8 results found
The transport of warm and salty waters from the Indian Ocean to the South Atlantic by the Agulhas Current constitutes a key return route of the meridional overturning circulation. Despite, the importance of the Agulhas Leakage on interoceanic exchange, its role on biogeochemical cycles is poorly documented. Here, we present the first lead (Pb) concentration and isotope data for surface seawater collected during the GEOTRACES cruise D357 in the Agulhas current system. Lead in surface waters of the Cape Basin is described by three distinct endmembers: the South African coast, open South Atlantic seawater, and Indian Ocean seawater. The latter stands out in its Pb isotopic composition and can be tracked within two distinct Agulhas rings. High Pb concentrations in the Agulhas rings further corroborate an Indian Ocean provenance of waters, and suggests that the Agulhas Leakage not only represents a major conduit for heat, but also for trace metals.
Paul M, Reisberg L, Vigier N, et al., 2011, Behavior of osmium at the freshwater–saltwaterinterface based on Ganga derived sedimentsfrom the estuarine zone, Geochemistry Geophysics Geosystems, Vol: 12, Pages: 1-13
Leaching experiments performed on several sediments of the Ganga river suggest that between 10 and 20% of the osmium is in an easily exchangeable position. Analyses of sediments from two estuarine rivers (the Pussur and the lower Meghna) reveal no enrichment of Os in the salt/fresh water mixing zone relative to Ganga sediments of similar Al2O3/SiO2 ratio, suggesting that osmium is not significantly trapped in this estuary. However, a significant decrease of the 187Os/188Os ratio is observed in the Pussur sediments relative to the Ganga composition. These latter are derived entirely from the Ganga or from erosion of the Ganga paleodelta, and thus would be expected to have similar Os isotopic compositions. Nd isotopic results from the Pussur are indistinguishible from those of the Ganga, while the Sr isotopic results are at the lower end of the Ganga range, confirming the absence of a major source difference between Ganga and Pussur sediments. It thus seems unlikely that the difference in Os isotopic signature can be entirely explained by a change in provenance, suggesting instead that the Os compositions have been modified. Our results show that the less radiogenic Os compositions of the Pussur sediments cannot result simply from desorption of radiogenic Os or from scavenging of river or seawater Os. Instead, the decrease of the 187Os/188Os ratio could imply a complex exchange between dissolved Os, derived partly from seawater, and Os in the leachable fraction of sediments. This mechanism could therefore constitute both a source and a sink for seawater osmium and may significantly influence the osmium marine budget.
Paul M, Reisberg L, Vigier N, et al., 2010, Dissolved osmium in Bengal plain groundwater: Implications for the marine Os budget, Geochemica et Cosmochemica Acta, Vol: 74, Pages: 3432-3448
We report osmium concentrations and isotopic compositions of 40 groundwater samples from the Bengal plain. Groundwaters have Os concentrations (16.9–191.5 pg/kg), about 5–10 times higher than those published for most rivers or seawater. 187Os/188Os varies widely (from 0.96 to 2.79) and is related to the isotopic signatures of the sediments constituting local aquifers. Os contents are correlated with those of soluble elements such as Sr, Mg, and Ca, suggesting that differing extents of solid-solution interaction explain most of the variation in measured Os concentrations. The covariation between Os and Sr allows us to estimate the mean Os content of Bengal groundwater (not, vert, similar70 pg/kg). This concentration is too low to allow Bengal groundwater to significantly influence the marine Os isotopic composition, if likely fresh groundwater discharge rates to the Bay of Bengal are assumed. However, if Bengal groundwater Os concentrations are typical, the global Os groundwater flux would be expected to be around 180 kg/year, making it the second largest input of Os to the ocean after the river flux. Including this flux in the current Os marine budget, and assuming that this and other fluxes have remained constant with time, would decrease the calculated residence time of Os in the ocean by about 30%.
