan. Because erosion is rapid and the sediment in the Indus River is relatively chemically fresh, as demonstrated by the overall low CIA, it seems more likely that strongly weathered, primitive material would be coming from the Deccan Plateau where chemical weathering has been impacting the landscape since the end of the eruption in the Paleocene (Widdowson & Cox 1996;Dessert et al. 2001). ...
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Measuring chemical weathering histories in submarine fan deposits is critical if the impact of orogenic erosion on atmospheric CO2 levels is to be understood, yet existing records are often noisy and hard to interpret. In this study, we selected mudstones from two International Ocean Discovery Program (IODP) sites from the Indus submarine fan and carefully removed the biogenic carbonate. The resulting records of chemical weathering show two trends, one of reducing chemical alteration since ∼8 Ma and which is associated with the Indus River, while a second trend is linked to sediment delivery from peninsular India. The second trend shows little temporal variation. Sediment deposited at IODP Site U1456 in the central Laxmi Basin is preferentially, but not exclusively, Indus-derived, while that at Site U1457 on the eastern flank of Laxmi Ridge is more peninsula-derived. Both trends show much less variability than seen in earlier studies in which various grain-size fractions were integrated together. The efficiency with which CO2 is removed from the atmosphere during chemical weathering has decreased over time in the Indus River-derived material. This reflects both lower degrees of alteration in the sediment since the late Miocene and increasing derivation of sediment from Himalayan sources, rather than more mafic Karakoram-Kohistan rocks. Previous estimates of CO2 consumption have overestimated the contribution that the Indus Basin has made to reducing atmospheric CO2 by ∼28–68%. This work emphasizes the importance of analysing appropriate largely silt-sized sediment when considering submarine fan records and in rigorously removing biogenic carbonate.
... (2021), and C the updated HVLC thickness map from most available OBS-derived velocity structure. Three discrepancies marked by brown rectangles a, b and c are recognized and discussed in the context For example, the Deccan Trap is one of the largest continental LIPs on Earth's surface, with an area of 5 × 10 5 km 2 and a volume of 10 6 km 3 (Dessert et al. 2001;Holbrook and Kelemen 1993;Kelemen and Holbrook 1995). Oceanic LIPs comprise oceanic plateaus and ocean basin flood basalts, including Ontong Java (44 × 10 6 km 3 ), Manihiki (8.8 × 10 6 km 3 ), Shatsky Rise (2.5 × 10 6 km 3 ), and Hikurangi (2.7 × 10 6 km 3 ) (Kerr 2014). ...
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High-velocity lower crust (HVLC) near the continent–ocean transition zone is significant for recording magmatic activity and early rifting history and is likely to be sourced from various types of magma supplies, including passive decompression melting, hot mantle plume, active small-scale mantle convection, and fertile source mantle. In the northern margin of the South China Sea (composed of the Dongsha Passive margin to the west and the SW Taiwan active margin to the east), a large amount of geophysical data, including refraction seismic data, enable us to prepare a map of the thickness distribution of the HVLC and estimate its volume. After revisiting published data, the volume of the HVLC in the northern margin of the South China Sea is found to be comparable but slightly less than that in continental large igneous provinces such as the Deccan Trap of west-central India or oceanic plateaus such as the Shatsky Rise in the northwest Pacific Ocean, probably indicating that they may have similar active source origins. Based on a more feature-based analysis ( h–v p diagram; a scatter plot showing mean velocity of lower crust versus corresponding crustal thickness) to identify the source of the underplating material in a magma-involved margin, we tentatively suggest that the HVLC along the Dongsha passive margin showing no h–v p correlation is most likely dominated by small-scale mantle convection of the Peikang–Dongsha mantle convection cell, and the HVLC along the SW Taiwan active margin showing a negative h–v p correlation is more likely to be dominated by the pre-existing distal domain of the continental margin. We propose that rather than being dominated by a hot mantle plume, small-scale mantle convection may be more likely to be responsible for the opening of the South China Sea, and suggest that a more inclusive geodynamic model to reconcile geophysical and geochemical observations should be further investigated.
... Strontium in seawater derives mainly from weathering of the crust and mid-ocean ridge hydrothermal fluids (Palmer and Elderfield, 1985;Palmer and Edmond, 1989;Dessert et al., 2001;Butterfield et al., 2011). At the same geological time, the strontium isotope composition of seawater is uniform on a global scale and can be compared within and between basins and across continents (Wickman, 1948;McArthur et al., 2001;Mazumdar and Strauss, 2006;Waltham and Gröecke, 2006;Nieto et al., 2008;Burla et al., 2009), because the residence time of strontium in seawater (~2.5 Ma) is far longer than the time required for uniform mixing in seawater (~1.6 ka) (Eglington et al., 2003;Dickin, 2005). ...
... Studies on weathering patterns of river basins have widespread implications to source ascription, paleoclimate interpretation, sediment accumulation in oceans and CO 2 budget (Gaillardet et al., 1999;Gibbs et al., 1999;Dessert et al., 2001;Clift, 2006;Jha et al., 2009;Tripathy et al., 2014;Kulkarni et al., 2014). The quantification of source ascription and studies on chemical weathering in response to past regional climatic changes in the river basins have provided diverging views (Meybeck, 1987;Galy and France-Lanord, 1999;Gaillardet et al., 1999;Tripathy and Singh, 2010;Lupker et al., 2012Lupker et al., , 2013Bouchez et al., 2012;. ...
Article
The drainage basins of Peninsular India are characterized by silicate-dominated lithologies, and influenced by Indian Summer Monsoon (ISM) precipitation. The Godavari River Basin (GRB), the largest river basin in Peninsular India situated within the ISM region, represents an ideal case for assessing weathering and climate interaction at different timescales. In this contribution, major and trace elemental geochemistry of a radiocarbondated sediment core (CY; 54.2 m long) from the Godavari delta region was investigated to reconstruct erosional changes in the Godavari basin in response to ISM variations during the Late Holocene. Comparison of geochemical data for the CY sediments and their possible sources confirm dominant sediment supply from the Deccan basalts and Archean Gneisses to the site. A distinct increase in Ti/Al, Ca/Al, and Cr/Al, along with a decrease in CIA* and LREE/HREE at 3.2 ka BP, point to relative increase in sediment supply from the Deccan Traps. Inverse model calculations of Al-normalized ratios of selected elements (Ti, Fe, V, Cr, Cu, Co) estimate that the core site on average receives ~41 % sediments from the Deccan regions, which increased by ~20% since last 3.2 ka BP. This accelerated erosion is attributed to the coupled effect of aridity-induced Deccan upland erosion with a relative decrease from the Archean rock source. This period of accelerated erosion coincides with the abandonment of Chalcolithic settlements.
... This has been argued to mean that other regions may significantly contribute to the global CO 2 budget and not just the Himalayan erosion-induced silicate weathering (Caves et al., 2016). Consequently, the focus of weathering research pivoted toward stronger consideration of tropical arc and ophiolite complexes (Jagoutz et al., 2016;Macdonald et al., 2019;Park et al., 2020;Bayon et al., 2023;Martin et al., 2023) that are characterized by more reactive, soluble Caand Mg-rich mafic and ultramafic rock compositions (Dessert et al., 2001). Nonetheless, the sheer volume of material derived from the Himalaya, plus the fact that this collision zone exposes mafic and ultramafic lithologies, and transited through tropical latitudes for a significant portion of its paleogeographic history (van Hinsbergen et al., 2012) merits that any comprehensive global Cenozoic CO 2 budget must involve consideration of the India-Eurasia collision and the remnant Himalayan orogen today. ...
... Chemical weathering of silicate rocks is a process of decomposition and alteration of parent rocks in the Earth's epigenetic environment. It acts as a first-order control on elemental mobilization and redistribution in the critical zone and on input fluxes to river waters and the oceans (Blum & Erel, 1995;Das et al., 2022;Dessert et al., 2001;Gaillardet et al., 1999;Li & Elderfield, 2013;Palmer & Edmond, 1989;Pearce et al., 2015). Effective constraints on continental weathering fluxes are crucial for understanding the mass weathering output of the Earth's surface and the global mass cycle. ...
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The fluxes of metal cations and isotopes released by weathering of silicate rocks are crucial and a prerequisite for constraining geochemical fluxes to rivers and oceans. This study presents mineral and elemental compositions along with ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr data from a basaltic weathering regolith on Hainan Island, South China to elucidate Sr isotope fractionation and weathering fluxes. The ⁸⁷Sr/⁸⁶Sr ratios vary from 0.703936 to 0.706338 as a result of differential weathering of the minerals. δ⁸⁸Sr values in the weathering regolith range from −0.29 to 0.37‰, with the majority of the weathering regolith having lower δ⁸⁸Sr values than the parent rock. Sr is leached into the soil solution during plagioclase decomposition, while ⁸⁶Sr is preferentially adsorbed on the surface of secondary minerals. As weathering progresses, smectite decomposes and kaolinite desorbs under weakly acidic conditions, releasing the previously adsorbed ⁸⁶Sr into the soil solution. The differential weathering of kaolinite and smectite controls the δ⁸⁸Sr values of the weathering regolith, with pH being an important determinant of isotope fractionation. Furthermore, Sr elemental fluxes (SrFlux) and Sr isotopic fluxes (δ⁸⁸SrFlux) of this weathering regolith were calculated using a mass balance model, yielding mean values of 0.20 (mg cm⁻³ Myr⁻¹) and 0.052 (‰ (mg cm⁻³ Myr⁻¹)), respectively. The δ⁸⁸SrFlux exhibits a nonlinear positive correlation with the Chemical Index of Alteration (CIA), indicating that enhanced weathering leads to significant stable Sr isotope fractionation at CIA values below 95%. Our research promotes the understanding of Sr recycling and the fractionation behavior of stable Sr isotopes during consecutive weathering.
... We also suggest that the bio-recovery was fast, erratic and unstable in the terrestrial and surface ocean systems, but slow and stable in the deep marine realm. Alternatively, climate and environmental perturbation may have been much more significant in the surface ocean and terrestrial realm than in the deep sea, resulting in a relatively shorter and faster ecosystem recovery than expected in surface realms (e.g., ~1-2 Myr; Dessert et al., 2001;Birch et al., 2016;Donovan et al., 2016;or > 3 Myr;D'Hondt et al., 1998;Adams et al., 2004). This may imply that the terrestrial and surface ocean realms were much more sensitive to climatic changes after the K/ PgB than the deep ocean, which may have been relatively buffered due to the time lag for such major temperature changes to infiltrate into the deep sea; and the recovery capability of terrestrial ecosystems could be much more powerful than we expect (Keller et al., 2016). ...
