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Strontium isotope variations in the lower Mississippi River and its estuarine mixing zone

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Abstract

The spatial and temporal variations of elemental and isotopic Sr in the Mississippi River (MR) mixing zone are examined in an attempt to address the extent to which estuarine environments modify the fluvial Sr flux on its way to the ocean. We collected samples from the MR delta and its outflow region during four cruises that took place from 1999 to 2002. MR discharge varied from about 7600–30,600 m3 s−1. Mixing patterns revealed minimal non-conservative behavior of Sr in the dissolved pool. Monthly measurements of the Lower MR at an anchored site during 2000 shows that Sr input to the Gulf of the Mexico from the MR varies temporally in both elemental concentration and isotopic composition, and that the flux of Sr is largely a function of water discharge. The extent of the non-conservative geochemical behavior of Sr does not compromise the use of Sr isotope ratios for stratigraphic dating purposes.

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... Studies have been conducted on trace metals for the Mississippi River [41,42], which has a high volume of freshwater discharge pushing into the far-shore open water. On the other hand, little is known about trace metal dynamics in Louisiana's small estuarine rivers like the Calcasieu that has low freshwater discharge and is strongly affected by tides and saltwater intrusion. ...
... Linear or nearly linear mixing of Sr and Ca across salinity gradients has been reported in estuaries worldwide [41,51,[64][65][66]. In our study, mixing curves of both the Ca and Sr concentrations across the salinity gradient in the Calcasieu River were linear ( Figure 6). ...
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Strontium and barium to calcium ratios are often used as proxies for tracking animal movement across salinity gradients. Many estuarine rivers face saltwater intrusion due to sea level rise, potentially causing changes in mobility and distribution of these metals upstream. From May 2013 to August 2015, monthly water samples were collected and in-situ measurements were performed at six sites along an estuary strongly affected by saltwater intrusion, the Calcasieu River, with salinity ranging from 0.02 to 29.50 Parts Per Thousand (ppt). Results showed that the total Sr concentration and the Sr/Ca ratio both increased significantly with increasing salinity. The average Sr concentration at the site closest to the Gulf of Mexico (Site 6) was 46.21 µmol/L, about 130 times higher than that of the site furthest upstream (Site 1, 0.35). The average Sr/Ca ratio at Site 6 (8.41 mmol/mol) was about three times the average Sr/Ca ratio at Site 1 (2.89). However, the spatial variation in total Ba concentration was marginal, varying from 0.36 to 0.47 µmol/L. The average Ba/Ca ratio at Site 1 (4.82 mmol/mol) was about 54 times the ratio at Site 6 (0.09), showing a negative relation between the Ba/Ca ratio and salinity. All elemental concentrations and ratios had considerable seasonal variations, with significant differences among sampling months for the Sr and Ba concentrations and the Ba/Ca ratio (p < 0.01). The results suggest that for low-gradient estuarine rivers such as the Calcasieu River, water chemistry upstream would experience substantial Sr and Ca enrichment, potentially affecting aquatic environments and biological communities.
... These deviations may be accepted due to biological or microbial process in the estuary. Xu and Marcantonio (2007) reported that the Sr has three sources in the Mississippi River mixing zone. Apart from river and sea, the third source could be from (1) submarine ground water discharge, (2) radiogenic particulate Sr that is dissolved and released into the mixing zone and (3) may be the greater concentration of radiogenic colloidal organic and/or inorganic particles. ...
... The major contributions of silicate and carbonate particles in suspended particulate matters are preferentially from dissolution of carbonate basin rock weathering. The estuary-mixing zone has the highest concentration of suspended particulate matter than river zone, which comes under (2) hypothesis reported by Xu and Marcantonio (2007). This signifies that the role of Sr in dissolved as well as in particulate forms could cause minor deviation from the conservative nature at the mixing zone by its adsorption-desorption mechanism. ...
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Water samples were collected for 23 different stations along a cross section profile of an estuary extending over to adjacent sea. The collected water samples were filtered and analyzed for major–minor ions and strontium isotope using the standard procedure to understand the geochemical behaviors of major and minor elements. The normalized values indicated that all riverine elements were entering to adjacent coastal sea with some significant variations at the estuary. The seawater dilution and regression lines explain about the overall patterns for seven elements. Removal processes were detected on calcium, magnesium, strontium and sulfate in the estuarine region. No significant mineral precipitation observed to release magnesium with respect to calcium. Minor variations of strontium and sulfate ions could be attributed to the presence of organic matter in the study area. Comparing seven elements with total suspended matters revealed that the total suspended matters played crucial role in either adsorption or absorption of all the elements in estuary before it reaches to coastal sea. Mixing patterns of strontium isotope showed minimal non-conservative with an evidence of active geochemical process in the estuary.
... The concentration and isotopic composition of aqueous Sr (Sr and 87 Sr/ 86 Sr) have been used as lithochemical proxies to characterize sources of dissolved solutes, the geochemistry of surface water– groundwater interactions, and mixing dynamics in rivers, groundwaters , and estuaries (Palmer and Edmond, 1989; Krishnaswami et al., 1992; Andersson et al., 1994; Abrerg, 1995; Négrel et al., 1997; Négrel and Grosbois, 1999; Aubert et al., 2002; Ojiambo et al., 2003; Xu and Marcantonio, 2004; Xu and Marcantonio, 2007). However, relatively few studies have utilized Sr isotopes to understand water sources, mixing and stratification in lakes and impoundments (Stein et al., 1997; Nakanao et al., 2005; Eckardt et al., 2008; Jin et al., 2010). ...
... In another study, the suspended load was also found to be higher than the dissolved load due to occurrences of minerals enriched with Rb associated with particulates during a flood event in the Garonne River and its tributaries in SW France (Semhi et al., 2000). Similarly, suspended sediment (N0.2 μm) in the Mississippi and Atchafalaya rivers at the Gulf of Mexico is more radiogenic than the dissolved load (Xu and Marcantonio, 2004; Xu and Marcantonio, 2007). In our study, for the sample LS-LT0803 1–2, collected at the station closest to where the Red River enters Lake Texoma (Fig. 2), the unfiltered aliquot has a more radiogenic 87 Sr/ 86 Sr value than the filtered aliquot. ...
Article
Strontium concentration and isotope composition (87Sr/86Sr) are important proxies for tracing solute and suspended load origins in surface water systems, but have not been widely applied to lakes. We determined these proxies, in addition to major ion compositions, in waters collected from Lake Texoma, a large crescent-shaped (concave-west) impoundment on the border of Texas and Oklahoma that marks the downstream culmination of the Upper Red River Basin (URRB), and its major influent rivers. These rivers enter the lake at its southerly and northerly arms and have distinctly different salinities and discharges, respectively: the more saline Red River (mean total dissolved solids, TDS = 2930 mg/L; 1290 mg/L weighted for discharge, Gainesville gauging station) and the fresher Washita River (TDS = 634 mg/L; 376 mg/L weighted for discharge, Dickson gauging station). These two influent river systems combine to mix in the deepest (main) part of the lake. The URRB involves a heterogeneous mix of bedrock geologies, notably high solubility Permian marine evaporites (gypsum, halite) and carbonates, and riverine solute load and composition can vary accordingly with regional precipitation. Most river and lake waters show an strong positive correlation between 87Sr/86Sr and 1/Sr. Red River system waters have variable 87Sr/86Sr values, ranging from 0.7076 to 0.7097 (n = 34), whereas the Washita River system appears to be less isotopically variable, with 87Sr/86Sr values ranging from 0.7082 to 0.7087 (n = 16). Lake Texoma 87Sr/86Sr varies spatially between 0.7087 and 0.7092, reflecting mixing of the two input rivers. Variations in Sr isotope composition, Ca/Na and Sr/Ca of river and lake waters are consistent with the majority of the solute load being derived from dissolution of Permian marine chemical sediments, dominantly halite with lesser gypsum. Waters with Sr isotope composition more radiogenic than Permian seawater reflect the contribution of Permian and Pennsylvanian siliciclastic sediments in the case of the Red River watershed, and Precambrian granite in the Washita River watershed. In the main lake water column, Sr concentrations and filtered 87Sr/86Sr increase substantially with depth during summer stratification (from 0.7087 to 0.7091), likely reflecting desorption of Sr from suspended solid surfaces and breakdown of oxyhydroxides below the metalimnion. The differences in 87Sr/86Sr between filtered and unfiltered aliquots suggest that Sr associated with the suspended load (calcite) is either less radiogenic than the dissolved load or more radiogenic than surface waters but only released to the dissolved pool under deeper reducing conditions below the metalimnion. Either possibility has important implications for Sr cycling in binary mixing models and paleolake reconstruction from Sr isotopes.Research highlights► 87Sr/86Sr & major ion ratios indicate halite and gypsum dissolution in the watershed. ► Sr (mg/L) and filtered 87Sr/86Sr increase with depth during summer stratification. ► Increase reflects desorption of Sr & breakdown of oxyhydroxides below the metalimnion. ► 87Sr/86Sr more radiogenic in filtered than unfiltered aliquots below the metalimnion. ► Suspended load (calcite) is therefore less radiogenic than the dissolved load.
... Meteoric water along the Gulf Coast usually exhibits radiogenic Sr isotope composition due to alteration of siliciclastic minerals, deriving from the weathering of continental rocks with a radiogenic Sr composition [75,76]. However, 87 Sr/ 86 Sr ratios of GC53 LMC are significantly lower than those of modern seawater, suggesting that Sr isotope compositions of meteoric water have been modified by pore fluid-sediment interactions. ...