Paul M, Reisberg L, Vigier N, 2009, A new method for osmium isotopic measurement of water samples, Chemical Geology, Vol: 258, Pages: 136-144
We describe a new method for the determination of osmium (Os) content and 187Os/188Os isotopic ratio in water samples, particularly adapted to analysis of reducing waters such as groundwaters. The critical feature of the method is the improved oxidation step. We use a high pressure asher (HPA-S) to achieve oxidation of all Os species at high pressure (~130 bar) and high temperature (250°C). A series of tests were performed on two groundwater samples in order to study the general behaviour of Os during oxidation and to optimize the various parameters of the method. At high temperature (≥250°C), using hydrogen peroxide as an oxidant, complete spike-sample equilibration and oxidation of Os species to the highest oxidation state (OsO4) is obtained in less than 10 hours. A strong contrast in oxidation behavior was observed between tests performed at high (250°C) and low temperature (100°C). The results for both samples indicate a difference in the oxidation kinetics between spike and sample. They also demonstrate that a non negligible proportion of each sample is easily oxidized at low temperature whereas high temperatures are required to oxidize the rest of the sample. This last observation could provide evidence for the existence of several species of osmium in groundwater samples. Our results underscore the need for high temperatures to assure spike-sample equilibration and complete oxidation of all Os species potentially present in water. The method presented here, which is adapted to various matrices, opens the possibility of analyzing Os concentrations and isotopic compositions in groundwater and thus to better constrain the potential sources of osmium delivered to the ocean.
Paul M, Reisberg L, Vigier N, et al., 2008, Os behaviour in groundwater and its effect on the Os residence time of the ocean, International Geological Congress
We have investigated Os behavior in groundwater sampled at different locations in the Bengal plain. Bangladesh groundwater chemistry is dominated by carbonate weathering, but also includes a variable seawater contribution. Redox conditions are mainly anoxic, and the analyzed samples display highly variable iron and arsenic contents. Due to the reducing conditions and the presence of several Os species, efficient spike-sample equilibration, and thus accurate Os concentrations, are difficult to attain by conventional methods. Therefore, Os contents and 187Os/188Os ratios were determined by a method specifically developed for groundwater samples (heating with H2O2, in a high pressure asher HPA-S at 250°C and ∼120 bars). The high temperatures allow complete Os oxidation and thus optimal spike-sample equilibration to be obtained in a few hours for different aqueous matrices (seawater, groundwater and river water).The analyzed groundwaters have very high Os concentrations (30-250 pg/L) compared to those of seawater and river water (∼10pg/L). Surprisingly, Os content variations are not linked with those of iron and redox sensitive elements but instead are correlated with Sr and Ca concentrations. This may indicate that Os contents in groundwater are not controlled by dissolution or precipitation of iron and manganese oxide/hydroxide, but instead behave like most moderately soluble elements. 187Os/188Os isotopic ratios vary greatly (from 0.9 to 2.60) and strongly depend on the sampled area. Aquifers located in the active and inactive Ganges delta are highly radiogenic (187Os/188Os between 1.80 and 2.60) while the Brahmaputra and Meghna flood plains have 187Os/188Os ratios close to those of typical continental crust. This isotopic contrast corresponds to that found in sediments carried by the Ganges and Brahmaputra rivers. Therefore, the Os isotopic ratios observed in Bengal groundwater probably reflect those of the corresponding sediments at depth.High Os conten
Najman Y, Bickle M, BouDagher-Fadel M, et al., 2008, The Paleogene record of Himalayan erosion: Bengal Basin, Bangladesh, Earth and Planetary Science Letters, Vol: 273, Pages: 1-14
A knowledge of Himalayan erosion history is critical to understanding crustal deformation processes, and the proposed link between the orogen's erosion and changes in both global climate and ocean geochemistry. The most commonly quoted age of India–Asia collision is ~ 50 Ma, yet the record of Paleogene Himalayan erosion is scant — either absent or of low age resolution. We apply biostratigraphic, petrographic, geochemical, isotopic and seismic techniques to Paleogene rocks of the Bengal Basin, Bangladesh, of previously disputed age and provenance. Our data show that the first major input of sands into the basin, in the > 1 km thick deltaic Barail Formation, occurred at 38 Ma. Our biostratigraphic and isotopic mineral ages date the Barail Formation as spanning late Eocene to early Miocene and the provenance data are consistent with its derivation from the Himalaya, but inconsistent with Indian cratonic or Burman margin sources. Detrital mineral lag times show that exhumation of the orogen was rapid by 38 Ma. The identification of sediments