... The release of CO 2 that accompanies eruption drives global warming; however, on long time scales (10 5 -10 7 yr), this is counteracted by enhanced silicate weathering that can sequester enough CO 2 to cause global cooling (e.g., Dessert et al., 2003). Many studies, from laboratory experiments to field observations, have found that mafic silicates weather five to 10 times faster than felsic material under the same climatic conditions (e.g., Dessert et al., 2001;Allègre, 1997, 1998;Rad et al., 2007;Wolff-Boenisch et al., 2004Ibarra et al., 2016). This work suggests that continental basalt weathering disproportionately affects the terrestrial silicate weathering flux and therefore may have played a key role in regulating Earth's climate and nutrient availability throughout history (e.g., Dessert et al., 2003;Horton, 2015;Ibarra et al., 2016). ...
Article
The rhenium-osmium (187Re-187Os) system is a highly versatile chronometer that is regularly applied to a wide range of geological and extraterrestrial materials. In addition to providing geo- or cosmo-chronological information, the Re-Os system can also be used as a tracer of processes across a range of temporal (millennial to gigayear) and spatial scales (lower mantle to cryosphere). An increasing number of sulfide minerals are now routinely dated, which further expands the ability of this system to refine mineral exploration models as society moves toward a new, green economy with related technological needs. An expanding range of natural materials amenable to Re-Os geochronology brings additional complexities in data interpretation and the resultant translation of measured isotopic ratios to a properly contextualized age. Herein, we provide an overview of the 187Re-187Os system as applied to sedimentary rocks, sulfides, and other crustal materials and highlight further innovations on the horizon. Additionally, we outline next steps and best practices required to improve the precision of the chronometer and establish community-wide data reduction procedures, such as the decay constant, regression technique, and software packages to use. These best practices will expand the utility and viability of published results and essential metadata to ensure that such data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR).
... Carbonates precipitated from these waters will thereby also have a similar 87 Sr/ 86 Sr ratio. (Dessert et al., 2001). There are five distinct sub-horizontal formations with the Bushe formation at the base and the Panhala formation at the top. ...
... 8 ka (Fig. 4C, D), which suggests a transition from sea-ice drifting to the dominance of surface ocean currents in sediment transport during the Holocene. With the enhanced chemical weathering volcanic input from the Okhotsk-Chukotka volcanic belt and the southern Kuril Islands/Hokkaido during the Holocene (Dessert et al., 2001(Dessert et al., , 2009Zhong et al., 2021), even a minor addition of volcanic materials (characterized by the most radiogenic εNd values) from these regions could significantly impact the εNd values in the sediments of core LV55-40-1 (Fig. 9A). ...
Article
The Okhotsk Sea, located between the Asian continent and the western Pacific, is a natural laboratory for investigating sediments source-to-sink influenced by atmosphere–ocean–land–sea ice interactions. However, despite their paleoenvironmental significance, changes in sediment provenance within the Okhotsk Sea are still debated due to the diversity of sediment sources and transport processes. Here, we investigate Sr and Nd isotope compositions in surface sediment samples from regions across the Okhotsk Sea, as well as Sr and Nd isotope compositions and elemental abundances in the sediments of core LV55-40-1, collected from the southwestern Okhotsk Sea. Our results reveal an increasing trend of εNd values over the past ~110 kyr, with an abrupt increase during the transition into the last deglacial/early Holocene (MIS 1). We also observed minor variations characterized by less radiogenic εNd during the glacial/stadial (G/S) periods (MIS 5d, MIS 5b, MIS 4, MIS 2), and more radiogenic εNd during the interglacial/interstadial (I/I) periods (MIS 5c, MIS 5a, MIS 3). The major terrigenous sediment sources in the northwestern Okhotsk Sea are via the Amur River and from Sakhalin Island (~60–80%), with a minor volcanic contribution (~20–40%) from the Okhotsk-Chukotka volcanic belt and the southern Kuril Islands/Hokkaido. Provenance variations are primarily controlled by the interplay of sea-ice drifting and ocean circulation, reflecting a dominant transport mode shift from sea ice to surface current dominance since the early Holocene. During the G/S periods with severe sea-ice condition, terrigenous materials from the Amur River and Sakhalin Island were transported to the study site, due to the southeastward drift of sea ice driven by the prevailing northwesterly winds and enhanced Okhotsk Sea Intermediate Water circulation. In contrast, during the I/I periods with moderate sea-ice conditions, an increased contribution from the Okhotsk-Chukotka volcanic belt was caused by sea-ice drifting southwestwards under the prevailing northerly and northeasterly winds, supplemented by surface currents. This study provides the first Sr-Nd isotopes record spanning the last glacial cycle in the Okhotsk Sea and it offers new insights into the relationship between sediment provenance and the interplay of sea ice and ocean currents in this region.
... Weathering of silicate rocks is an important feedback for consuming atmospheric CO 2 and regulating the Earth's temperature over geologic timescales (Raymo and Ruddiman, 1992;Kump et al., 2000;Penman et al., 2020). Volcanic rock weathers at the highest rates among silicates (Louvat and Allègre, 1997;Dessert et al., 2001;Rad et al., 2007;Louvat et al., 2008). Although it encompasses only 5% of continental surface area, it consumes 30% to 40% of atmospheric CO 2 globally (Gaillardet et al., 1999). ...
Article
Dendritic network models have proven valuable in predicting variation in strontium concentrations [Sr] and isotope ratios (87Sr/86Sr) across large river systems. These models also support inference about sources and processes controlling Sr fluxes from such systems, although this capacity has not been exploited in previous work. Here, we apply these methods to model [Sr] and 87Sr/86Sr values of a large river draining accreted volcanic arc terranes of the northern Cordillera of North America. The [Sr] and 87Sr/86Sr river models have excellent fit to measured river waters with RMSE = 0.05: r2 = 0.67 and RMSE = 0.0004: r2 = 0.87, respectively. The results strongly indicate that mixed siliciclastic sedimentary units dominate the Sr weathering flux within these systems. Extrapolation using global arc sediment distributions suggests arc sedimentray units could source ∼30% of the modern riverine Sr flux, constituting a major source of juvenile Sr to the ocean. Because the weathering of arc sediments can lag arc growth and may persist over 100's of millions of years, we suggest that the preservation and accretion of mixed siliciclastic units formed in these settings should be considered when scaling arc weathering contributions through geologic time. Contributions from such sediments could amplify or smooth the arc-derived rock weathering and 87Sr/86Sr flux over long timescales, altering interpretations of biogeochemical cycles and the silicate weathering feedback.
... The Cretaceous is a classical example of a greenhouse climate [16] and serves as a perfect guide to modern and future global warming [28,110]. The closure of the Cretaceous coincides with the KT (Cretaceous-Tertiary) mass extinction and large-scale volcanic activity, which have explicitly affected Earth's environment in many ways [19,81]. Therefore, the interest of researchers in particular and the public in general has been ever increasing to decode the paleoenvironmental conditions prevalent during the Cretaceous period. ...
Article
The Maastrichtian age dinosaur-bearing Lameta Formation in Central India is a pedogenically modified sedimentary succession. This succession records various pedogenic signatures such as the obliteration of bedding, root traces, burrows, soil structures, and soil horizons, indicating distinct paleosol types, which provide information about the associated paleoclimatic and paleoenvironmental conditions. In the present study, combining soil morphology and micromorphology, we recognized, classified, and interpreted eleven paleosol profiles preserved in the Lower Limestone and Mottled Nodular Bed Unit of the Lameta Formation, exposed in the Jabalpur subregion in Central India. The studied paleosols were divided into five different pedotypes (soil types) based on the majority/stage of development of pedogenic features. The observed pedotypes are compound and composite paleosol profiles, composed of entisols, hydromorphic calcic-inceptisols, gleyed alfisols, and well-developed alfisols. This variation in observed pedotypes represent the characteristics of a soil catena. In this soil catena, the variation in paleosol profiles is related to the variation in hydrology and relief of the landscape. The better-drained paleosols are forming on higher grounds, whereas poorly drained paleosols are forming in lowlands. The identified pedotypes with varying drainage and maturity are used to reconstruct the paleolandscape as an agglomeration of seasonal wetlands and dry lands. Further, based on several pedogenic features, the Lameta sediments are interpreted to have been deposited in tropical warm, humid climatic conditions.
... The pH related weakly to salinity (up to a salinity of ~30 × 10 − 3 ) in the SE and NW estuaries, which may be due to the smaller size of the drainage basins of the rivers and similarity in the lithological and weathering factors. The Narmada (NW) which is long (1312 km), drains a variety of basins e.g., the Deccan basalts, Vindhyan carbonates, sediments, and alluvial deposits (Sharma and Subramanian, 2008;Dessert et al., 2001) affecting the pH. Modifications also occur along the river system's long course e.g., by a number of hydrothermal springs which have been reported along the Narmada-Tapti lineament (Minissale et al., 2000), besides the human-caused factors (Qasim et al., 2022). ...
Article
This study investigates the coupling between Chromophoric Dissolved Organic Matter (CDOM) and Dissolved Inorganic Carbon (DIC) in eighteen Indian estuaries across salinity gradient of the east and west coasts during the monsoon season, characterized by significant river discharge. The hypothesis that humic acids (HA) and fulvic acids (FA), prominent in estuarine CDOM, closely correspond to the ‘organic alkalinity’ (Aorg) component of total alkalinity is examined. In most estuaries, specifically those along the northeast coast (NE) and southwest coast (SW), a significant linear relationship exists between DIC, CDOM abundance, and pH level. Notably, minor estuaries along the southeast coast (SE) and northwest coast (NW) exhibit elevated DIC levels beyond what this relationship predicts. These estuaries also reveal heightened ammonium levels, increased δ15N values, and decreased δ13C values, indicative of anthropogenic influence. CDOM properties, such as spectral slope (S300–500) and spectral slope ratio (SR, S275–295:S350–400), align with these findings, with SE and NW estuaries displaying higher values. On average, CDOM contributes 110.5 μM (6.8 %) to DIC in NE, 390.7 μM (11 %) in SE, 24.4 μM (4.8 %) in SW, and 122.2 μM (4 %) in NW estuaries. The relationship between total alkalinity minus DIC (TADIC) and pH25 suggests that CDOM, mediated by HA/FA, buffers the inorganic carbon system in estuaries. This buffering capacity weakens at elevated DIC levels, and this condition is marked by anomalous SR values compared to the baseline salinity-SR linear regression. This Study suggests that estuarine CDOM could largely represent “organic alkalinity” and could help monitor acidification in estuaries.