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Authigenic low-magnesium calcite (LMC)—a mineral phase that should precipitate in calcite seas rather than today’s aragonite sea—was recently discovered at the seafloor of the Gulf of Mexico (GoM) at water depths of 65 m (site SS296) and 189 m (site GC53). This study investigates the mineralogical, petrographic, and geochemical characteristics of LMC from both sites to reveal its formation process. The δ18O values of LMC from site SS296 cluster in two groups (−0.6‰ to 1.7‰; 6.3‰ to 7.5‰) and the presence of cone-in-cone texture in the samples with lower δ18O values suggest precipitation at higher temperatures and greater depth. Low δ18O values of LMC from site GC53 ranging from −9.4‰ to −2.5‰ indicate an influence of meteoric waters during formation. LMC at both sites reveals a wide range of δ13C values (−17.4‰ to 2.6‰), indicating various carbon sources including seawater and/or organic matter. This interpretation is further supported by the δ13C values of organic carbon extracted from the LMC lithologies (δ13Corg: from −26.8‰ to −18.9‰). Relatively low Sr concentrations of LMC samples regardless of variable 87Sr/86Sr ratios, ranging from 0.707900 to 0.708498 for site GC53 and from 0.709537 to 0.710537 for site SS396, suggest the exchange of Sr between pore fluids and ambient sediments/rocks. The observed wide range of 87Sr/86Sr ratios and the enrichment of Fe and Mn in LMC is in accordance with pore fluids deriving from the dissolution of Louann salt. Overall, this study reveals that the formation of LMC at sites SS296 and GC53 was favored by the presence of low Mg/Ca ratio pore fluids resulting from salt dissolution in subsurface environments when sufficient dissolved inorganic carbon was available. These results are essential for understanding the formation of marine LMC at times of an aragonite sea, highlighting the role of formation environments—open environments close to or at the seafloor vs. confined subseafloor environments typified by pore waters with a composition largely different from that of seawater.
... We have compared the Sr isotope ratio in carbonate nodules with the Sr isotope ratios in global river water. The 87 Sr/ 86 Sr of the major global rivers water ranges from 0.7089 to 0.7291 in which the Ganga river water is characterized by the highest Sr isotopic ratio (0.7291) whereas the Danube river water have lowest Sr isotope ratio (0.7089) (Goldstein and Jacobsen, 1987;Krishnaswami et al., 1992;Palmer and Edmond, 1992;Pande et al., 1994;Trivedi et al., 1995;Yang et al., 1996;Gaillardet et al., 1999;Millot et al., 2003;Tipper et al., 2006;Moon et al., 2007Moon et al., , 2009Pattanaik et al., 2007;Xu and Marcantonio, 2007;Peucker-Ehrenbrink et al., 2010;Noh et al., 2009). The rivers like Amazon, Orinoco etc. are characterized by intermediate values. ...
Article
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The 87Sr/86Sr values in carbonate nodules and 87Sr/86Sr and εNd values in silicate samples from the valley-fill deposits of the Ganga river have been used to ascertain temporal changes in sediment provenance during the last glacial–interglacial cycle. To understand the changes, a 25 m long core named as the Firozpur core has been sampled for carbonatenodules and sediments. The 87Sr/86Sr values in carbonate nodules (0.71846 to 0.71965) of the Firozpur core are significantly more radiogenic during Last Glacial Maxima (LGM) and pre-LGM time and suggest that the Ganga river is characterized by higher 87Sr/86Sr values compared to global average river water (0.7119) at all times in the past and present. Sr and Nd isotopic data in silicate vary significantly with depth, 0.73580 to 0.77894 and –14.3 to –17.6, respectively and falling within the range of silicate rocks of the Higher Himalaya (HH) and the Lesser Himalaya (LH), the two major sediment sources to the Ganga river. The strong anti-correlation between 87Sr/86Sr and εNd again confirm this hypothesis. Together with the δ18O values and 87Sr/86Sr and εNd values of the Firozpur core suggest low precipitation over the HH during LGM caused less sediments supply from it and enhanced sediments supply from the LH. It shows significant influence of climate on erosion in the Himalaya.
... Although in restricted basins the 87 Sr/ 86 Sr ratio can be altered by local river influence, the 87 Sr/ 86 Sr ratio of water in Biscayne Bay (a popular nesting site for Crocodylus acutus) is indistinguishable from seawater (Stalker et al. 2009). Although we are aware of no coastline measurements of seawater from RWR, measurements near the Mississippi Delta in eastern LA (where river influence should be much higher than at RWR) indicate that by the time water reaches 35 psu, the 87 Sr/ 86 Sr ratio is the same as seawater (Xu & Marcantonio 2007). 87 Sr/ 86 Sr ratios of FL crocodylians (Table 2) are consistent with those of terrestrial mammals collected across FL by Hoppe et al. (1999). ...
Article
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Alligators and crocodiles differ in their physiological capacity to live in saline waters. Crocodiles can tolerate high-salinity water, at least for limited timeframes, whereas alligators and their close relatives cannot. Experiments have placed different crocodylians in various water salinities to document physiological responses, but no study has estimated the extent to which natural populations of crocodylids can live independent of fresh water. Here we estimated marine food and perhaps seawater contributions to a population of American crocodile Crocodylus acutus in southernmost Florida, USA. We evaluated the use of carbon, oxygen, and strontium isotopes as tracers of marine versus terrestrial sources. We compared C. acutus isotopic values to those of marine reptiles (marine iguanas and Pacific loggerhead turtles) and to American alligators, which require fresh water. We found that freshwater reptiles can be discriminated from those that drink seawater (or survive on metabolic and prey-included water in saline habitats) based on the magnitude of population-level oxygen isotope variation in bioapatite, whereas mean carbon isotope values discriminate between marine versus terrestrial food consumption. We used a 2 endmember (seawater and fresh water) mixing model to calculate percentage of marine resources used by C. acutus. Results indicate that adult C. acutus in southern Florida use marine food about 65% of the time and seawater or water gleaned from marine food about 80% of the time. This suggests that behavioral osmoregulatory techniques (i.e. seeking fresh water specifically for drinking, as suggested by other researchers) may not be necessary and that C. acutus is capable of being largely ecologically independent of fresh water.
... Study of the watershed geochemical characteristics, therefore, has long been an scientific interest (Chetelat et al., 2008;Fiege et al., 2009;Gaillardet et al., 1995Gaillardet et al., , 1999Han et al., 2010;Hosono et al., 2007;Meybeck, 1987;Nakano et al., 2005;Ryu et al., 2008;Wu et al., 2009). Numerous researches have been done in the past decades on rivers worldwide such as the Amazon River Gaillardet et al., 1997;Gibbs, 1972;Edmond, 1981, 1983), Mississippi River (Shiller, 1997;Shiller and Boyle, 1987;Xu and Marcantonio, 2007), Orinoco River (Lewis and Saunders, 1989) and Congo River Dupré et al., 1996;Gaillardet et al., 1995;Negrel et al., 1993). In China, most of the investigations are carried out in the Yangze River, Huanghe River and Himalayan area (Dalal et al., 2003;Galy et al., 1998;Jacobson and Blum, 2000;Moon et al., 2007;Wang et al., 2007;Wu et al., 2005). ...
... However, most previous studies of 87 Sr/ 86 Sr in otoliths have examined low-order tributaries with relatively small upstream catchments (e.g., Kennedy et al. 2000;Barnett-Johnson et al. 2008). In large river networks, the relative contributions of water originating from upstream tributaries with different chemical signatures often varies drastically in response to factors, including localized rain events and managed flow releases, potentially increasing temporal variation in 87 Sr/ 86 Sr (Goldstein and Jacobsen 1987;Åberg et al. 1989;Xu and Marcantonio 2007;Walther and Thorrold 2009). The degree to which this temporal variability affects the efficacy of direct matching of otolith and water chemistry to identify fish recruitment sources in large rivers is currently uncertain. ...
Article
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The contributions of nursery habitats to recruitment of non-native common carp (Cyprinus carpio) were estimated via analysis of water and otolith 87Sr/86Sr and otolith trace element concentrations (Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca) over 3 years in the Lachlan River, Australia. Water samples and otoliths of postlarval carp were analyzed to characterize 87Sr/86Sr and multi-elemental signatures of nursery habitats. Considerable temporal variation occurred in both water 87Sr/86Sr and otolith multi-elemental signatures, which limited our ability to directly match water and otolith 87Sr/86Sr in nurseries of the lower catchment. However, spatial variation in multi-elemental signatures was sufficient to allow accurate classification of nurseries within years. Assignment analysis of young-of-year fish suggested that several wetland and floodplain systems made significant contributions to young-of-year recruitment in the lower catchment. These contributions were strongly influenced by river flows and water management. Nurseries contributed fewer recruits to the main channel as distance from the nursery increased. Fish from the upper catchment originated from local sources, and there was no evidence of mixing of recruits between the upper and lower catchments. We conclude that identification of recruitment “hotspots” via otolith chemical analysis can assist in developing strategies to control invasive fishes in large river networks.
... However, most previous studies of 87 Sr/ 86 Sr in otoliths have examined low-order tributaries with relatively small upstream catchments (e.g., Kennedy et al. 2000;Barnett-Johnson et al. 2008). In large river networks, the relative contributions of water originating from upstream tributaries with different chemical signatures often varies drastically in response to factors, including localized rain events and managed flow releases, potentially increasing temporal variation in 87 Sr/ 86 Sr (Goldstein and Jacobsen 1987;Åberg et al. 1989;Xu and Marcantonio 2007;Walther and Thorrold 2009). The degree to which this temporal variability affects the efficacy of direct matching of otolith and water chemistry to identify fish recruitment sources in large rivers is currently uncertain. ...
... Walther, unpubl. data), the Florida coast (Surge & Lohmann, 2002) and the Mississippi Delta, U.S.A. (Xu & Marcantonio, 2007), and the North Sea (Jarvie et al., 2000), to name just a few. Again, although Sr and Ca are by themselves conservative, a mixing plot of their ratio (Sr:Ca) will be curved for most systems (Dodd & Crisp, 1982;Phillis et al., 2011). ...
Article
Chemical constituents in otoliths have become a valuable tool for fish ecologists seeking to reconstruct migratory patterns and life-history diversity in a wide range of species worldwide. This approach has proved particularly effective with fishes that move across substantial salinity gradients over the course of their life, including many diadromous species. Freshwater endmembers of several elemental and isotope ratios (e.g. Sr:Ca, Ba:Ca and (87)Sr:(86)Sr) are typically identifiably distinct from marine values, and often differ among freshwater tributaries at fine spatial scales. Because these chemical tags are generally incorporated in proportion to their ambient dissolved concentrations, they can be effective proxies for quantifying the presence, duration and frequency of movements between freshwater and marine habitats. The development of high precision probe-based analytical techniques, such as laser ablation inductively coupled plasma mass spectrometry (ICP-MS) and microbeam methods, has allowed researchers to glean increasingly detailed life-history profiles of these proxies across otoliths. Researchers are also combining multiple chemical proxies in an attempt to refine interpretations of habitat residence patterns. A thorough understanding of the spatial and temporal variation in water chemistry as well as environmental and physiological controls on incorporation of specific elements into otoliths is required for confident estimation of lifetime salinity experience. First some assumptions, methodological considerations and data processing options that are particularly relevant to diadromous otolith chemistry studies are discussed. Insights into diadromous migrations obtained from decades of otolith chemistry research, highlighting the increasingly recognized importance of contingent behaviour and partial migration are then discussed. Finally, areas for future research and the need to integrate otolith chemistry studies into comprehensive assessments of the effects of global environmental change are identified.