shed from the rapidly exhuming southern flanks of the eastern–central Himalaya at 38 Ma, provides a well dated accessible sediment record 17 Myr older than the previously described 21 Ma sediments, in the foreland basin in Nepal. Discovery of Himalayan detritus in the Bengal Basin from 38 Ma: 1) resolves the puzzling discrepancy between the lack of erosional evidence for Paleogene crustal thickening that is recorded in the hinterland; 2) invalidates those previously proposed evidences of diachronous collision which were based on the tenet that Himalayan-derived sediments were deposited earlier in the west than the east; 3) enables models of Himalayan exhumation (e.g. by mid crustal channel flow) to be revised to reflect vigorous erosion and rapid exhumation by 38 Ma, and 4) provides evidence that rapid erosion in the Himalaya was coincident with the marked rise in marine 87Sr/86Sr values since ~ 40 Ma. Whether
Paul M, Reisberg L, Vigier N, et al., 2007, Osmium groundwater flux from the Bengal basin: implications for the osmium marine budget, Goldschmidt Conference
The Os residence time in the oceans is currently poorly constrained, in part because the sources of Os to ocean water have not yet all been explored. One such source, potentially important, may be groundwater flux. Basu et al.  suggested that Bengal Basin groundwaters may provide a large flux of radiogenic Sr to the oceans. By analogy, and considering that the Ganges has the most radiogenic 187Os/188Os ratio yet measured in the world's rivers , groundwater of the Ganges alluvial plain may provide a significant flux of radiogenic Os to the oceans. We present Os isotopic and concentration data for groundwaters collected along a travese in the Bengal alluvial plain. As these waters are highly reducing, a specific method was developed (heating with CrVI in a high pressure asher, HPA-S at 250°C) to allow complete oxidation of all of the Os, and thus equilibration with the Os isotopic tracer. Oxidation at lower temperatures, in the HPA-S or in teflon vessels , led to underestimation of the Os concentration by as much as a factor of two.Os concentrations are high (30-250 pg/l) and display 187Os/188Os ratios (2.2-2.6) in agreement with those of Ganges water and sediments. Os contents correlate strongly with those of Sr and several other elements, which allows us to roughly estimate the groundwater Os flux from the Bengal alluvial plain. Assuming conservative behaviour and both the mean Sr concentration and the groundwater flux of , we estimate an 188Os groundwater flux of ~ 9 M/yr. This estimate represents more than three times the Os flux from the Ganges and a radiogenic contribution similar to that of the Amazon . If realistic, this estimate would significantly reduce the estimate of the Os residence time in the ocean, and may suggest that Himalayan derived Os has had an important effect on the 187Os/188Os ratio of seawater. On the other hand, prelimimary results on groundwaters from the fresh/salt water transition area show that a substantial quan
Paul M, Reisberg L, Vigier N, et al., 2006, Sediment sources and transport in the Ganga-Brahmaputra basin: Information from Os isotopes, Goldschmidt Conference
We present Re–Os isotopic results for sediments (bank deposits, suspended matter and bedload) collected in various locations in the Ganga–Brahmaputra basin. A radiogenic signature (187Os/188Os ≈ 2.8) is observed for Himalayan tributaries (Gomati, Ghaghara, Gandak, Kosi), underscoring the importance of weathering of Lesser Himalaya black shales (Pierson-Wickmann et al., 2000). 187Os/188Os increases markedly in the downstream direction along the Ganga (from 1.3 to 1.5 at Varanasi Ghat, India, to 2–2.5 at Harding bridge, Bangladesh) suggesting a large contribution of radiogenic osmium from Himalayan tributaries. For the Brahmaputra, our new results confirm the non-radiogenic signature determined by Singh et al. (2003), indicating a significant contribution from ultrabasic rocks.Two tidal rivers (Pussur and lower Meghna) were also analyzed. The Pussur sediments, influenced only by the Ganga, have a mean 187Os/188Os ratio of 1.7. This value is substantially lower than that of the nearest measured Ganga sediments (2–2.5), suggesting remobilization and/or exchange with seawater osmium (187Os/188Os not, vert, similar 1.05). The 187Os/188Os ratio of the lower Meghna sediments (≈1.1) is consistent with both mixing between Ganga, Brahmaputra and Meghna river sediments, with a dominant contribution from the Brahmaputra, and a seawater contribution.Depth profiles of suspended matter were carried out at Harding bridge (Ganga) and at Sirajganj (Brahmaputra). Both profiles demonstrate a systematic 187Os/188Os variation with depth. However, 187Os/188Os ratios decrease with depth in the Ganga, but increase with depth in the Brahmaputra. This suggests that in the Ganga, the finest particles are enriched in material derived from Lesser Himalaya black shales. In contrast, in the Brahmaputra, the finest particles are enriched in the erosional products of mafic and ultramafic terrains. More generally, a better understanding of the Os isotopic variation
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