... Parental rocks, climate, biosphere, relief and time play important roles during chemical weathering processes [60,61]. The temperature and runoff are the main parameters controlling the chemical weathering of igneous and metamorphic rocks, but other factors must be considered to explain the water/rock interactions, such as the variations in landforms and landscapes [64][65][66][67][68][69]. Both study areas have the same geological, climatic and geomorphological settings, as well as savannah vegetation. ...
Article
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In equatorial and tropical regions, supergene mineral deposits created during water/rock interactions are found. Simply put, these supergene deposits are formed through the accumulation of low solubility ions or through the preservation of primary minerals. The supergene manganese (Mn) deposits are examples of the economic importance associated with the chemical weathering processes. In Brazil, the Southern Brasilia Orogen (SBO) was generated during the collision between the Paranapanema Craton and the passive margin of the São Francisco Craton. In the southern Minas Gerais (MG), several supergene Mn occurrences are hosted in the SBO, which were originated during the chemical weathering of gondites belonging to the Amparo Complex. Here, we studied the supergene Mn occurrences in the southern MG, more specifically in the municipalities of Ouro Fino and Careaçu. The MnO contents ranged from 25.50 to 28.40 wt% at Ouro Fino and from 16.80 to 21.20 wt% at Careaçu. These supergene Mn deposits have a diverse mineral assemblage, being composed of spessartine, quartz, Mn-oxides, goethite and kaolinite. The various Mn minerals formed due to spessartine incongruent dissolution were hollandites, cryptomelanes, romanechites, pyrolusites and lithiophorites. Both study areas are relevant for the possible opening of mines for the commercialization of Mn.
... Basalt weathering is a critical factor for regulating atmospheric CO 2 levels on Earth over the long-term carbon cycle, as (ultra-)mafic minerals have a high propensity for weathering and enable net CO 2 fixation (Amiotte Suchet et al. 2003;Dessert et al. 2001Dessert et al. , 2003Li et al. 2022;Taylor and Lasaga 1999;Xiong et al. 2022). Nevertheless, carbonate weathering is also emphasized in basalt weathering (Georg et al. 2007;Jacobson et al. 2015;Nelson et al. 2022). ...
Article
The (ultra-)mafic mine tailings pond revealed a weathering discrepancy in the tailings profile, which provided a valuable analog to assess the role of carbonate and silicate weathering of the basalt. In this study, drill-cores samples were selected from the Wanniangou V–Ti–Fe mine tailings pond (Sichuan province, China) to investigate the mineralogicand geochemical characteristics in the tailings profile. The results reveal (1) the tailings pond profile consist of upper and lower layers. The upper layer is composed of carbonate weathering (1.4 %), which was formed in the initial stages of tailings exposure and represented a minimal weathering degree. (2) The lower layer was primarily observed at the aquifer zone of the tailings pond, and was consistent with 0.45 % carbonate weathering and 48.4 % silicate weathering. (3) The weathering discrepancy in the tailings profile could be due to the sulfide oxidation and aerobic/flowing aquifer, which facilitate the water-tailings reactions. The tailings profile provides an analog to studying basalt weathering, as it spans both carbonate and silicate weathering. This research reinforces the idea that silicate weathering is predominant in basaltic areas and plays a crucial role in regulating atmospheric CO2 (carbon dioxide) levels on Earth.
... However, the challenge remains the complex chemistry of these resources due to the water-rock interaction, ion-ion interaction, and a wide range of chemical transport mechanisms occurring in their genesis. Understanding ionic interactions in multi-component electrolytes is therefore essential to understanding the chemistry of geothermal brines and seawater with respect to lithium extraction [56][57][58]. Therefore, the purpose of this review is to examine existing relevant literature in the subject areas of sorption [59][60][61], solar/vacuum evaporation [62,63], electrodialysis [64][65][66], precipitation [67][68][69][70][71], and solvent extraction [72,73], which will serve as a foundational framework for future lithium extraction from aqueous sources such as seawater and geothermal brines. ...
Article
The increasing adoption of lithium in clean energy technologies has promoted significant development of novel and environmentally sustainable techniques for lithium extraction from secondary sources. In this review, we evaluate seawater and geothermal brines as potential secondary lithium resources for supplementing the rising demand. The review examines relevant literature to understand key aspects pertaining to lithium extraction from these systems in which the fundamental chemistry, the efficacy of different potential extraction techniques, and the associated impacts of each technique are critically reviewed. The extensive research in conventional closed basin brines is utilized as a baseline to demonstrate the current research progress, providing guidelines for future research direction in lithium extraction from seawater and geothermal brines. Based on the literature, it is suggested that sorption and ion-exchange will have high potential for prospective lithium extraction from aqueous resources like salars, seawater, and geothermal brines, and that the integration of activated carbon materials or microorganisms with these techniques will enhance the selectivity of lithium extraction from aqueous resources.
... Magmatic release of SO 2 can drive cooling over the time scale of eruptions (Guex et al., 2016), whereas longer-term warming (<10 5 years) results from the release of massive amounts of CO 2 (e.g. Schaller et al., 2012), and subsequent cooling (>10 5 years) can be caused by accelerated silicate weathering of fresh continental flood basalts (Dessert et al., 2001). The release of massive amounts of CO 2 is predicted to trigger a negative δ 13 C anomaly that corresponds with the initial phase of global warming, while subsequent drawdown of CO 2 due to weathering of the juvenile basalt may be detected by declines in marine 87 Sr/ 86 Sr and 187 Os/ 188 Os (Cox et al., 2016). ...
Article
The Franklin igneous event (ca. 718 Ma) emplaced basalts and mafic intrusions across Arctic Laurentia just prior to the onset of the ca. 717-661 Ma Sturtian Snowball Earth event. Given the close association in time between these two events, it has been widely argued that the Franklin event contributed to the initiation of global glaciation. However, a lack of accurate high-precision geochronology has hampered establishing the timeframe of Franklin magmatism and discerning its precise temporal and possible causative relationship to glaciation. Here we present new high-precision U-Pb CA-ID TIMS zircon ages for two dykes and two sills from Baffin Island, Canada, and Avannaata, northwestern Greenland, related to the Franklin event. Combined with field relationships showing that the dykes crosscut the sills, these four dates indicate magmatic activity over 1.10 ± 0.74 Myr at ca. 718 Ma, immediately prior to Sturtian glaciation, which we estimate to have initiated at 717.19 +0.26/−0.33 Ma. Our results are consistent with recent high precision U-Pb CA-ID TIMS age estimates for the Franklin magmatic activity in the central and western parts of the province. Combining our new dates with the other high precision CA-ID-TIMS ages yields a duration for the Franklin event of 2.14 ± 0.75 Myr which is shorter than the previous estimates but somewhat longer than that for typical Phanerozoic large igneous events.
... Many studies have shown the carbonate and silicate dominance in the river water chemistry in the Canadian Shield including the Bow River, southern Alberta (Grasby & Hutcheon 2000;Millot et al. 2002) and some depict the runoff as the controlling agent (Tipper et al. 2006). In some cases, temperature replaces runoff as a controlling factor (Dessert et al. 2001;Dalai et al. 2002). Erosion and rock-water interaction are also acting as major contributors (Oliva et al. 2003;Hagedorn & Cartwright 2009). ...
Article
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Hydro-geochemical characteristics of Kodku River of Lalitpur, Nepal were determined through analysis of physicochemical parameters including major anions and cations. This study also characterizes and evaluates the suitability for drinking and irrigation. The results showed that river water is slightly alkaline toward the downstream. The concentration of BOD, COD, and total coliform was much higher than its prescribed limits for drinking purposes in downstream, whereas a few samples in upstream are fit for drinking with reference to WQI value. The estimated sodium percentage (Na%), sodium adsorption ratio (SAR), permeability index (PI), magnesium adsorption ratio (MAR), and Kelly's ratio (KR) depict the excellent and safe categories for irrigation purposes. The obtained Wilcox diagram also revealed that river water falls under the C1S1 category which is safe for crops. Hydrogeochemical facies in Piper diagrams signifies the dominance of Ca–HCO3 type. Similarly, Gibbs maps indicate mineral (rock) weathering, whereas mixing diagram reveals carbonate weathering as the main controlling factor of major ion species. This study provides information on the hydrogeochemical properties of the river and its suitability for drinking and irrigation. Despite the comprehensiveness, the study can be an effective approach for countries with similar levels of water resources management. HIGHLIGHTS Hydro-geochemical analysis revealed the carbonate weathering dominates in Kodku River.; Concentration of BOD, COD, total coliform and WQI value reflect few samples suitable for drinking.; Na%, SAR, PI, MAR, KR and EC value were shown to be good for irrigation purposes and good for crops.;
... According to a recent study [9], the active and diffuse carbon outflux at arcs above subducted slabs (23 MtC a -1 ) is comparable to that at mid-ocean ridges (16 MtC a -1 ) and intraplate volcanoes (40 MtC a -1 ). Though intraplate volcanoes driven by mantle plumes have a high CO 2 flux, the relatively short emplacement time of large igneous provinces (LIPs) makes this unlikely to be the decisive factor for long-term climate variations during the Cenozoic [2], and associated spikes of increasing atmospheric CO 2 concentration are consumed via the chemical weathering feedback on a time scale of less than 1 Ma [51]. Self et al. [52] estimated the CO 2 flux released during the emplacement of the Deccan province ($65 Ma) and concluded that the individual flood basalt eruptions event had very little effect on global warming. ...