Chapter
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Chapter
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Chapter
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Chapter
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Chapter
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Article
To better constrain the Sr isotope budget in marginal domains without any fluvial inputs, we analyzed the chemical composition and 87Sr/86Sr ratio of waters and shells from four locations: two coastal lagoons, one hemipelagic platform and one open marine shelf. Our results highlight homogeneous 87Sr/86Sr ratios typical of oligotrophic oceanic waters (OOW) (i.e., 0.709172 ± 0.000023) in the Pacific Tatakoto atoll and along a Mediterranean shore to offshore transect (~25 km off Banyuls-sur-Mer, BSM). This attests that oceanic inputs from oligotrophic areas remain the main Sr source in open shelf areas compared with submarine groundwater discharges (SGD) or particulate dissolution influences. In BSM, only foreshore data are more radiogenic, possibly due to rainwater mixing, local groundwater springs or more efficient particle dissolution in the intertidal zone. In restricted areas, we report variable 87Sr/86Sr ratios between the Salses-Leucate (France) and Oualidia (Morocco) lagoons. The first one has homogeneous 87 Sr/ 86 Sr ratio typical of OOW except close to SGD. In Oualidia, 87Sr/86Sr ratios decrease by 1.2 × 10 −3 from OOW values close to the Atlantic inlet to progressively less radiogenic ones upstream within the interior of the lagoon. These differences depend on several factors including the leaky, restricted or choked morphology of lagoons modulating the oceanic Sr inputs, but also SGD fluxes whose 87 Sr/ 86 Sr ratios and Sr concentrations are highly variable according to the nature of rocks leached in karstic aquifer. In Oualidia, the low 87 Sr/ 86 Sr ratios correspond to high Sr concentrations (up to 150 μmol·l −1) issued from the dissolution of Mesozoic evaporites, leading to SGD fluxes accounting for 60% of the local Sr budget. Through data compilation, we show that similar 87 Sr/ 86 Sr gradients and processes prevail at the whole Mediterranean scale. Finally, we postulate that high coastal water retention times can also account for anomalous coastal 87 Sr/ 86 Sr ratios and that the combination of water mass restriction, SGD, bioadsorption and early diagenetic processes could decrease seawater Sr concentrations in some marginal areas.
Article
Multiple isotope systematics incorporating paired carbon isotopes (δ¹³C and ∆¹⁴C), strontium isotopes (⁸⁷Sr/⁸⁶Sr) and water isotopes (δ²H and δ¹⁸O) are used to investigate the coherent relationships among flow paths, chemical weathering regimes, and Sr export fluxes from the Lower Mississippi River. Monthly water samples were collected at a site near Baton Rouge, Louisiana, during 2006–2008 for measurements of water isotopic composition, the concentration and isotopic composition of dissolved inorganic carbon (DIC), and the concentration and isotopic ratio of Sr along with other selected major elements. Both δ²H and δ¹⁸O followed a similar seasonal pattern with a steady increase from a minimum in March to a maximum in July, indicating a shift of water sources from the snowmelt-dominant uppermost Upper Mississippi River during spring freshet to rainfall-induced midcontinent surface runoff and groundwater during other seasons. Values of δ¹³C-DIC ranged from −8.67‰ to −5.96‰ while Δ¹⁴C-DIC varied from −56.8‰ to 27.9‰, corresponding to a ¹⁴C age from contemporary to 415 yr BP. Generally, Δ¹⁴C-DIC increased with increasing δ¹³C-DIC, suggesting variations in bicarbonate sources in response to the shifts of flow paths and chemical weathering regimes. Depleted Δ¹⁴C-DIC and δ¹³C-DIC values during the wet seasons are likely contributed by carbonate mineral dissolution involving soil-derived CO2, while the higher Δ¹⁴C and relatively enriched δ¹³C-DIC values during the dry seasons mirror the atmospheric CO2 signatures, implying the supply by silicate weathering and a seasonal CO2 exchange between riverwater and the atmosphere which is enhanced by high primary production. Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios averaged 1.80 ± 0.26 μmol L⁻¹ and 0.709866 ± 0.000248, respectively. Both Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios show a significant correlation with δ¹⁸O values, supporting a hydrologic control of the Sr provenance in the Mississippi River basin. Indeed, the radiogenic ⁸⁷Sr from the Archean and early Proterozoic terrain in the uppermost Upper Mississippi River, the Sr released from carbonate-mineral dissolution, and the radiogenic ⁸⁷Sr from silicate weathering are manifested in the Lower Mississippi River with modifications from the snowmelt flow, rainfall-induced surface runoffs, and subsurface/groundwater, respectively. Overall, our results suggest that different hydrological flow regimes play unique roles in regulating the chemical weathering processes and therefore the seasonal variations in isotope systematics and in the concentrations and export fluxes of both DIC and Sr from the Mississippi River.
Article
Submarine groundwater discharge (SGD) is an important component of the marine ⁸⁷Sr/⁸⁶Sr budget, which is currently in an imbalance with a missing source. In this contribution, dissolved Sr concentrations and ⁸⁷Sr/⁸⁶Sr of the Chilika lagoon (India), the largest brackish-water lagoon in Asia, have been investigated for three different seasons (pre-monsoon (May 2017), monsoon (Aug., 2017) and post-monsoon (Jan., 2018)) to infer coastal behavior of Sr and estimate the SGD fluxes to the coastal ocean. Major source waters (groundwater, river and seawater) and suspended sediments from the lagoon system have also been analyzed for source characterization. Salinity and Sr concentrations of the Chilika samples show wide variations during pre-monsoon (0.2–35.8; 1–93 μM), monsoon (0.1–20.1; 0.8–55 μM) and post-monsoon (0.3–7.7; 1–20 μM) seasons. Despite of these variations, salinity and Sr concentrations of the lagoon co-vary linearly as expected for conservative mixing between river and seawater. In contrast, the mixing plot between 1/Sr and ⁸⁷Sr/⁸⁶Sr during the monsoon and pre-monsoon seasons deviate from the river-seawater mixing trend, indicating an additional source/sink of ⁸⁷Sr to this lagoon. The non-conservative behavior of ⁸⁷Sr/⁸⁶Sr during monsoon has largely been restricted to low salinity (< ~2) regime, which could be attributed to subsurface ion-exchange process. During the pre-monsoon, the SGD can explain the non-conservative isotopic behavior that requires additional water supply with higher ⁸⁷Sr/⁸⁶Sr ratios. The SGD fluxes have been estimated using two separate approaches, (i) using an inverse model with fixed SGD composition and (ii) using a source-mixing computation using variable SGD compositions within the lagoon. These computations estimate that the SGD contributes ~20% of total water supplied to the Chilika lagoon during the pre-monsoon season. This SGD contribution corresponds to a flux of 1.51 × 10⁶ m³/d to the lagoon. Data from this and earlier studies indicate that the ⁸⁷Sr/⁸⁶Sr ratios of the SGD to the western Bay of Bengal, which receives water from several large rivers from the Himalaya and Peninsular Indian regions, are relatively higher (~0.715) than the seawater value (0.7092). The SGD flux to the east coast of India, therefore, would not contribute in reducing the oceanic imbalance, which requires a less-radiogenic source.
Article
We develop a broad geochemical dataset from 50 samples of Pleistocene glacial till matrix (including three lacustrine samples) representing four sediment provenances collected from across Minnesota, USA. Such a dataset is useful both in the context of regional studies of glaciation, and in efforts to understand how provenance and glacial processes impact the geochemistry of sediment archives. The principal sediment sources of the four provenances include materials from the Archean-Proterozoic Canadian Shield, the Proterozoic Midcontinent Rift, Paleozoic carbonates, and Cenozoic and Mesozoic clastic sediments. We measured major element geochemistry in all till samples in both <2 mm and <63 μm size fractions, the trace element geochemistry in most samples, radiogenic isotopic compositions (Sr, Nd) in 13 representative samples, and Hf-isotope composition in 6 samples. Differences in source rock composition explain the primary variations in the geochemistry of our samples. In untreated (carbonate-bearing) samples, Na2O + K2O versus Fe2O3 or CaO are distinct in tills sourced from crystalline rocks versus sedimentary basins, and mechanical mixing from different source areas is evident on 100-km glacial transport length scales. Glacial materials originating from sedimentary rocks have higher chemical index of alteration (CIA) relative to materials sourced from the predominately igneous and metamorphic Canadian Shield. Increased values of Rb + Sr, Zr/Sc, and Cr + Ni are also associated with tills derived from crystalline rocks relative to tills derived from sedimentary rocks. Both the Hf- and Nd-isotopic composition of glacial sediments distinguish crystalline rock sources (less radiogenic) versus sedimentary rock sources (more radiogenic). The Sr-, and, to a lesser degree, the Hf-isotopic composition of lacustrine samples is influenced by subtle changes in sample mineral composition, reflecting both source rock variability and the sorting of clay and heavy mineral components during sediment transport. A carbonate-free composite of our till samples was found to be broadly representative of a Canadian Shield source. The ability to discern provenance and sediment transport process controls on glacial sediment geochemistry presents an opportunity to extend our understanding of past ice-sheet dynamics, and validates approaches that use tills as a proxy for continental crustal composition, provided that the influences of sediment recycling are carefully considered.