Article
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Volcanic arc degassing contributes significantly to atmospheric CO2 levels and therefore has a pivotal impact on paleoclimate changes. The Neo-Tethyan decarbonation subduction is thought to have played a major role in Cenozoic climate changes, although there are still no quantifiable restrictions. Here we build past subduction scenarios using an improved seismic tomography reconstruction method and calculate the subducted slab flux in the India-Eurasia collision region. We find remarkable synchronicity between calculated slab flux and paleoclimate parameters in the Cenozoic, indicating a causal link between these processes. The closure of the Neo-Tethyan intra-oceanic subduction resulted in more carbon-rich sediments subducting along the Eurasia margin, as well as continental arc volcanoes, which further triggered global warming up to the Early Eocene Climatic Optimum. The abrupt termination of the Neo-Tethyan subduction due to the India-Eurasia collision could be the primary tectonic cause of the ∼50-40 Ma CO2 drop. The gradual decrease in atmospheric CO2 concentration after 40 Ma may be attributed to enhance continental weathering due to the growth of the Tibetan Plateau. Our results contribute to a better understanding of the dynamic implications of Neo-Tethyan Ocean evolution and may provide new constraints for future carbon cycle models.
Article
Western Yunnan is an important area for Mesozoic marine strata, development of which helps to reconstruct paleoenvironments in the eastern Tethys. To bring knowledge of eastern Tethys up to that of western Tethys, this study focuses on its sedimentary characteristics and paleoenvironmental significance concentrating on the Xiushan section of the Simao Basin with analyses of the petrology, element geochemistry, and C‐O‐Sr isotopes. Samples are micrite, bioclastic limestone, marl, and mixed calcareous clastic rocks with the ⁸⁷ Sr/ ⁸⁶ Sr values of limestone ranging from 0.708255 to 0.708933; the δ ¹³ C values of the limestone range from –5‰ to +1.5‰ and the δ ¹⁸ O values range from –13.7‰ to –9.1‰. Based on the results, it is concluded that the Middle Jurassic limestones in the Simao Basin were deposited in a tidal flat environment with a strong influence of terrigenous input. At least one transgression event occurred during the depositional period. The paleoclimate as a whole changed from dry hot to humid hot and back to dry hot again. These enriched and improved results provide further data support for a comparison of the paleoenvironments between the eastern and western Tethys.
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Clean river water is an essential and life-sustaining asset for all living organisms. The upper Ganga and Yamuna river system has shown signs of rejuvenation and tremendous improvement in the water quality following the nationwide lockdown due to the coronavirus pandemic. All the industrial and commercial activity was shut down, and there was negligible wastewater discharge from the industries. This article addresses the water quality assessment from the study area, which is based on the original data of physical parameters, major and trace elements, and stable isotopes (hydrogen and oxygen) systematics during the nationwide lockdown. The impact of the lockdown could be seen in terms of an increase in dissolved oxygen (DO). Water samples were collected from the Upper Ganga and Yamuna river basins (Alaknanda, Bhagirathi, and Tons rivers) during an eight-week lockdown in Uttarakhand, India. We discussed the signs of rejuvenation of riverine based on physical parameters, major ions, trace elements, isotopic ratios, and water pollution index (WPI). Results reveal that the water quality of the entire upper basins of the Ganga has significantly improved by 93%, reflecting the signs of self-rejuvenation of the rivers. Multivariate analysis suggests a negative factor loading for an anthropogenic element (NO3-NO3{NO}_{3}^{-}), implying that they contribute little to the river water during the lockdown. Further, bicarbonate (HCO3-HCO3{HCO}_{3}^{-}) is a dominant element in both river basins. The geochemical facies are mainly characterized by the (Ca2+:Mg2+:HCO3-Ca2+:Mg2+:HCO3{{Ca}^{2+} :{Mg}^{2+} : HCO}_{3}^{-}) type of water, suggesting that silicate rock weathering dominates with little influence from carbonate weathering in the area. The positive factor loadings of some cations, likeHCO3-HCO3{HCO}_{3}^{-},Ca2+Ca2+{Ca}^{2+}, and Mg2+Mg2+{Mg}^{2+} reflect their strong association with the source of origin in the lockdown phases. Stable isotopic reveals that the glaciated region contributed the most to the river basin, as evidenced by the low d-excess in riverine water compared to anthropogenic contributions. Rivers can self-rejuvenate if issues of human influence and anthropogenic activities are adequately resolved and underline our responsibility for purifying the ecosystem. We observed that this improvement in the river water quality will take a shorter time, and quality will deteriorate again when commercial and industrial activity resumes.
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The Indian Coastal Current is the only channel for material exchange between the two largest marginal seas in the northern Indian Ocean: the Bay of Bengal and the Arabian Sea. However, its past history is poorly known, limiting accurate predictions of its future changes. Here, we present a new clay mineral record from south of India supported by interpretations of model simulations to trace its variability over the last 18 000 years. Decreased smectite/(illite + chlorite) ratios during the cold intervals suggest that a stronger northeasterly wind led to a mean southward flow of the Indian Coastal Current in the Bay of Bengal. In contrast, increased smectite/(illite + chlorite) ratios during the warm intervals suggest the opposite scenario. Combining the proxy record with model simulations, we infer that atmospheric circulation changes were the main driver of the changes. Moreover, a possible link is observed between a positive Indian Ocean Dipole (IOD) and weakened southward flow of the Indian Coastal Current in the Bay of Bengal during the Holocene. These findings imply that future warming scenarios, if associated with more intense positive IOD events as proposed, may lead to a reduction in fresh water transport from the Bay of Bengal to the Arabian Sea.
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Anthropogenic influences significantly modify the hydrochemical properties and material flow in riverine ecosystems across Asia, potentially accounting for 40-50% of global emissions. Despite the pervasive impact on Asian rivers, there is a paucity of studies investigating their correlation with carbon dioxide (CO2) emissions. In this study, we computed the partial pressure of CO2 (pCO2) using the carbonate equilibria-based model (pCO2SYS) and examined its correlation with hydrochemical parameters from historical records at 91 stations spanning 2013-2021 in the Ganga River. The investigation unveiled substantial spatial heterogeneity in the pCO2 across the Ganga River. The pCO2 concentration varied from 1321.76 μatm, 1130.98 μatm, and 1174.33 μatm in the upper, middle, and lower stretch, respectively, with a mean of 1185.29 μatm. Interestingly, the upper stretch exhibited elevated mean pCO2 and FCO2 levels (fugacity of CO2: 3.63 gm2d-1) compared to the middle and lower stretch, underscoring the intricate interplay between hydrochemistry and CO2 dynamics. In the context of pCO2 fluctuations, nitrate concentrations in the upper segment and levels of biological oxygen demand (BOD) and dissolved oxygen (DO) in the middle and lower segments are emerging as crucial explanatory factors. Furthermore, regression tree (RT) and importance analyses pinpointed biochemical oxygen demand (BOD) as the paramount factor influencing pCO2 variations across the Ganga River (n=91). A robust negative correlation between BOD and FCO2 was also observed. The distinct longitudinal patterns of both parameters may induce a negative correlation between BOD and pCO2. Therefore, comprehensive studies are necessitated to decipher the underlying mechanisms governing this relationship. The present insights are instrumental in comprehending the potential of CO2 emissions in the Ganga River and facilitating riverine restoration and management. Our findings underscore the significance of incorporating South Asian rivers in the evaluation of the global carbon budget.
Chapter
The exploration of the universe, of the earth's interior and of the oceans are three great endeavors of humanity challenging nature in order to improve living conditions for humans. Scientific drilling is a significant exploration tool in current earth science research. We can directly observe the lithosphere which we know little about and obtain the rock record from its deeper parts through scientific drilling programs. Drilling provides important information about natural resources, disaster prediction and a variety of environmental problems currently facing modern human society. The Deep Sea Drilling Project (DSDP) in 1968, followed by the Ocean Drilling Program (ODP) and the Integrated Ocean Drilling Program (IODP), led early to a major scientific breakthrough and geosciences revolution which has changed our views of the Earth.
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River Mahi drains through semi-arid regions (Western India) and is a major Arabian Sea draining river. As the principal surface water source, its water quality is important to the regional population. Therefore, the river water was sampled extensively (n = 64, 16 locations, 4 seasons and 2 years) and analyzed for 11 trace elements (TEs; Sr, V, Cu, Ni, Zn, Cd, Ba, Cr, Mn, Fe, and Co). Machine learning (ML) and multivariate statistical analysis (MVSA) were applied to investigate their possible sources, spatial–temporal-annual variations, evaluate multiple water quality parameters [heavy metal pollution index (HPI), heavy metal evaluation index (HEI)], and health indices [hazard quotient (HQ), and hazard index (THI)] associated with TEs. TE levels were higher than their corresponding world average values in 100% (Sr, V and Zn), 78%(Cu), 41%(Ni), 27%(Cr), 9%(Cd), 8%(Ba), 8%(Co), 6%(Fe), and 0%(Mn), of the samples. Three principal components (PCs) accounted for 74.5% of the TE variance: PC-1 (Fe, Co, Mn and Cu) and PC-2 (Sr and Ba) are contributed from geogenic sources, while PC-3 (Cr, Ni and Zn) are derived from geogenic and anthropogenic sources. HPI, HEI, HQ and THI all indicate that water quality is good for domestic purposes and poses little hazard. ML identified Random forest as the most suitable model for predicting HEI class (accuracy: 92%, recall: 92% and precision: 94%). Even with a limited dataset, the study underscores the potential application of ML to predictive classification modeling.
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Potassium isotopes are a novel tracer for continental weathering. Previous K isotope studies on chemical weathering generally targeted weathering profiles under a particular climate region, yet the effects of chemical weathering on K isotopes under different climatic backgrounds remain unclear. Moreover, little is known about the K isotope signatures of modern unconsolidated detrital sediments. Here, we report K isotopic data of surficial seafloor sediments from continental shelves along the east coast of China (ECC), as well as those around the tropical Hainan island in the northern South China Sea. The ECC sediments have a relatively narrow distribution of δ41K (with reference to NIST3141a) values, which range from (−0.40±0.01)‰ to (−0.57±0.04)‰, with an average of (−0.51±0.09)‰. By contrast, δ41K values of Hainan offshore sediments display a larger variation, ranging from (−0.28±0.07)‰ to (−0.67±0.02)‰. The δ41K values of Hainan offshore sediments exhibit negative correlations with the chemical index of alteration (CIA), Al/K, Ti/K, and total iron (FeT), which underlines the control of chemical weathering on K isotopic signatures of detritus inputs into oceans. We also measured Mg isotope compositions for the same samples; interestingly, the variability in δ26Mg of the samples is small (~0.24‰) for all ECC and Hainan offshore sediments, and δ26Mg values do not show clear correlations with indexes of chemical weathering. Our study demonstrates the link between K isotopic variability of detrital sediments and climatic conditions including rainfall intensity, which indicates that K isotopes of the detrital component of marine sediments could be applied to study Earth’s climate in deep time. The δ41K values of the offshore detrital sediments are significantly less variable than those of pelagic marine sediments, highlighting the importance of distinguishing the effects of diagenesis and neoformation of clay minerals from continental weathering in attempts to study deep-time climate-weathering link by K isotopes in detrital sedimentary records.