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This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Article
The chemical and isotopic characteristics of the water and suspended particulate materials (SPM) in the Yellow River were investigated on the samples collected from 29 hydrological monitoring stations in the mainstem and several major tributaries during 2004 to 2007. The δD and δ18O values of the Yellow River water vary in large ranges from -32‰ to -91‰ and from -3.1‰ to -12.5‰, respectively. The characters of H and O isotope variations indicate that the major sources of the Yellow River water are meteoric water and snow melting water, and water cycle in the Yellow River basin is affected strongly by evaporation process and human activity. The average SPM content (9.635 g/L) of the Yellow River is the highest among the world large rivers. Compared with the Yangtze River, the Yellow River SPM has much lower clay content and significantly higher contents of clastic silicates and carbonates. In comparison to the upper crust rocks, the Yellow River SPM contains less SiO2, CaO, K2O and Na2O, but more TFe2O3, Co, Ni, Cu, Zn, Pb and Cd. The abnormal high Cd contents found in some sample may be related to local industrial activity. The REE contents and distribution pattern of the Yellow River SPM are very close to the average value of the global shale. The average δ30SiSPM in the Yellow River (-0.11‰) is slightly higher than the average value (-0.22‰) of the Yangtze River SPM. The major factors controlling the δ30Sispm of the Yellow River are the soil supply, the isotopic composition of the soil and the climate conditions. The TDS in the Yellow River are the highest among those of world large rivers. Fair correlations are observed among CI-, Na+, K+, and Mg2+ contents of the Yellow River water, indicating the effect of evaporation. The Ca2+ and Sr2+ concentrations show good correlation to the SO42- concentration rather than HCO3- concentration, reflecting its origin from evaporates. The NO3- contents are affected by farmland fertilization. The Cu, Zn and Cd contents in dissolved load of the Yellow River water are all higher than those of average world large rivers, reflecting the effect of human activity. The dissolved load in the Yellow River water generally shows a REE distribution pattern parallel to those for the Yangtze River and the Xijiang River. The δ30Si values of the dissolved silicon vary in a range from 0.4‰ to 2.9‰, averaging 1.34‰. The major processes controlling the DSi and δ30Sidiss of the Yellow River water are the weathering process of silicate rocks, growth of phytolith in plants, evaporation, dissolution of phytolith in soil, growth of fresh water diatom, adsorption and desorption of aqueous monosilicic acid on iron oxide and human activities. The average δ30SiDiss value of the Yellow River is significantly lower than that of the Nile River, Yangtze River and Siberia rivers, but higher than those of other rivers, reflecting their differences in chemical weathering and biological activity. The δ34SSO4 values of the Yellow River water range from -3.8‰ to 14.1‰, averaging 7.97‰. There is some correlation between SO42- content and δ34Sso4. The factors controlling the δ34SSO4 of the Yellow River water are the SO4 in the meteoric water, the SO4 from gypsum or anhydrite in evaporite rocks, oxidation and dissolution of sulfides in the mineral deposits, magmatic rocks and sedimentary rocks, the sulfate reduction and precipitation process and the sulfate from fertilizer. The 87Sr/86Sr ratios of all samples range from 0.71041 to 0.71237, averaging 0.71128. The variations in the 87Sr/86Sr ratio and Sr concentration of river water are primarily caused by mixing of waters of various origins with different 87Sr/86Sr ratios and Sr contents resulting from water-rock interaction with different rock types.
Article
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Chapter
Full-text available
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Chapter
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
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􀀶􀁈􀀃 􀁏􀁏􀁄􀁐􀁄􀀃 􀂳􀁐􀁒􀁑􀁌􀁗􀁒􀁕􀁈􀁒􀂴􀀃 􀁄􀀃 􀁘􀁑􀀃 􀁖􀁌􀁖􀁗􀁈􀁐􀁄􀀃 contínuo de observación y control de medidas y evaluaciones para un propósito definido. El monitoreo es una herramienta importante en el proceso de evaluación de impactos, y en cualquier programa de seguimiento y control (Sors, 1987). Actualmente existe un fuerte consenso sobre que el monitoreo ambiental no es un fin en sí mismo, sino un paso esencial en los procesos de administración del ambiente (Stelzenmüller et al., 2013). Teniendo en cuenta los conceptos previamente mencionados, se puede observar la importancia que actualmente tiene el monitoreo en los diversos procesos de la actividad humana. Además, y tal como acertadamente se menciona, es una herramienta fundamental dentro de todo aquel desarrollo o procedimiento que se desee llevar a cabo de manera controlada y segura (Collins et al., 2012). El monitoreo no es una actividad independiente, sino más bien una parte de un proceso más amplio que puede estar científicamente orientado a la conservación o la gestión (Nichols y Williams, 2006). El paradigma dominante sobre el que se basa la ciencia del monitoreo es la evaluación de los cambios en las variables ambientales seleccionadas entre tratamientos y áreas de control, así como su evolución en el tiempo tras la aplicación de un régimen de gestión (Tulloch et al., 2011). Los programas de monitoreo consisten en realizar una vigilancia permanente de un sistema natural, controlando el estado y evolución de sus variables, cuantificando las variaciones que se producen, e identificando los motivos que las generaron. Hay diferentes estrategias de trabajo, tanto espaciales como temporales, y pueden desarrollarse exclusivamente en base a datos de campo, en base a datos experimentales, o combinando ambos. Cuando estos programas se llevan a cabo por plazos prolongados (por ejemplo, decadales), el conjunto de información que van proveyendo constituye una base sólida sobre la cual es posible caracterizar con precisión la estructura y funcionamiento del sistema bajo estudio. Siempre y cuando se mantenga la continuidad del monitoreo, se puede sostener que a medida que el programa tiene más antigüedad (por lo tanto más cantidad de información) es más sencillo identificar una anomalía o variación significativa en alguno de los parámetros 􀁈􀁖􀁗􀁘􀁇􀁌􀁄􀁇􀁒􀁖􀀏􀀃􀁗􀁈􀁑􀁌􀁈􀁑􀁇􀁒􀀃􀁈􀁑􀀃􀁆􀁘􀁈􀁑􀁗􀁄􀀃􀁔􀁘􀁈􀀃􀁏􀁄􀀃􀁇􀁌􀁖􀁗􀁕􀁌􀁅􀁘􀁆􀁌􀁹􀁑􀀃􀁇􀁈􀀃􀁙􀁄􀁏􀁒􀁕􀁈􀁖􀀃􀂳􀁑􀁒􀁕􀁐􀁄􀁏􀁈􀁖􀂴􀀃􀁖􀁈􀁕􀁩􀀃􀁐􀁘􀁜􀀃􀁇􀁈􀁑􀁖􀁄􀀏􀀃􀁜􀀃􀁓􀁒􀁕􀀃 lo tanto los que se alejen de ese modelo serán fácilmente distinguibles. Otro tipo de alternativa de trabajo es la aplicación de bancos de muestras ambientales (
Article
The chemical and isotopic characteristics of the water and suspended particulate materials (SFM) in the Yangtze River were investigated on the samples collected from 25 hydrological monitoring stations in the mainsteam and 13 hydrological monitoring stations in the major tributaries during 2003 to 2007. The water samples show a large variation in both D (-30 parts per thousand to -112 parts per thousand) and O-18 (-3.8%. to -15.4 parts per thousand) values. Both D and O-18 values show a decrease from the river head to the Jinsha Jiang section and then increase downstream to the river mouth. It is found that the oxygen and hydrogen isotopic compositions of the Yangtze water are controlled by meteoric precipitation, evaporation, ice (and snow) melting and dam building. The Yangtze SPM concentrations show a large variation and are well corresponded to the spatial and temporal changes of flow speed, runoff and SPM supply, which are affected by the slope of the river bed, local precipitation rate, weathering intensity, erosion condition and anthropogenic activity. The Yangtze SPM consists of clay minerals, clastic silicate and carbonate minerals, heavy minerals, iron hydroxide and organic compounds. From the upper to lower reaches, the clay and clastic silicate components in SPM increase gradually, but the carbonate components decrease gradually, which may reflect changes of climate and weathering intensity in the drainage area. Compared to those of the upper crust rocks, the Yangtze SPM has lower contents of SiO2, CaO, K2O and Na2O and higher contents of TFe2O3 and trace metals of Co, Ni, Cu, Zn, Pb and Cd. The LREE in the Yangtze SPM is also slightly higher than that of the upper crust From the upper to lower reaches, the CaO and MgO contents in SPM decrease gradually, but the SiOz content increases gradually, corresponding to the increase of clay minerals and decrease of the carbonates. The Si-30(SPM) values (-1.1 parts per thousand to 0.3 parts per thousand) of the Yangtze SPM are similar to those of the average shale, but lower than those of the granite rocks (-0.3 parts per thousand to 0.3 parts per thousand), reflecting the effect of silicon isotope fractionation in silicate weathering process. The Si-30(spm) values of the Yangtze SPM show a decreasing trend from the upper to the middle and lower reaches, responding to the variation of the clay content. The major anions of the river water are HCO3-, SO42-, Cl-, NO3-, SiO(4)(4-)and F- and the major cations include Ca2+, Na+, Mg2+, K+ and Sr2+. The good correlation between HCO3- content and the content of Ca2+ may suggest that carbonate dissolution is the dominate contributor to the total dissolved solid (TDS) of the Yangtze River. Very good correlations are also found among contents of Cl-, SO42-, Na+, Mg2+, K+ and Sr2+, indicating the important contribution of evaporite dissolution to the TDS of the Yangtze River. High TDS contents are generally found in the head water, reflecting a strong effect of evaporation in the Qinghai-Tibet Plateau. A small increase of the TDS is generally observed in the river mouth, indicating the influence of tidal intrusion. The F- and NO3- contents show a clear increase trend from the upstream to downstream, reflecting the contribution of pesticides and fertilizers in the Chuan Jiang section and the middle and lower reaches. The D-si shows a decrease trend from the upstream to downstream, reflecting the effect of rice and grass growth along the Chuan Jiang section and the middle and lower reaches. The dissolved Cu, Zn and Cd in the Yangtze water are all higher than those in world large rivers, reflecting the effect of intensive mining activity along the Yangtze drainage area. The Yangtze water generally shows similar REE distribution pattern to the global shale. The Si-30(Diss) values of the dissolved silicon vary from 0.5 parts per thousand to 3.7 parts per thousand, which is the highest among those of the rivers studied. The Si-30(Diss) values of the water in the Yangtze mainsteam show an increase trend from the upper stream to downstream. Its D-Si and Si-30(Diss) are influenced by multiple processes, such as weathering process, phytolith growth in plants, evaporation, phytolith dissolution, growth of fresh water diatom, adsorption and desorption of aqueous monosilicic acid on iron oxide, precipitation of silcretes and formation of clays coatings in aquifers, and human activity. The (S-34(SO4) values of the Yangtze water range from -1.7 parts per thousand to 9.0 parts per thousand. The SO4 in the Yangtze water are mainly from the SO4 in meteoric water, the dissolved sulfate from evaporite, and oxidation of sulfide in rocks, coal and ore deposits. The sulfate reduction and precipitation process can also affect the sulfur isotope composition of the Yangtze water. The Sr-87/Sr-86 ratios of the Yangtze water range from 0.70823 to 0.71590, with an average value of 0.71084. The Sr-87/Sr-86 ratio and Sr concentration are primary controlled by mixing of various sources with different Sr-87/Sr-86 ratios and Sr contents, including the limestone, evaporite and the silicate rocks. The atmospheric precipitation and anthropogenic inputs can also contribute some Sr to the river. The B-11 values of the dissolved B in the Yangtze water range from 2.0 parts per thousand to 18.3 parts per thousand, which is affected by multifactors, such as silicate weathering, carbonate weathering, evaporite dissolution, atmospheric deposition, and anthropogenic inputs.