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Characteristics of large-scale landforms have been widely studied due to their importance for understanding the interaction between tectonics and climate changes. However, increasing studies suggest that localized landforms, previously poorly focused, is more helpful for revealing mechanisms of geologic hazards and can provide insights for making mitigation schemes. This paper used the linear trough at three sites developed on the southern Daliangshan, southeastern Tibetan Plateau as an example, to provide implications for choosing a safer construction site in topographic relief region. Based on field investigations, geomorphic analysis, and numerical simulation, we suggest that characteristics of gullies have been affected by the linear trough. The three gullies above the trough show a normal sequence in hillslope angles, which is consistent with the width of the corresponding outlets at the linear trough. Conversely, the part below the trough along the gullies show a reversed sequence in hillslope angles for the three sites. The aforementioned characteristics are further evidenced by the numerical simulation by PFC3D. Therefore, we suggest that linear troughs can be considered natural barriers that hinder transportation of the gullies, which might provide some new insights for gully-related debris mitigation in mountainous areas.
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The flux of atmospheric/soil CO 2 consumed by chemical weathering of the continents ( F CO2 ) can be estimated from river fluxes of bicarbonates. Using published data for 232 small monolithologic watersheds, empirical relationships between F CO2 and runoff ( Q ) have been determined for the major rock types outcropping on the continents. For validation, the models fitted to these relationships are applied to the Garonne, Congo and Amazon river basins to calculate the average CO 2 consumption on these large river basins; the model results are close to previous estimates based on field measurements. The model (GEM-CO 2 : Global Erosion Model for CO 2 fluxes) is then applied at the global scale and allows the determination of latitudinal variations of the consumption of atmospheric/soil CO 2 by chemical erosion. The main results show that the consumption of CO 2 is mainly localized in the Northern hemisphere, because of large continental area with a high proportion of carbonate rocks, and in equatorial regions which are very humid. Finally, the mean annual CO 2 consumption for the whole continents amount to 0.26 Gt C y -1 , and 72% of this flux is removed in the Northern hemisphere. DOI: 10.1034/j.1600-0889.47.issue1.23.x
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Compared to the global average, the chemical weathering rates of basalt in southwest Iceland are high and rather variable. This can be attributed to soluble rock type (basalt) and mechanical weathering, variation in runoff and age of rocks, and variable vegetative/ glacial cover. The average temperature of the catchments in this study is near constant, 5°C. Chemical weathering of the basalt is incongruent. Some of the primary minerals do not dissolve, and secondary minerals form, resulting in the fact that fluxes of all elements increase with runoff, and there is an enormous variation in the relative mobility of elements in the basalt during weathering. The relative mobility, in decreasing order, is: S > F > Na > K ≫ Ca > Si > Mg > P > Sr ⋙ Mn > Al > Ti > Fe. Relative to Na, close to 90 percent of Mg and Ca in the original rocks is left behind at the weathering site. The runoff dependence of fluxes and the variation in relative mobility is less in old rocks than in young ones. In old rocks the number of saturated minerals with respect to soil solutions has decreased because of lesser amount of soluble basaltic glass and an increased vegetative cover on old rocks. The saturation state of basaltic minerals is the most important variable for the dissolution and precipitation rate of minerals during weathering in southwest Iceland and is dictated by the pH of the weathering solutions. The overall rate of chemical denudation rate in southwest Iceland is independent of vegetative cover. However, fluxes of Ca, Mg, and Sr increase with increasing vegetative cover at constant runoff, whereas fluxes of Na and K decrease. With a continuous vegetative cover the pH of the soil solutions tends to be low (<7), and glass, olivine, pyroxene, and plagioclase are unstable, but the solutions are decreasingly saturated or more undersaturated with respect to zeolites and smectite, thus increasing the relative mobility and fluxes of Ca, Mg, and Sr. Since the weathering of Ca-Mg silicate rocks is the principal process by which CO2 is removed from the atmosphere on a geological time scale (Berner, 1992), the spread of vascular plants on the continents during the mid-Paleozoic may have resulted in a drop in CO2, not necessarily because of greatly enhanced bulk chemical weathering, as suggested by Trendall (1966) and Berner (1993), but rather due to the enhanced relative mobility and fluxes of Ca and Mg.
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Thirty-six different geochemical and foraminiferal analyses were conducted on samples collected at closely spaced intervals across the Cretaceous/Tertiary (K/T) boundary exposed at Caravaca, Spain. A rapid reduction in the gradient between delta13C values in fine fraction carbonate and benthic foraminiferal calcite and a decrease in the abundance of phosphorus (a proxy for organic carbon) and calcium were recorded in sediments 0-0.5 cm above the K/T boundary. These trends imply that an abrupt mass mortality occurred among pelagic organisms, leading to a significant reduction in the flux of organic carbon to the seafloor. In addition, variations in sulfur isotope ratios, the hydrocarbon-generating potential of kerogen (measured as the hydrogen index), and foraminiferal indices of dissolved oxygen level all imply that a rapid decrease in dissolved oxygen was coincident with the delta13C event. Evidence of the low oxygen event has also been recognized in Japan and New Zealand, suggesting that intermediate water oxygen minima were widely developed during earliest Danian time. A threefold increase in the kaolinite/illite ratio and a 1.20/00 decrease in delta18O (carbonate fine fraction) were recorded in the basal 0.1-2 cm of Danian age sediments. These trends suggest that atmospheric warming and an increase in surface water temperature occurred 0-3 kyr after the delta13C event. Recovery in the difference between delta13C values in the carbonate fine fraction and in benthic foraminiferal calcite as well as increases in phosphorus and calcium contents occur at the base of planktonic foraminiferal Zone Pla, implying that an increase in primary productivity commenced some 13 kyr after the K/T boundary.
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A detailed record of the strontium-87 to strontium-86 ratio in seawater during the last 100 million years was determined by measuring this ratio in 137 well-preserved and well-dated fossil foraminifera samples. Sample preservation was evaluated from scanning electron microscopy studies, measured strontium-calcium ratios, and pore water strontium isotope ratios. The evolution of the strontium isotopic ratio in seawater offers a means to evaluate long-term changes in the global strontium isotope mass balance. Results show that the marine strontium isotope composition can be used for correlating and dating well-preserved authigenic marine sediments throughout much of the Cenozoic to a precision of ±1 million years. The strontium-87 to strontium-86 ratio in seawater increased sharply across the Cretaceous/Tertiary boundary, but this feature is not readily explained as strontium input from a bolide impact on land.
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Rapid detachment between Main Central thrust and South Tibetan detachment system occurred 21-17 Ma. Enhanced weathering from this event (6 km of unroofing) increased delivery of Sr, and P, stimulating productivity in oceanic upwelling zones and expanision of OMZ thus enhancing preservation. Drawdown of CO2 associated with chemical weathering event between 20-16 Ma. Runaway CO2 drawdown is balanced by net global organic carbon cycle, which acted as a net source of CO2 over the late Cenozoic. After 16 Ma the percent fraction of organic carbon burial decreased. This could be due to lowered sea-level and increased weathering or a colder ocean and increased O2 available for oxidation. In addition cooler mid mIocene cliamte could have suppressed chemical weathering rates in Himalayas.
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Weathering and erosion can affect the long-term ocean-atmosphere budget of carbon dioxide both through the consumption of carbonic acid during silicate weathering and through changes in the weathering and burial rates of organic carbon. Recent attention has focused on increased silicate weathering of tectonically uplifted areas in the India-Asia collision zone as a possible cause for falling atmospheric CO2 levels in the Cenozoic era. The chemistry of Neogene sediments from the main locus of sedimentary deposition for Himalayan detritus, the Bengal Fan, can be used to estimate the sinks of CO2 from silicate weathering and from the weathering and burial of organic carbon resulting from Himalayan uplift. Here we show that Neogene CO2 consumption from the net burial of organic carbon during Himalayan sediment deposition was 2-3 times that resulting from the weathering of Himalayan silicates. Thus the dominant effect of Neogene Himalayan erosion on the carbon cycle is an increase in the amount of organic carbon in the sedimentary reservoir, not an increase in silicate weathering fluxes.
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Many of the world's flood basalt provinces form elevated plateaux at the margins of continents, although in most cases their present large elevation is not the result of mountain building processes. Several explanations have recently been put forward to explain such occurrences of epeirogeny. The Deccan Trap basalt province forms one such elevated plateau, and results are presented here showing how the epeirogenic uplift in this region, combined with crustal subsidence probably associated with the rifting of the Indian continental margin, has affected the structure of the basalt sequence. Trace element analytical data are used for samples from numerous vertical sections through the Deccan Traps lava series along and around the Western Ghats ridge in India. The results reinforce the previously defined stratigraphy of the Mahabaleshwar area, and extend it over a region covering some 36 000 km2, reaching as far south as Belgaum and the Trap/basement contact. These results show that the lava pile is not flat lying, but forms a very low amplitude anticlinal fold structure plunging southwards by up to 0.3 over most of the area, although in the south there is evidence of a reversal of this plunge. The fold is interpreted as being the result of two tilting processes: (1) westward tilting near the coast, due to the foundering of the passive continental margin, and (2) epeirogenic uplift along the whole west coast of India producing the observed topography and the peninsula-wide drainage patterns, and also the easterly component of dip. Variations in the magnitude of the latter effect along the western continental margin may also be important in generating the plunge of the fold, although the possibility of some component of depositional dip may also be important. This latter possibility can be modelled using a simple computer program. The results of this modelling show that a migrating linear volcanic edifice fits the observations best.
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Seawater 87Sr/86Sr values increase abruptly by 28 × 10−6 across the Cretaceous/Tertiary boundary (KTB). This small, but rapid shift is superimposed on the larger scale structure of the seawater Sr isotope curve. The time scale of radiogenic Sr addition appears to be too rapid to reconcile with sources associated with volcanism, and we show that the amount of Sr required to produce even this small increase is too large to be derived from: (1) a KT bolide of the size constrained by the Ir anomaly, (2) continental crust ejecta from the impact of such a bolide, (3) soot from global wildfires initiated by an impact, or (4) any combination of these sources. The probable source of the radiogenic Sr is enhanced continental weathering, but the high rate of increase appears to rule out processes such as sea level regression, glaciation or tectonism. A plausible mechanism for rapid addition of radiogenic Sr to the oceans is enhanced weathering associated with globally distributed acid rain (pH ∼ 1) which is a proposed by-product of a bolide impact [51, EPSL Vol. 83].
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La composition chimique naturelle des eaux (pH, cond., Ca⁺⁺, Mg⁺⁺,K⁺, Sr⁺⁺, NH₄⁺, CI, SO₄⁻⁻,HCO₃⁻,NO₃⁻, PO₄⁻³, F⁻, B, Corg) est étudiée sur 232 bassins versants monolithologiques représentant 25 types de roches, des granites aux évaporites. Chaque bassin a été choisi sans aucune source de pollution directe et loin de rejets atmosphériques. Le bassin moyen est forestier, il a 8 km2, un débit de 8 l.s'¹.km2, une altitude moyenne de 650 m et a été échantillonné 1,4 fois. Les aérosols océaniques influencent la plupart des éléments, sauf les nutriments (Corg, N, P) et la silice, sur une bande côtière de 200 km, notamment dans les Landes et en Bretagne. En dehors de ces régions, chaque analyse a été corrigée des apports atmosphériques (Meybeck, 1983), pour examiner les liaisons entre lithologie et composition des eaux. La lithologie est responsable de variations très marquées des concentrations, de 1 à 10 pour SiO₂ et K⁺, et de 1 à 1000 pour NA⁺, Cl⁻, SO₄⁻⁻, et la somme des cations Σ⁺( de 70 à 70 000μeq/I). Pour chaque roche, les eaux superficielles sont caractérisées par SiO₂, Σ⁺, les proportions de cations, certains rapports (Ca⁺⁺/Mg⁺⁺). Trois origines principales sont définies : (1 ) l'altération des silicates des roches cristallines et sédimentaires correspond à un «bruit de fond » en Ca⁺⁺, Mg⁺⁺, Na⁺, K⁺, Sr⁺, F, В ; ces eaux sont caractérisées par 100 <Σ⁺+ < 800 μeq, 0,35 < SiO₂, < 1, Ca⁺⁺/Na⁺< 5 ; (2) l'altération des carbonates (Ca⁺⁺, Mg⁺⁺, HCO₃⁻ ; 2000 < Σ⁺ < 5000 μeq/I/1 ; Ca⁺⁺/Na⁺ > 10 ; SiO₂/Σ⁺+< 0,1) ; (3) la dissolution des évaporites gypseuses (Ca⁺⁺, Mg⁺⁺, Sr⁺⁺, SO₄₋₋, B, F ; Σ⁺ — 10 000 μeq et salées (Na⁺, Cl⁻ ; Σ⁺ > 20 000 μeq/I). Les sulfates proviennent également de l'oxydation des minéraux sulfurés (schistes), В et Cl⁻ des argiles marines, SiO₂ de la silice amorphe (craie). L'altération des silicates, appréciée par SiO₂ , décroît avec l'altitude dans chaque catégorie d'eau. Presque toutes les combinaisons ioniques entre Ca⁺⁺, Mg⁺⁺, Na⁺ et Cl⁻, SiO₄⁻⁻ , HCO⁻₃ ont été trouvées, mais K⁺ est toujours secondaire. Les nutriments sont surtout liés aux conditions hydropédologiques. L'ordre relatif de minéralisation des eaux est : granite 1, gneiss et micaschistes 1,2, grès 1,3, basaltes-trachyandésites-rhyolites 3, gabbro 3,5, pélite 5, amphibolites et serpentinites 10, carbonates 20, gypse 60, halite 120. Rivières, Composition chimique, Apport atmosphérique, Altération, France
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The aim of this article is to characterise the present state of knowledge and outstanding problems of the Deccan. A description of the physical features, stratigraphy and age constraints on volcanism is followed by sections covering the Deccan's relationship to plate tectonics and the petrological-geochemical characteristics of the rocks and their petrogenesis. The final section discusses the relationship between the Rajmahal and Deccan traps and the possible link between the Deccan traps and Cretaceous-Tertiary extinctions. -A.W.Hall
Chapter
The Indian subcontinent is blanketed with the remnants of at least five continental flood basalt provinces ranging in age from the Middle Proterozoic to the Late Cretaceous-Early Tertiary. Not surprisingly, the youngest of these, the Deccan plateau (derived from a Sanskrit word meaning “south” or “southern”) is by far the best-preserved and most extensive, presently occupying some half a million square kilometers of western and central India and southernmost Pakistan (Fig. 1). Until quite recently comparatively little was known about the geology of the body of this vast province, save that it was comprised of thick successsions of nearly flat-lying, subaerially erupted, predominantly tholeiitic lava flows, at least some of which were traceable for distances of more than 100 km. Neither the gross features nor the details of flow stratigraphy, eruptive source locations, regional structure, or chemical and isotopic variations were understood; ages and age distributions were imperfectly known, and ideas on mantle sources and petrogenesis remained largely in the realm of speculation. Up to and throughout the 1970’s it was not unusual to see arguments in the literature that relied heavily on pioneering studies done in the 1800’s or early years of this century. A voluminous amount of work did exist on the scattered, small alkalic and acidic intrusive suites dotting the western and northern areas of the Deccan. Although lithologically distinctive in a sea of tholeiitic lava, they are volumetrically insignificant and for the most part appear to post-date the main phase of eruptive activity. Studies of these restricted occurrences therefore were unable to shed much light on the fundamental nature of the plateau itself. Only in the last several years have detailed, systematic investigations been conducted on a regional scale, utilizing a combination of modern methods.
Article
Detailed stratigraphy based on whole-rock geochemistry is presented for a 1200 m sequence of basaltic lava flows in the Western Ghats escarpment near Mahabaleshwar. Five separate sections are used to define a regional dip of approximately 0-5° to the SW. From the base upwards the following formations are described: Bushe, Lower Poladpur, Upper Poladpur, Ambenali, and Mahabaleshwar. Inter-formation boundaries, with the exception of the Upper Poladpur-Ambenali, are sharp, and are particularly well defined by breaks in Sr-isotopic composition. Two of the formation bases are marked by abnormally mafic flows- the Kamshedi picrite horizon at the base of the Upper Poladpur, and the Kelghar mafic unit at the base of the Mahabaleshwar. Major element compositions are controlled throughout largely by the degree of gabbro fractionation. Intense crustal contamination further modifies compositions in the lower part of the sequence (Bushe-Upper Poladpur) and has strong effects on trace elements and Sr-isotopes. Contamination decreases up-sequence leading to the comparatively uniform Ambenali rocks. The Mahabaleshwar Formation represents a change towards magmatism generated in an enriched mantle with many characteristics similar to those of oceanic island basalts. The geochemical discussion deals mainly with two well-developed mixing lines, one between Ambenali magmas and granitic crust, the other between ambenali magmas and the products of the postulated enriched mantle source. The detailed stratigraphic sequences strongly support the RTF (replenished, tapped, fractionated) magma chamber model of O'Hara &Mathews (1981) and the idea of periodical replenishment by picritic magmas (e. g. Huppert & Sparks, 1980b). This is believed to be the first demonstration of such processes operating on a large scale in a continental basalt province.
Article
The Congo river Basin is the second largest drainage basin in the world, after the Amazon. The materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the African continent. This paper presents the chemical composition of the different phases carried in the Congo rivers and is followed by a companion paper, devoted to the modelling of major and trace elements.The Congo river between Bangui and Brazzaville as well as its main tributaries, including a few organic-rich rivers, also called Black rivers, were sampled during the 1989 high water stage. The three main phases (suspended load, dissolved load, and bedload) were analysed for twenty-five major and trace elements.Concentrations normalized to the upper continental crust show that in each river, suspended sediments and dissolved load are chemical complements for the most soluble elements (Ca, Na, Sr, K, Ba, Rb, and U). While these elements are enriched in the dissolved loads, they are considerably depleted in the corresponding suspended sediments. This is consistent with their high mobility during weathering. Another type of complementarity is observed for Zr and Hf between suspended sediments and bedload, related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads.Compared to other rivers, absolute dissolved concentrations of Ca, Na, Sr, K, Ba, Rb, and U are remarkably low.Surprisingly, high dissolved concentrations are found in the Congo waters for other trace elements (e.g., REEs), especially in the Black rivers. On a world scale, these concentrations are among the highest measured in rivers and are shown to be pH dependent for a number of dissolved trace elements. The dissolved loads are systematically normalized to the suspended loads for each river, in order to remove the variations of the element abundances owing to source rock variations. Normalized diagrams for REEs are presented and extended to the other elements. They strongly support the argument that the apparent higher solubility of trace elements in the Congo waters is due to the presence in the dissolved load of a colloidal phase (as a result of 0.2 μm filtration). An important result is that these colloids are strongly depleted in Fe and Al with respect to the other elements.Finally, the comparison of the dissolved, suspended, and sandy transport fluxes of each element in the Congo Basin rivers shows that, although the proportions of, for example, the REEs in the dissolved loads of the majority of the Congo Basin rivers is close to 10% of the total transport flux, up to 80% of the REEs are transported by the so-called “dissolved”load in the Black rivers.
Article
Paleomagnetic data from the African plate from the Early Jurassic onward are used to produce a set of 30 paleopoles, from which an apparent polar wander path is obtained. Based on this path, other continents bordering the Indian Ocean and blocks involved in the collision of India with Eurasia are situated in an absolute paleogeographic frame fixed with respect to Africa. Paleogeographic maps have been developed for the Early Jurassic, the Jurassic-Cretaceous boundary, the Early Cretaceous, the Late Cretaceous, the Cretaceous-Tertiary boundary, the Early Eocene, the Middle Eocene, and the Oligocene-Miocene boundary. Particular attention is given to the episodic nature of the breakup of Gondwanaland and the formation of the Indian Ocean.