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Dissolved Sr and 87Sr/86Sr are measured in the Narmada, Tapi and the Mandovi estuaries linked to the eastern Arabian Sea. The concentration of dissolved Sr and 87Sr/86Sr in the river water endmembers show significant differences reflecting the lithologies they drain. The distribution of Sr in all these estuaries shows a near perfect two endmember mixing between river water and seawater suggesting that there is no discernible net addition/removal of Sr from the estuarine waters. In contrast, 87Sr/86Sr shows non-conservative behaviour in all these estuaries, its distribution exhibits significant departure from the theoretical mixing lines. A likely mechanism for this difference in the behaviour between dissolved Sr and its 87Sr/86Sr is the discharge of submarine groundwater (SGD) which can modify the 87Sr/86Sr of the estuarine waters by exchange with sediments without causing measurable changes in Sr concentration. The impact of such an exchange process on the 87Sr/86Sr of the estuaries and therefore on the Sr isotope composition of dissolved Sr entering the Arabian Sea differs among the three estuaries and also between seasons in the Narmada. The non-conservative behaviour of 87Sr/86Sr provides a handle to estimate the quantum of SGD to these estuaries. The Sr concentration, 87Sr/86Sr ratio and salinity of the submarine groundwater and estimate of its fluxes to the Narmada estuary have been made using inverse model calculations. The model derived SGD flow rates are ∼5 and 280 cm/day during pre-monsoon and monsoon, respectively. The more radiogenic Sr isotope composition of SGD relative to the seawater suggests that SGD acts as an additional source of 87Sr to the Arabian Sea.
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The applications of radiogenic isotopes to investigate chemical and physical erosion processes, particularly in river basins of the Himalaya, have led to interesting inferences on the relationship between tectonics, weathering and climate. The chemical weathering studies rely more on Sr isotopes because of their widely different ratios in various end members, their uniform distribution in the oceans and the availability of continuous and robust record of marine ⁸⁷Sr/⁸⁶Sr through much of the geological past. The record for the Cenozoic shows steady increase in ⁸⁷Sr/⁸⁶Sr; one of the hypotheses suggested to explain this is enhanced continental silicate weathering due to the uplift of the Himalaya. This hypothesis linking tectonics-weathering-climate, based on ⁸⁷Sr/⁸⁶Sr as an index of silicate weathering, however, is being challenged by the recent observations that there are a variety of carbonates in the river basins of the Himalaya with ⁸⁷Sr/⁸⁶Sr similar to that of silicates which have the potential to contribute significantly to the high ⁸⁷Sr/⁸⁶Sr of rivers such as the Ganga-Brahmaputra. Further, the non-stochiometric release of Sr isotopes during chemical weathering of minerals and rocks, the imbalance of Sr isotope budget in the oceans and temporal variations in riverine fluxes due to impact of glaciations all have compounded the problem.
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Strontium and particularly 87Sr/86Sr ratios in stream water have often been used to calculate weathering rates in catchments. Nevertheless, in the literature, discharge variation effects on the geochemical behavior of Sr are often omitted or considered as negligible. A regular survey of both Sr concentrations and Sr isotope ratios of the Strengbach stream water draining a granite (Vosges mountains, France) has been performed during one year. The results indicate that during low water flow periods, waters contain lower Sr concentrations and less radiogenic Sr isotope ratios (Sr=11.6 ppb and 87Sr/86Sr=0.7246 as an average, respectively) than during high water flow periods (Sr= 13 ppb and 87Sr/86Sr=0.7252 as an average, respectively). This is contrary to expected dilution processes by meteoric waters which have comparatively lower Sr isotopic ratios and lower Sr concentrations. Furthermore, 87Sr/86Sr ratios in stream water behave in 3 different ways depending on moisture and on hydrological conditions prevailing in the catchment. During low water flow periods (discharge < 9 l/s), a positive linear relationship exists between Sr isotope ratio and discharge, indicating the influence of radiogenic waters draining the saturated area during storm events. During high water flow conditions, rising discharges are characterized by significantly less radiogenic waters than the recession stages of discharge. This suggests a large contribution of radiogenic waters draining the deep layers of the hillslopes during the recession stages, particularly those from the more radiogenic north-facing slopes. These results allow one to confirm the negligible instantaneous incidence of rainwater on stream water chemistry during flood events, as well as the existence in the catchment of distinct contributive areas and reservoirs. The influence of these areas or reservoirs on the fluctuations of Sr concentrations and on Sr isotopic variations in stream water depends on both moisture and hydrological conditions. Hence, on a same bedrock type, 87Sr/86Sr ratios in surface waters can be related to flow rate. Consequently, discharge variations must be considered as a pre-requisite when using Sr isotopes for calculating weathering rates in catchments, particularly to define the range of variations of the end-members.
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Seawater Sr and Sr/Ca exhibit spatial gradients of 2–3% globally, with the deep ocean more enriched relative to the surface. In latitudinal transects, the highest surface values are found at high latitudes and associated with areas of upwelling. A pronounced upper ocean vertical Sr gradient is attributable to the production of celestite skeletons by surface-dwelling acantharia, coupled to a shallow dissolution cycle. The upper ocean residence time of Sr with respect to celestite cycling is much shorter than its global oceanic residence time. Although the magnitude of seawater Sr/Ca variability is relatively small, it is significant with respect to high-precision paleoceanographic applications. Sr/Ca gradients in the contemporary ocean also complicates evaluating Quaternary changes in seawater Sr/Ca that may have resulted from other processes, such as aragonite recrystallization during sea-level low stands.
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Measurements of marine carbonate samples indicate that during the past 2.5 million years the 87Sr/86Sr ratio of seawater has increased by 14 x 10-5. The high average rate of increase of 87Sr/86Sr indicates that continental weathering rates were exceptionally high. Nonuniformity in the rate of increase suggests that weathering rates fluctuated by as much as ±30 percent of present-day values. Some of the observed shifts in weathering rates are contemporaneous with climatic changes inferred from records of oxygen isotopes and carbonate preservation in deep sea sediments.
Article
Integration of foraminiferal biostratigraphy, ⁸⁷ Sr/ ⁸⁶ Sr isotope stratigraphy, and traditional physical stratigraphy has provided a refined age control of a poorly known Oligocene-to-Pleistocene sedimentary sequence nonconformably covering the crystalline basement complex of the Calabrian microplate, a continental block which rifted off the southern margin of the European plate during Neogene time. In spite of the fossil-poor content of the sequence, the simultaneous use of paleontological and geochemical techniques have resulted in the following conclusions. 1) The age of an unnamed, thin calcarenite unit locally present at the base of the sequence, previously considered Rupelian to early Aquitanian in age, has been refined to Chattian (27.8–24.8 Ma). This calcarenite was considered a basal, conformable member of the overlying Stilo-Capo d'Orlando Formation. However, this study indicates that it is separated from the Stilo-Capo d'Orlando Formation either by an angular unconformity or by a disconformity representing a significant time interval. 2) The Stilo-Capo d'Orlando Formation has a latest Chattian–earliest Aquitanian to Burdigalian age. Previously published reports suggested deposition over a much longer time span, ranging from late Rupelian to Langhian. 3) An unnamed deep-marine siliciclastic unit mostly composed of conglomerate and sandstone and previously considered Tortonian in age is, in fact, Serravallian to Tortonian. 4) The depositional interval of the “trubi,” fine-grained marine deposits, has been independently confirmed to span the Pliocene-Pleistocene. The results of this study provide a framework for future sequence–stratigraphic and paleotectonic studies in the area, and prove the effectiveness of an integrated paleontological and geochemical ( ⁸⁷ Sr/ ⁸⁶ Sr) approach in the study of fossil-poor sedimentary sequences.
Chapter
Paleomagnetic data have been obtained from heterogeneous, shallow-water, miogeoclinal carbonate rocks of the Pogonip Group (Early Ordovician) in the Desert Range of southern Nevada, the Egan Range of east-central Nevada, and the southern House Range of western Utah. These rocks locally contain abundant replacive chert that preserves relict textures from the host limestones as well as clearly detrital grains (e.g., blue-luminescing feldspars). Stylolites are abundant and are interpreted as late diagenetic features, as they cut late cements and truncate bedding lamination. Differential compaction along stylolites wrapping around the chert masses has resulted in macroscopic deformation, as evidenced by tilting of bedding of over 25° about chert masses in some cases. We have used the differential compaction fabrics in these rocks to test for the age of acquisition of a generally well-grouped and well-defined characteristic magnetization. All three carbonate sections give a low-inclination, southerly to southeasterly magnetization residing in magnetite (e.g., Decl. = 152°, Incl. = −21°, α95 = 3°, kl = −61, k2 = −21, N = 48 independent samples, site 12; Pogonip Group, Sawmill Canyon, Egan Range). The magnetization is interpreted to be secondary and acquired after local compaction because directions of magnetizations from different samples are not dispersed by the compaction deformation. The uniform reversed polarity in addition to the direction of the magnetization, moreover, is interpreted to suggest a late Paleozoic age of remagnetization. In the Desert Range, the remagnetization had been previously attributed to a viscous partial thermoremanent magnetization (VPTRM) from deep burial. Based on several observations, we now argue for a chemical origin from late diagenetic magnetite, such as is now well-documented in the Appalachians and mid-continent. The cherts are almost nonmagnetic, as would be expected from their impermeability if the magnetite were precipitated from late fluids. Abundant authigenic alkali feldspar in the Desert Range is also consistent with late metasomatism. Finally, in the Egan Range, the remagnetization extends through a section exceeding 3 km in thickness, into rocks as young as Mississippian, which were never buried as deeply and thus not heated to the same degree as lower Paleozoic strata. These results underscore the utility of integrating observations based on paleomagnetic data with carbonate textures. “Micro”-field tests can constrain both the timing of magnetization acquisition and of diagenetic events. The micro-fold tests discussed apply to features that are not formed by tectonic deformation. The availability of field tests from early formed textures in sedimentary rocks is especially important given the recent recognition of widespread remagnetization in ancient rocks.