Article
Rivers carry the products of continental denudation either in a dissolved form (chemical erosion) or in a solid form (physical erosion). We focus in this paper on the relationship between physical erosion and chemical erosion. We establish the mass budget of the Congo Basin Rivers using chemical complementarities between river suspended sediments, sandy bedload, and dissolved load of the Congo Basin rivers reported in a previous paper (Dupré et al., 1995). A series of equations are presented, assuming that the physical and chemical erosion processes are in a steady state during one year. The total mass of river-borne material (dissolved and particulate) transported in the river over a given period of time should balance the mass of upper continental crust eroded during this time.
Article
The two major river systems on the west coast of India, Narbada and Tapti, their estuaries and the coastal Arabian sea sediments have been extensively studied for their uranium concentrations and 238 U / 238 U activity ratios. The 238 U concentrations in the aqueous phase of these river systems exhibit a strong positive correlation with the sum of the major cations, Na + K + Mg + Ca, and with the HCO 3 - ion contents. The abundance ratio of dissolved U to the sum of the major cations in these waters is similar to their ratio in typical crustal rocks. These findings lead us to conclude that 238 U is brought into the aqueous phase along with major cations and bicarbonate. The strong positive correlation between 238 U and total dissolved salts for selected rivers of the world yield an annual dissolved 238 U flux of 0.88 × 10 10 g / yr to the oceans, a value very similar to its removal rate from the oceans, 1.05 × 10 10 g / yr , estimated based on its correlation with HCO 3 - contents of rivers. In the estuaries, both 238 U and its great-grand daughter 234 U behave conservatively beyond chlorosities 0.14 g/l. These data confirm our earlier findings in other Indian estuaries. The behavior of uranium isotopes in the chlorosity zone 0.02-0.14 g/l, was studied in the Narbada estuary in some detail. The results, though not conclusive, seem to indicate a minor removal of these isotopes in this region. Reexamination of the results for the Gironde and Zaire estuaries (Martin et al ., 1978a and b) also appear to confirm the conservative behavior of U isotopes in unpolluted estuaries. It is borne out from all the available data that estuaries beyond 0.14 g/l chlorosities act neither as a sink nor as a source for uranium isotopes, the behavior in the low chlorosity zones warrants further detailed investigation. A review of the uranium isotope measurements in river waters yield a discharge weighted-average 238 U concentration of 0.22 g/l with a 234 U / 238 U activity ratio of 1.20 ± 0.06 is missing . The residence time of uranium isotopes in the oceans estimated from the 238 U concentration and the 234 U / 238 U A. R. of the rivers yield conflicting results; the material balance of uranium isotopes in the marine environment still remains a paradox. If the disparity between the results is real, then an additional 234 U flux of about 0.25 dpm/cm 2 ·10 3 yr into the oceans (about 20% of its river supply) is necessitated.
Article
A suite of basaltic rocks sampled over a vast exposure and stratigraphic thickness in the Deccan traps has been investigated for Os isotopic systematics. The results plot on a very well defined Re–Os isochron corresponding to an age of 65.6±0.3 Ma (2σ uncertainty). This age is in excellent agreement with previous K–Ar and Ar–Ar data. Os data also imply a short duration of volcanism, which should have important implications on mantle geodynamics. The 187Os/188Os initial ratio is typically chondritic: 0.12843±0.00047 (2σ) and indicates that metasomatism and crustal contamination played only a very minor role in the Re–Os budget during formation of the Deccan traps.
Article
A numerical model describing the coupled evolution of the biogeochemical cycles of carbon, sulfur, calcium, magnesium, phosphorus and strontium is developed to describe the changes in chemical weathering, atmospheric CO2, climate, and oceanic chemistry over Phanerozoic time.
Article
Using a coupled fluid flow–mass transfer model to calculate the Sr flux resulting from the weathering of the young Columbia River basalts, it can be shown that the dissolution of the reactive phases in the extensive continental flood basalt could have caused the significant inflection in the marine Sr isotope record that occurred approximately 15 million years ago. The results of our model demonstrate the important and often overlooked role that the weathering of large igneous provinces can play in geochemical cycles. Furthermore, by approaching the question of continental Sr fluxes to the ocean using a mass transfer model, we are able to establish a quantitative relationship between the marine Sr record, chemical weathering rates and atmospheric CO2 concentrations. The CO2 drawdown rates that we calculate for the weathering of the Columbia River basalt suggest that, on a several million-year timescale, the formation of large igneous provinces represents a net sink for atmospheric CO2. The removal of CO2 via the rapid dissolution of the Columbia River basalts represents an alternative explanation for the glacial period believed to have followed its formation. We also consider the potential role that basalt weathering could play in the evolution of the 87Sr/87Sr ratio of the oceans since the early Cretaceous. Rapid Sr release during basalt dissolution combined with variations in the relative exposure of young vs. old lithologies could explain changes in the 87Sr/86Sr ratio of the oceans without calling on dramatic increases in the continental Sr flux over that time period. Our results demonstrate the importance of accounting for changes in the relative exposure of different lithologies when considering the nature of global geochemical cycles in the geologic past.
Article
The knowledge of dissolution and precipitation rates of rocks and minerals is important to understand the effect of natural as well as human influences on the soil system. The combination of mass-balance calculations and water sample geochemistry allows us the computation of weathering rates for small watersheds. The basaltic area of Ribeirao Preto (Parana, Brazil) provides an opportunity to study solute acquisition by meteoric waters in basaltic terranes, to quantify the water-basalt interaction, to address the impact of biomass on the weathering and to estimate the age of the weathering processes affecting the basalts under humid tropical climate conditions. The mass-balance approach is extended to Si and trace elements as well as neoformed minerals. The computations are showing that the transformation rates calculated with a biomass factor are 1.3-5 times faster than the rates achieved for the model not taking into account the biomass. The rates of release of Si (log k, k in mol m(-2) s(-1) Si) comp
Article
In an attempt to develop the formulation for weathering in long-term carbon cycle models, equations are drawn for calculating various possibilities of a CO2-productivity feedback upon weathering. Results of an analysis of the dependence of weathering rate on T and CO2 P, the connection between CO2 P in the atmosphere and in soils, the effects of a variable terrestrial biosphere on weathering, and other effects demonstrate the capacity of the terrestrial biosphere for moderating changes in climate and CO2 P and delineate the limitations to the strength of the CO2-productivity feedback functioning through the direct effects upon soil CO2. -M.S.
Article
In the Amazon Basin, substrate lithology and erosional regime (seen in terms of transport-limited and weathering-limited denudation) exert the most fundamental control on the chemistry of surface waters within a catchment. Secondary effects,, such as the precipitation of salts within soils and in stream beds, biological uptake and release, and cyclic salt inputs, are more difficult to discern. Samples can be separated into four principal grouping based on relationships between total cation charge (TZ+) and geology. (1) Rivers with O
Article
The flux of atmospheric/soil CO2 consumed by chemical weathering of the continents (FCO2) can be estimated from river fluxes of bicarbonates. Using published data for 232 small monolithologic watersheds, empirical relationships between FCO2 and runoff (Q) have been determined for the major rock types outcropping on the continents. For validation, the models fitted to these relationships are applied to the Garonne, Congo and Amazon river basins to calculate the average CO2 consumption on these large river basins; the model results are close to previous estimates based on field measurements. The model (GEM-CO2: Global Erosion Model for CO2 fluxes) is then applied at the global scale and allows the determination of latitudinal variations of the consumption of atmospheric/soil CO2 by chemical erosion. The main results show that the consumption of CO2 is mainly localized in the Northern hemisphere, because of large continental area with a high proportion of carbonate rocks, and in equatorial regions which are very humid. Finally, the mean annual CO2 consumption for the whole continents amount to 0.26 Gt C y−1, and 72% of this flux is removed in the Northern hemisphere.
Article
0We present river chemistry data for a network of rivers draining the western and central Nepal Himalaya. Our sampling locations cover the system from the sources of rivers in Tibet to the Gangetic plain. Water samples were collected throughout the year, including the monsoon season, for rivers in Nepal and for the Ganges and Brahmaputra in Bangladesh. Rivers draining the North Himalaya are characterized by low discharge under a cold and arid climate. Main stream waters have delta(13)C(DIC) near 0 parts per thousand and high [SO42-]/([SO42-]+[HCO3-]) ratios (X-SO4) With values around 40 Eq% and high [Cl-]. Ca is the dominant cation (Ca2+/Sigma cations = 55 to 75 Eq%, after correction of sodium by chloride). Dissolved sulfate is produced in waters from the Tethian Sedimentary Series whereas chlorine is related to thermal waters. Dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution. delta(13)C(DIC) values of up to 3.9 parts per thousand show that metamorphic CO2 is an important weathering agent. Rivers of the North Himalayan basins have about 50% of their alkalinity derived from carbonate dissolution, 20% from biogenic activity and 30% from metamorphic CO2. On the south flank of the Himalaya, rivers are more depleted in C-13 and have, on average, lower X-SO4. Most rivers have delta(13)C(DIC) and X-SO4 values compatible with a simple mixing between soil CO2 and sulfate derived from sulfide oxidation. During the monsoon, discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced. X-SO4 and delta(13)C decrease while Ca/Sigma cations increases, implying enhanced dissolution of pedogenic calcite. As a whole, the G-B riverine flux of alkalinity derived from silicate weathering is around 2.7 x 10(11) mol/year, modest at the global scale. Sulphuric acid controls 20-30% of the weathering reaction in the Brahmaputra and 6 to 9% in the Ganges. Na+ and K+ balance 60 to 65% of the silicate-derived alkalinity flux, and the long term CO2 consumption by Ganges and Brahmaputra is near 6.4 x 10(10) mol/year. The flux of metamorphic CO2 converted to alkalinity via weathering reactions is ca. 1 x 10(10) mol/year. (C) 1999 Elsevier Science B.V. All rights reserved.