Article
Over the last century, the river-dominated Mississippi delta has received increasing attention from geoscientists, biologists, engineers, and environmental planners because of the importance of the river and its deltaic environments to the economic well-being of the state of Louisiana and the nation. Population growth, subsurface resource extraction, and increased land-water use have placed demands on the delta's natural geologic, biologic, and chemical systems, therefore modifying the time and spatial scales of natural processes within the delta and its lower alluvial valley. As a result, the combined effects of natural and human-induced processes, such as subsidence, eustatic sea level rise, salt water intrusion, and wetland loss, have produced a dynamically changing landscape and socioeconomic framework for this complex delta. Under natural conditions, the fundamental changes that result in land-building and land loss in the Holocene Mississippi River delta plain are rooted in the systematic diversion of water and sediment associated with major shifts in the river's course-the process of delta switching. Research over the last half century has shown that major relocations of the Mississippi's course have resulted in five Holocene delta complexes and a sixth one in an early stage of development as a product of the latest Atchafalaya River diversion. Collectively, these Holocene deltas have produced a delta plain that covers an area of ~30,000 km2 and accounts for 41% of the coastal wetlands in the United States. After a river diversion takes place, the resulting delta evolves through a systematic and semipredictable set of stages generally characterized by: (a) rapid progradation with increasing-to-stable discharge, (b) relative stability during initial stages of waning discharge, (c) abandonment by the river in favor of a higher gradient course to the receiving basin, and (d) marine reworking of a sediment-starved delta as it undergoes progressive submergence by the combined processes of subsidence. Delta switching has taken place every 1000 to 2000 years during Holocene times, and resulting deltas have an average thickness of approximately 35 m. Within a single delta there are subdeltas, bayfills, and crevasse-splays that have higher frequency delta cycles ranging from several hundred years to a few decades. These depositional features are usually less than 10 m thick, and some have produced marshland areas of over 300 km2. The net result of these delta-building events is a low-lying landscape with components that are changing (building and deteriorating) at different rates. Geologically, these depositional cycles produce a thick accumulation of coarsening, upward deltaic deposits that have various thicknesses in response to development on a variety of temporal and spatial scales. In this river-dominated delta system, distributaries can prograde seaward at rates of over 100 m/year. The cumulative effect of the Holocene depository has been to depress the underlying Pleistocene surface. In a local setting, e.g., the modern Balize Lobe, differential loading causes the vertical displacement of underlying clay-rich facies (shale diapirs-mudlumps). The delta front of this lobe, which has prograded into deep water of the outer continental shelf, is characterized by rapid deposition of silt- and clay-rich sediments and slope instability, which results in seaward displacement of sediments by a variety of mass-movement processes. Superimposed on the natural processes and forms of the Mississippi deltaic plain and its associated estuarine environments, are human impacts, most of which have been imposed in this century. The most significant impacts have resulted from a decrease in sediment input to the river from its tributaries and the alteration of the river's natural sediment dispersal processes through the construction of levees. Measures are now being taken to reinstate some of the delta's natural processes, thereby mitigating landloss so that decline in animal and plant productivity can be mitigated.
Article
Measurements of 87Sr/86Sr ratios of planktonic foraminifers from paleomagnetically dated Deep Sea Drilling Project (DSDP) cores indicate that the 87Sr/86Sr ratio of seawater increased linearly between 6 and 4 Ma with a slope of 87 x 10-6/m.y. This suggests that 87Sr/86Sr variations in this time interval are useful for estimating ages in marine sequences. The theoretical stratigraphic resolution of the technique is 0.23 m.y., but the practical resolution is estimated to be 0.3-0.5 m.y. Strontium isotope ratios of five samples at the base of the Miocene/Pliocene (M/P) boundary stratotype at Capo Rossello, Sicily, have a mean 87Sr/86Sr values of 0.708995 ±0.00002. Applying the regression equation derived from our 87Sr/86Sr vs. age data set for deep-sea sequences yields an age of 4.94 ±0.50 Ma (95% confidence interval) for the M/P boundary stratotype, similar to ages estimated by biostratigraphic and magnetostratigraphic methods. Correlation of this mean 87Sr/86Sr ratio to the section at DSDP Site 519 suggests that the boundary is coincident with the onset of a major early Pliocene transgression, as recorded by a decrease in oxygen isotope values for benthic foraminifers. This study demonstrates the potential application of strontium isotope stratigraphy for correlating and dating problematic sequences, such as those deposited in restricted, shallow-water, or high-latitude environments.
Article
The effects of Quaternary sea level changes on the Sr budget of the ocean are investigated using coupled numerical models of the seawater Sr and Ca budgets. Glacial/interglacial sea level cycles influence the Sr concentration of seawater directly through the periodic exposure and weathering of aragonite on continental shelves and indirectly by modulating the location and extent of carbonate deposition in the ocean. As sea level recedes, brief pulses of Sr are added to the ocean from recrystallization of shelf aragonite; this flux at its maximum is nearly ten times larger than the combined riverine and hydrothermal influx. The large shelf recrystallization fluxes during glacial maxima produce rapid increases in the Sr/Ca ratio of seawater despite the long residence time of Sr in the ocean. Variations in the ratio of aragonitic vs. calcitic sedimentation also influence on the Sr content of seawater because the Sr partitioning coefficient is higher in aragonite; however, this effect is minor compared to shelf aragonite recrystallization. Based on a variety of sensitivity tests, the amplitude of Sr/Ca variations ranges from 1 to 3 % and is maximized by constant rates of total carbonate deposition, extensive aragonite recrystallization, and carbonate budgets which include dissolution of shelf carbonates. Such variation is sufficient to produce up to 1.5°C errors in paleotemperatures calculated from coral Sr/Ca ratios since the last glacial maximum.
Article
Clastic sediments in the Bengal Fan contain a Neogene history of erosion and weathering of the Himalaya. We present data on clay mineralogy, major element, stable and radiogenic isotope abundances from Lower Miocene-Pleistocene sediments from ODP Leg 116. Nd and Sr isotope data show that the Himalayan provenance for the eroded material has varied little since > 17 Ma. However, from 7 to 1 Ma smectite replaces illite as the dominant clay, while sediment accumulation decreased, implying an interval of high chemical weathering intensity but lower physical erosion rates in the Ganges-Brahmaputra (GB) basin. O and H isotopes in clays are correlated with mineralogy and chemistry, and indicate that weathering took place in the paleo-Gangetic flood plain. The 87Sr 86Sr ratios of pedogenic clays (vermiculite, smectite) record the isotopic composition of Sr in the weathering environment, and can be used as a proxy for 87Sr 86Sr in the paleo-GB basin. The Sr data from pedogenic clays shows that river 87Sr 86Sr values were near 0.72 prior to 7 Ma, rose rapidly to ≥ 0.74 in the Pliocene, and returned to ≤ 0.72 in the middle Pleistocene. These are the first direct constraints available on the temporal variability of 87Sr 86Sr in a major river system. The high 87Sr 86Sr values resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate-silicate weathering ratio. Modeling of the seawater Sr isotopic budget shows that the high river 87Sr 86Sr values require a ca. 50% decrease in the Sr flux from the GB system in the Pliocene. The relationship between weathering intensity, 87Sr 86Sr and Sr flux is similar to that observed in modern rivers, and implies that fluxes of other elements such as Ca, Na and Si were also reduced. Increased weathering intensity but reduced Sr flux appears to require a late Miocene-Pliocene decrease in Himalayan erosion rates, followed by a return to physically dominated and rapid erosion in the Pleistocene. In contrast to the view that increasing seawater 87Sr 86Sr results from increased erosion, Mio-Pliocene to mid-Pleistocene changes in the seawater Sr budget were the result of reduced erosion rates and Sr fluxes from the Himalaya.
Article
The 87 Sr / 86 Sr ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of 87 Sr / 86 Sr ratios. The measurements by et al. (Geochim. Cosmochim. Acta 34 , 105-120, 1970) of the value of the marine 87 Sr / 86 Sr ratio have been confirmed by several other workers and by some new measurements on JOIDES samples. They form the basis of a model calculation of the relative proportions of `basaltic' ( 87 Sr / 86 Sr = 0.704) and `granitic' ( 87 Sr / 86 Sr = 0.718) strontium being supplied to the sea. For the last 200 million years, the proportions of these two sources appear to reflect the history of global tectonics; `basaltic' during rifting and increasingly `granitic' during the present episodes of uplift and continental collision
Article
The present published inventory of fluvial Sr and87Sr/86Sr data, combined with new information from the big rivers of Eastern Siberia (a combined total of ∼ 1,000 measurements), is used to investigate the probable origin of the large rise in the marine isotopic ratio, recorded in limestones, over the last ∼ 20 million years. With the exception of the data from the Ganga-Brahmaputra all measurements fall on what is proposed to be called the “Wickman trend”, essentially a mixing line between the limestone sink for Sr, with the integrated marine ratio, and the flux from the weathering of average continental crust. However, time-variations along this trend, i.e. changes in relative weathering intensity, cannot explain the observations from limestones. They can only be caused by very high and radiogenic fluxes of Sr as are occurring from the present Himalayan orogeny, lying far above the Wickman trend and caused by metamorphic remobilization of radiogenic Sr during underthrusting and subsequent unroofing associated with the collision of India with Eurasia. In general the variations in the ratio are therefore caused by specific tectonic events, not by general climatic variations in the intensity of aluminosilicate weathering.