Article
Climatic effects on chemical weathering are evaluated by correlating variations On solute concentrations and fluxes with temperature, precipitation, runoff, and evapotranspiration (ET) for a worldwide distribution of sixty-eight watersheds underlain by granitoid rock types. Stream solute concentrations are strongly correlated with proportional ET loss, and evaporative concentration makes stream solute concentrations an inappropriate surrogate for chemical weathering. Chemical fluxes are unaffected by ET, and SiO2 and Na weathering fluxes exhibit systematic increases with precipitation, runoff, and temperature. However, warm and wet watersheds produce anomalously rapid weathering rates. A proposed model that provides an improved prediction of weathering rates over climatic extremes Os the product of linear precipitation and Arrhenius temperature functions. The resulting apparent activation energies based on SiO2 and Na fluxes are 59.4 and 62.5 kJ · mol-1, respectively. The coupling between temperature and precipitation emphasizes the importance of tropical regions On global silicate weathering fluxes, and suggests it is not representative to use continental averages for temperature and precipitation On the weathering rate functions of global carbon cycling and climatic change models.
Article
We use a high-resolution seawater Sr isotopic evolution curve for the last 100 m.y. in conjunction with modern riverine Sr flux measurements, and also geologic, tectonic and geochronological data, to make the case for a close relationship between seawater Sr isotopic composition and the India-Asia continental collision. Using a simple seawater Sr budget model we begin by showing that the Sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater Sr isotopic curve of the last 100 m.y. The flux of dissolved Sr carried by rivers originating in the Himalaya-Tibet region on the other hand is presently a significant fraction of the global Sr budget. We calculate how this riverine flux would have had to change with time in order to match the observed seawater Sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of India with Asia but then increases very significantly after collision. We note that the period of most rapid change in seawater Sr isotopic ratio, from 20 Ma to 15 Ma, is also a period of exceptionally high erosion in parts of the Himalayas and the Tibetan Plateau. As further evidence that Sr derived from the collision of India with Asia plays a major role in the Sr isotopic evolution of seawater we show that the total amount erosion of the Himalaya-Tibetan Plateau since collision, which we calculate separately, represents a total amount of Sr that is very nearly the same as the cumulative amount required by the Sr isotopic change of seawater since collision. The relationship between erosion and riverine Sr flux allows us to use the Sr isotopic evolution of seawater to reconstruct a history of erosion since collision, and we find that the erosion rate accelerates with time since collision, with the present having the largest rate.
Article
Geochemical and isotope results are presented from a new study of the most southern basalts in the Deccan Trap, India. Three chemical formations are recognised, two of which can be correlated with the established stratigraphy in Mahabaleshwar and imply a regional southerly dip of 0.06° over a distance of 250 km. In detail Sr-isotope variations within the Ambenali and Mahabaleshwar Formations can be shown to reflect three distinct end-members which provide new constraints for petrogenetic models. Pb-isotope data for selected basalts exhibit a wide range with206Pb/204Pb= 16.87–22.45, and a linear correlation on a Pb—Pb diagram. The least contaminated Ambenali basalts plot within the Pb-array, and interaction with mantle lithosphere involves a shift to less radiogenic Pb whereas contamination with crust is characterised by more radiogenic Pb. Unlike the Karoo and Parana continental flood basalt provinces only four flow units within the southern Deccan appear to contain a significant contribution from mantle lithosphere. The Mahabaleshwar and Ambenali Formation basalts exhibit a striking negative Pb—Sr isotope trend which is presently regarded as one of the features of interaction with shallow level lithospheric mantle. It further suggests that basalts from the Walvis Ridge, Kerguelen and Ninetyeast ridge all remobilised such shallow level material, and that the Deccan basalts which were not affected by crustal contamination reflect interaction between asthenospheric material similar to T-type MORB, but related to the Reunion hotspot, and continental mantle lithosphere of the Indian plate.
Article
The Western Ghats escarpment in the western coastal region of peninsular India is locally capped by plateau remnants on which a regional high-level laterite carapace is preserved. Geochemical “fingerprinting” studies show that the laterite has been developed from a protolith of Panhala Formation basalts, the youngest known formation of the Deccan sequence. The high-level laterite is in detail structurally concordant with its underlying basalts, and hence it is concluded that it represents the original top of the lava pile. A younger, low-level, laterite carapace is developed on the Konkan plain below the escarpment where it lies discordantly on basalts of the Ambenali and Poladpur formations, from which it developed in situ. The low-level laterite rises topographically northwards in response to differential uplift, probably at a maximum in the Nasik region. The main structures of the Ghats region, excluding the coastal monocline, are the Nasik dome and the Mahabaleshwar anticline. These have no effect on regional drainage patterns, and it is argued that they are largely secondary features, developing in response to post-volcanic uplift.
Article
Dissolved and suspended load river material represents the integrated products of the erosion of drainage basins. To enlarge the study of erosion processes we have determined87Sr/86Sr ratios and the Cl, Na, Mg, Ca and Sr contents for the main tributaries of the Congo River Basin, both for water and suspended sediment. We have also analyzed 30 streams draining monolithological terranes.A systematic study of precipitation has permitted the estimation of a good rain correction factor. Sr isotopic ratios have shown that the seawater input correction based on riverine Cl content is not valid in the Congo Basin because a large part of the Sr, Ca and Mg come from a terrestrial source. The conventional atmospheric input correction by reference to the marine ratios underestimates the real atmospheric input because of the crustal elements carried by rainwaters.Different erosion source parameters have been obtained for carbonates, evaporites and silicates. An inversion scheme has been developed to compute the multimixing equations and allows the quantification of the input of each main reservoir (atmosphere, carbonates, evaporites and silicates) for each tributary and each element. For Ca and Mg, rainfall and carbonate dissolution are the main inputs. For Sr, the input is mainly controlled by rains and silicate weathering.By using Sr isotopic systematics we have calculated the Sr isotopic composition of the silicate weathered crust for each of the main tributaries of the Congo Basin. We obtain uniform values for the main tributaries ranging between87Sr/86Sr= 0.7195 ± 0.001 and 0.7251 ± 0.005. These results allow the calculation of strontium model agesTSr, which differ from neodymium model agesTNd. UsingTNd, we have calculated the87Rb/86Sr of the silicate weathered crust. We obtain homogeneous values close to 0.75, which is in agreement with estimates for the average silicate crust. The discrepancy betweenTSr andTNd may be linked to the vegetation impact which fractionates Rb and Sr.
Article
Dissolved and suspended loads of the main streams on the island of Réunion have been analysed for their major and trace element contents in order to characterise both chemical and mechanical erosion products.The chemical composition of Réunion surface waters is controlled by partial dissolution of the basaltic rocks they interact with, by atmospheric input and, in some cases, by thermal spring inflow. The elemental contributions arising from these three processes have been calculated using typical concentration ratios for each endmember. The contribution from oceanic type rains is important for Cl and Na and minor for Mg, Ca, K, and Sr. Some thermal springs join their nearest rivers in Piton des Neiges calderas and influence the sulphate, Li, and B concentrations, and to a lesser extent Ca, Sr, and Rb. For all the rivers that are not affected by the thermal springs, the contribution from basalt weathering is higher than 70% of the total dissolved solids.The chemical composition of the suspended load in Réunion rivers reflects a low weathering state for basalt in each drainage basin, implying that mechanical erosion is more important than chemical erosion. Assuming a steady-state between these two complementary erosion processes, we propose a mass budget between the dissolved and suspended erosion products and the local unweathered basalt for each of the main river catchments. Given the lack of reliable long-term measurements of river suspended load at Réunion, this mass budget enables us to calculate the suspended load concentration that is required to counterbalance the concentration of total dissolved solids due to chemical weathering. The calculated suspended load concentration is much higher than that measured which implies that equilibrium between mechanical and chemical erosion was not attained at the time of sampling. However, we suggest that in such a tropical climate, most of the suspended load will be carried during cyclonic events, these short periods of time providing about 50% of the annual amount of river water. Thus, erosional steady-state must be achieved on the scale of the annual hydrologic cycle.From the total dissolved solid concentrations induced by basalt weathering, and given the annual discharge from Réunion rivers, we infer specific chemical erosion rates of 63–170 t/km2/yr and specific atmospheric CO2 consumption rates, during basalt erosion, of 1.3–4.4 × 106 mol/km2/yr. Mechanical erosion rates deduced from the calculated suspended load concentrations range from 1200 to 9100 t/ km2/yr. Total erosion (chemical plus mechanical) give basalt denudation rates of 470–3430 mm/kyr. All these erosion rates (chemical, mechanical, total, and atmospheric CO2 consumption) are among the highest global estimates and are due to high runoff, steep slopes, active volcanics, and related active tectonics, and young basaltic lithologies, which characterise Réunion.
Article
We present major and trace element data on the suspended and dissolved phases of the Amazon River and its main tributaries. The Sr isotopic composition of the dissolved load is also reported. Special attention is paid to the abundances of REE and to their fractionation between the dissolved and suspended phase. The rivers of the Amazon Basin are among the richest in dissolved REE and are similar to the rivers of the Congo system. However a greater range of fractionation between LREE and HREE is reported here. At a global scale the rivers have intermediate patterns between those of the Congo system and those of high pH rivers such as the Indus and Mississippi rivers. Only few elements (Rb, U, Ba, K, Na, Sr and Ca) are mobilized by silicate weathering. These elements are strongly depleted in the suspended phase with respect to upper continental crust. In the dissolved load, these elements are controlled by atmospheric inputs and the weathering of the main lithologies. We propose a model based on mass budget equations, that allow the proportions derived from the different sources to be calculated. As a consequence silicate, carbonate and evaporite weathering rates can be estimated as well as the consumption of CO2 by weathering of each of these lithologies. Physical weathering rates can be estimated by two complementary approaches. On the one hand, the multi-year average of suspended sediments yields can be used to estimate physical denudation. On the other hand, we have developed a steady-state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers. A mean crustal composition is assumed in this model for the rock sources of the drainage basins. Comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers, but a strong discrepancy for the rivers derived from the Andes. Andean rivers (Solimoes, Madeira and Amazon) have observed sediment yields much greater than those predicted according to the steady-state model of chemical and physical weathering. Two interpretations can account for this inconsistency. The first is that these rivers are not in steady state and hence that the soils are being destroyed. The second requires that the local continental crust is different from the average continental crust of Taylor and McLennan, and contains a large proportion of sedimentary rocks. Using the measured sediment yields, and assuming a steady state, we can estimate the amount of sediment recycling for each drainage basin. For the Amazon at Santarem, we find that at least 25% of the mass of the upper continental crust of the Amazon drainage basin is constituted of recycled material.
Article
Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events.