Article
Precise measurements of 786 marine carbonate, evaporite, and phosphate samples of known age provide a curve of seawater 87Sr/86Sr versus geologic time through the Phanerozoic. Many episodes of increasing and decreasing values of 87Sr/86Sr of seawater have occurred through the Phanerozoic. The Late Cambrian Early Ordovician seawater ratios are approximately equal to the modern ratio of 0.70907. The lowest ratios, ˜0.7068, occurred during the Jurassic and Late Permian. The configuration of the curve appears to be strongly influenced by the history of both plate interactions and seafloor spreading throughout the Phanerozoic. The curve provides a basis for dating many marine carbonate, evaporite, and phosphate samples. Furthermore, diagenetic modifications of original marine 87Sr/86Sr values are often interpretable. Analysis of 87Sr/86Sr data, therefore, may provide useful information on regional diagenetic patterns and processes. All of the Cenozoic samples and some of the Cretaceous samples are from Deep Sea Drilling Project (DSDP) cores. With the exception of the DSDP samples, the curve was constructed only from samples containing at least 200 ppm Sr and not more than 10% dilute acid insoluble material. All measurements are made by comparison with standard SrCO3 (NBS SRM 987) for which a 87Sr/86Sr of 0.71014 is assumed. Precision is estimated to be ± 0.00005 at the 95% confidence level. Measured ratios of 42 modern marine samples average 0.70907, with a standard deviation of 0.00004. *Present addresses: (Denison) Suite 616, One Energy Square. 4925 Greenville Avenue, Dallas, Texas 75206; (Nelson) 2516 West Five Mile Parkway, Dallas, Texas 75233
Article
The Quaternary deposits of the Lamone River Valley in the Northern Apennines are mainly composed of marine clays and sands. Micropaleontological investigations allow the reconstruction of the paleoenvironmental evolution of that succession by recognition of 13 foraminifer and ostracod biofacies diagnostic of upper slope to deltaic environments. 87Sr/86Sr measurements were performed in well-preserved fossils throughout most of the recognized biofacies. These are compared with the strontium (Sr) isotopic ratios of Pleistocene seawater. Samples collected within biofacies characteristic of slope to shallow marine environments, without significant riverine influence, provide Sr isotope ratios consistent with Pleistocene seawater. In contrast, fossils from shallow marine deposits of a river-influenced zone may exhibit Sr isotope ratios lower than Pleistocene seawater. These anomalous ratios may be due to dilution by low-87Sr/86Sr fluvial waters from the Lamone River that largely drain Tertiary sedimentary rocks. 87Sr/86Sr ratios measured in brackish and freshwater ostracods from backshore sediments are substantially lower than that in Pleistocene seawater and support this interpretation. Sr isotope stratigraphy is an important tool for dating marginal marine sequences; however, caution should be used in applying this technique in shallow marine river-influenced deposits.
Article
ABSTRACf: River sediments are distributed unevenly in space and time, and they are markedly susceptible to human influences. Half the world's river sediment is derived from the Himalayan region and its environs. Most of the remainder is derived from other tectonically active regions such as the western Pacific islands, the Andes, and southern Alaska. River-sediment loads are variable at many time scales: seasonal, annual, decadal, and longer. The storage of sediment in river systems confounds our ability to predict the delivery of sediment to coastal zones. Natural river-sediment loads are increased by deforestation and crop farming, and decreased by dams and reservoirs.
Article
ratios have been determined in modern rivers and freshwater limestones dating from 65 Ma to the present over an area extending from the Pyrénées to the Massif Central (southern France). About 40 Ma ago, the Alpine mountain range of the Pyrénées was strongly uplifted by the Alpine orogenic activity and its pre-Hercynian basement dominated by schists, gneisses and granites was rapidly exposed from beneath its blanket of Mesozoic marine limestones. Meanwhile, the ratio of the rivers has remained remarkably constant in both space (with the exception of the very few northernmost samples) and time: in this area, extreme variations through the orogenic period are small with lying between 0.7073 and 0.7084 at 55–65 Ma and 0.7077–0.7096 for the last 30 Ma. River Sr is controlled by a nearly binary mixing of silicate () and () end-members with negligible influence of evaporites.It is inferred that the silicate/sediment contribution ratio of the world-average rivers to seawater Sr changes at a very slow rate. Even taking major basaltic events into account does not alter this conclusion as their contribution to the river Sr is very subordinated to other components. It is inferred that the seawater secular main trend over the Phanerozoic is controlled by hydrothermal alteration of oceanic basalts and slow variation of the limestone/silicate contribution ratio to the riverine Sr. Oceanic events [changes in organic productivity and carbonate compensation depth (CCD)] may produce intense, short-term variations but recovery of steady-state seawater is delayed by 40–80 Ma through diagenetic Sr remobilization in carbonate sediments.
Article
In 1899, T. C. Chamberlin proposed that the CO2 content of the atmosphere decreased during times of enhanced continental erosion, ultimately resulting in glacial epochs. He ascribed the increase in the rate of chemical weathering (relative to the rate of supply of CO2 from Earth's interior) to increased orogenic activity and globally higher average elevations, which promoted rapid chemical erosion of silicates. The oceanic record of strontium isotopes, preserved in marine sediment, supports his suggestion that glacial climates during the Phanerozoic are in part linked to increases in the rate of global chemical erosion relative to outgassing from Earth's interior. Further, the close correspondence of the major tectonic episodes of the Late Ordovician and Early Silurian, the Devonian, the Carboniferous and Permian, and the late Cenozoic to times of increased continental erosion and glaciation suggests that Chamberlin's hypothesis of the cause of glacial periods should be revived.
Article
Fluctuations in the ratio of seawater accompanying glacial/interglacial climate changes have been reported in recent studies but remain highly controversial. To investigate these potential fluctuations we present very high precision (13 ppm) measurements on planktonic foraminifera from Indian Ocean and Pacific cores. ratios from three different foraminiferal species are indistinguishable from one another in both core-top and 50 ka samples, demonstrating that changes due to diagenesis or contamination do not influence the measurements. Average ratios for Pacific Ocean and Indian Ocean samples are also indistinguishable at three intervals (core-top, 50 ka and 300–370 ka), demonstrating that the oceans have remained well mixed with respect to Sr. Also, ratios are not affected by changes in the precleaning of samples.Measurements from Pacific core V28-238, that used in the study of Dia et al. [1], do not reproduce the cycles seen in the previous study and are statistically well explained by a linear increase in the seawater ratio. It seems likely that an analytical artefact caused the cycles in the previous study. Measurements from two key sections of core from ODP site 758, that used in the study of Clemens et al. [2], do not reproduce the largest shifts seen in the previous study, despite the considerably better precision reported here. The apparent cyclicity in the Clemens et al. [2] data is also suggested to be due to an unknown analytical artefact.The 48 measurements presented here are a statistically good fit to a straight line defined by Δ87Sr(ppm) = −0.0613 × age (ka). At the 13 ppm level of precision no evidence for a glacial/interglacial variation in the seawater ratio is seen. Glacial/interglacial variation may still exist but this data constraints its maximum amplitude to 6–9 ppm, which would correspond to changes in the riverine Sr flux of ≈ 30%.
Article
The 87Sr/86Sr ratio in 53 water samples were analysed, 26 from streams in Sweden, 22 from streams in Finland and 5 from the Bothnian Bay itself. The brackish seawater of the bay had the isotope ratio 0.7095, while the stream-water samples varied from 0.7177 to 0.7366. The weighted average isotope ratio with respect to discharges was 0.7313, this high ratio reflecting the dominance of granitic Proterozoic rocks in the region. For streams with an average discharge between 1-40 m3/s the isotope ratio was within the interval 0.718-0.736, while those with discharges >150 m3/s defined an interval of 0.728-0.735. The variations in isotope ratios are discussed with respect to bedrock geology of drainage basins, possible effects of seawater inundations and Postglacial uplift combined with the complex history of the Baltic Sea.
Article
The estimated path of seawater 87Sr/86Sr during the Cambrian and Ordovician is based on 172 analyses. The sample suite is largely from locations in Oklahoma, New York, Tennessee and Texas where rocks of this age are widely exposed and well studied. The oldest reliable samples examined in this study are of Middle Cambrian age and yield an 87Sr/86Sr near 0.70888 (sw-19). The estimated ratio rises to near 0.70915 (sw+8) in the lower Upper Cambrian, the highest value during the Phanerozoic and probably for all geologic time. The estimated path drifts lower to near 0.70903 (sw-4) at the Cambrian-Ordovician boundary. This further declines to near 0.70875 (sw-32) at the end of the Ibexian and reaches a value near 0.70821 (sw-86) at the Whiterockian-Mohawkian boundary. The decline in the seawater 87Sr/86Sr continues into later Mohawkian time to flatten abruptly near 0.70782 (sw-125). This ratio is essentially constant through the latest Ordovician. The curve rises near to a Ordovician-Silurian boundary value, poorly constrained from our data, near 0.70784 (sw-123). The early Paleozoic seawater curve shows the alternating dominance of continental and oceanic sources of Sr. Continental Sr dominated seawater from the beginning of the Cambrian into Late Cambrian. From that time, as the Cambrian transgression proceeded and radiogenic crystalline rocks were covered, oceanic Sr began to dominate. Sr from low ratio (oceanic) sources dominated through early Mohawkian time. An abrupt balance is struck between oceanic and continental sources to end the decline in the Late Ordovician and maintained until near the end of the Ordovician. The general curve shape follows the pattern suggested by proposed sealevel changes. The rise and fall of seawater 87Sr/86Sr during the Cambrian and Ordovician characterized seawater variation during all the Paleozoic.
Article
An introduction to Sr-isotope stratigraphy can be obtained from several recent reviews (Elderfield, 1986; Veizer, 1989; McArthur, 1991, 1992a). The purpose of this article is not to repeat these reviews but to give to those not expert in the field, but who may be prospective users, a summary of aspects of the method not covered elsewhere, and some examples of recent applications of the technique and the problems that affect it. Highlighting problems inevitably means highlighting the publications in which they occur; I hope the authors will forgive me for spotlighting their work. Neither is this article intended to be a review of all that has been written on the use of Sr isotopes in low-temperature geochemistry; it concentrates on their use in stratigraphy and dating, rather than how they may be used to elucidate geochemical processes. Many excellent papers are thereby excluded from consideration. Implicit in this article is the view that the successful application of strontium isotope stratigraphy requires an application of good methodology in both geochemistry and stratigraphy.
Article
Systematic sampling of the 39 largest Canadian rivers shows that the weighted average ratio in the dissolved load is 0.7111, similar to previous measurements on such large rivers as the Amazon and Mississippi. Consequently, we believe that the above estimate is likely representative of the global average. This imposes a limit of 6.5 × 1011gyr−1 on the amount of Sr exchanged with basalts in hydrothermal cells on mid-oceanic ridges. Evaluation of geological information suggests that Sr from silicate sources is of considerable importance for all but the largest Canadian rivers. The latter have chemical and isotopic composition consistent with ~4:1 carbonate vs. silicate derivation of Sr, but such interpretation is not unique. In terms of their water discharge, the 39 Canadian rivers studied account for 4.2% of the world total and their weighted average concentrations for other dissolved solutes are: TDS 176 ppm, Ca 18 ppm, Cl 6.8 ppm and Sr 84 ppb.
Article
A new generation 87Sr/86Sr isotope curve for the Phanerozoic seawater, based on a compilation of 3635 stratigraphically well defined and well preserved skeletal components, results in a substantially narrower band, with some discrepancies, if compared to the `curve' of Burke et al. (1982), which is based mostly on whole-rock samples. The results obtained from low-Mg calcite shells (foraminifers, belemnites, brachiopods and oysters) are mostly superior to those from phosphatic skeletal components, such as conodonts, even if the latter are exceptionally well preserved (CAI ≤1.5). At this stage, the Phanerozoic data set based on fossils is patchy for the Cambrian, Upper Ordovician–Lower Devonian and Upper Carboniferous–Triassic intervals.The major practical constraint that precludes delineation of an unequivocal Sr isotope curve for the Phanerozoic seawater is the uncertainty in the assignment of absolute ages to the samples. As a result the curve is in reality a band. This is a reflection of the limitations of biostratigraphy and geochronology that are inherited by any derivative correlation and dating technique, including isotope stratigraphy. It is therefore unlikely that, in the near future, the Sr isotope stratigraphy will surpass the resolution capability of biostratigraphy as a global correlation tool. On the other hand, if complemented by lithostratigraphy, it is potentially a valuable tool for correlation of sequences on intrabasinal and regional scales. Taking into account the above limitations, careful consideration must be given to assignment of higher order isotope peaks to specific `events' within a biozone and to the use of such events as a global correlation tool.
Article
The significance of weathering by Himalayan runoff for both the Sr-isotope marine record and the removal of atmospheric CO2 through silicate dissolution has been examined by systematic sampling of dissolved loads and bedloads from the Bhote Kosi, a tributary of the Ganges that rises in Tibet from Tethyan sediment bedrock and traverses the major Himalayan lithologies of eastern Nepal before debouching onto the Gangetic plains. Throughout the section, the cation geochemistry of water samples is dominated by Ca and Mg ions, suggesting that carbonates are the predominant lithology undergoing dissolution particularly within the Lesser Himalayas. As the river transects the metasedimentary and granitic lithologies of the High Himalayas the Sr-isotope ratio of the bedload rises rapidly, closely reflecting the isotope geochemistry of the bedrock. In contrast the 87Sr/86Sr ratio of the dissolved load remains roughly constant (0.719–0.723). Downstream of the Main Central Thrust, where the river transects the carbonate-bearing lithologies of the Lesser Himalayas, the 87Sr/86Sr ratio of the dissolved load rises sharply (>0.768). The relative contributions of silicate and carbonate weathering from each of the main Himalayan units has been estimated from major cation, Sr concentration and Sr isotope mass-balance equations. These calculations suggest that the high Sr and high 87Sr/86Sr characteristics of riverine analyses arise initially from a component dissolved from the Tibetan Sedimentary Series which is substantially enhanced by input weathering fluxes, particularly as the river traverses the Lesser Himalayas. Whilst mechanical erosion is maximised within the High Himalayan Crystalline Series, as confirmed by 143Nd/144Nd ratios from the bedload, at least 63% of the dissolved load is acquired by chemical weathering of bedrock lithologies and/or of transported particulates within the Lesser Himalayas, enhanced by higher ambient temperatures and slower discharge rates. This may involve continued dissolution of the High Himalayan Crystalline Series particulates in addition to Lesser Himalayan lithologies. Although Himalayan rivers collectively have a major influence on the Sr-isotope marine record, the high 87Sr/86Sr ratios of their dissolved load results from the mixing of a small component (<10%) of silicate-derived material with an unusually high 87Sr/86Sr ratio (0.75–1.0) and a large component (>90%) of carbonate-derived material some of which is characterised by a high 87Sr/86Sr ratio (up to 0.8). Elevated 87Sr/86Sr ratios in rivers are therefore not necessarily indicative of anomalously high dissolution rates of silicates.
Article
A study was conducted of the isotopic composition and concentration of Sr and of major elements in dissolved and suspended loads of fresh and brackish waters. The purpose was to establish the contributions of different parent rocks and minerals to Sr during weathering and transport and to identify the role of FeMn oxyhydroxides in the redistribution of Sr in the water column during the sedimentary cycle. Studies were conducted on a profile across an oxic-anoxic boundary in the Baltic and on rivers covering behavior over an annual cycle. In general, the 87Sr/86Sr ratios differ between particulate and dissolved loads, with more radiogenic Sr in the particulate loads. These differences are attributed to differential weathering of minerals, where high Rb/Sr minerals dominate the particulate load and low Rb/Sr the dissolved load. There is broad correlation of 87Sr/86Sr with K/Al in the suspended load. The differences in 87Sr/86Sr between suspended and dissolved load are highly variable and are related to the Fe or Mn concentration on the particulates. In samples with high Fe/Al, the difference becomes small. A good correlation was found between Sr/Al and Fe/Al or Mn/Al in the particulates both in brackish and fresh waters. Sr is removed from solution both in rivers and in the Baltic Sea whenever there is formation of FeMn oxyhydroxide particulates. This precipitation greatly diminishes the difference in isotopic composition of the dissolved and suspended loads. As the particles containing FeMn oxyhydroxides settle, they dissolve in anoxic zones and release Sr. This provides a mechanism for Sr redistribution in the water column. Sr is thus only quasi-conservative in environments where FeMn oxyhydroxides form or dissolve. From consideration of the isotopic differences in Sr between dissolved and suspended loads, it follows that the net Sr input depends upon weathering characteristics of the contributing mineral phases. Changes in weathering mechanisms due to climate change may cause Sr isotopic shifts in the marine environment.
Article
We report the isotopic composition and concentration of Nd and Sr in the dissolved load of rivers from North America, Australia, Japan, the Philippines and Venezuela. The data are used to: (1) characterize the behavior of Nd and Sr during weathering and river transport; (2) estimate the concentration and isotopic composition of Nd and Sr in river runoff to the oceans; and (3) construct an isotopic mass balance for Nd and Sr in seawater. In general, Nd is mobilized in rivers of low pH, whereas Sr concentrations in rivers are determined by the degree of chemical weathering of source rocks. The isotopic composition of Nd and Sr in river waters is strongly influenced by the age of materials in a drainage basin, and this results in a rough overall inverse correlation between and ϵNd. Preferential weathering of marine precipitates, such as carbonates and phosphates, significantly affects the , and to a lesser degree, the ϵNd of river waters. Established geochemical relationships and data in the literature are used to estimate the weighted average ϵNd of the primary river runoff to each of the ocean basins. New Sr isotope data for rivers (including new island arc data) lower the estimated mean of river runoff and reduces the calculated seawater-basalt Sr exchange near mid-ocean ridges. This yields a hydrothermal water flux of (2.9 ± 2.5) · 1016 g yr.−1. It is shown that river inputs are the dominant factor in determining the Nd and Sr isotopic signature of modern seawater.
Article
We have measured the Sr concentration and isotopic composition of most of the world's major rivers. These data greatly extend the range of geologic, tectonic and climatic drainage basin environments that have been studied and, overall, constitute a doubling of the fluvial Sr isotope data base. This has produced a more accurate estimate for the87Sr/86Sr ratio of global runoff of 0.7119; this is distinctly higher than obtained from previous studies. We have also determined the Sr concentration and87Sr/86Sr ratio of oceanic hydrothermal vent waters from diverse tectonic and volcanogenic environments that have allowed us to further constraint the Sr isotope systematic of this source. These new data have been combined with previously published riverine data and values for the diagenetic benthic flux of Sr from deep-sea sediments to obtain a contemporary oceanic Sr isotope budget. The results of steady-state calculations give a best estimate for the flux of seawater through high-temperature ridge crest hydrothermal system of1.2 ± 0.3 × 1014 kg/yr; this is substantially higher than recent estimates.
Article
The time evolution of the isotopic composition of seawater strontium (the ratio of strontium-87 to strontium-86) over the last 500 million years has the form of an asymmetric trough. The values are highest in the Cambrian and Recent (0.7091) and lowest in the Jurassic (0.7067). Superimposed on this trend are a number of smaller oscillations. Consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only Himalayan-style continental collisions can influence the isotope ratio on the scale observed. The contemporary Himalayan orogeny is by far the largest since the late Precambrian Pan-African event that produced the high in the Cambrian.
Strontium geochemistry in the estuarine mixing zone of the Mississippi River
  • Y Xu
Xu, Y., 2004. Strontium geochemistry in the estuarine mixing zone of the Mississippi River. PhD thesis, Tulane University, 124 p.
86 Sr in runoff from freshwater limestones of southern France
  • Sr
Sr/ 86 Sr in runoff from freshwater limestones of southern France. Chemical Geology 64, 55–65.
Effects of quaternary sea level cycles on strontium in seawater
  • Stoll
Strontium and its isotopes in Canadian river: fluxes and global implications
  • Wadleigh