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Mukul Sharma Current Research Interests in Isotope Geochemistry

Near-Surface Geochemistry of Osmium:

Seawater Os is derived from continental weathering, hydrothermal alteration of oceanic crust and from the dissolution of cosmic dust/micrometeorites. On average, 187Os/188Os ratio of MORB and cosmic material are nearly identical (~0.130) and about ten times lower than that of continental matter (~1.26). Therefore, variations in marine osmium isotopic composition could potentially provide quantitative insights into (1) the changes in the paleo-weathering rates, (2) the variations in the cosmic dust flux to the earth, and (3) glacial-interglacial variations in the source areas of eolian dust. Understanding the aqueous geochemistry of osmium, however, had been an intractable problem due to its low abundance and multiple oxidation states in natural waters. By developing a new osmium separation technique and combining it with highly sensitive negative thermal ionization mass spectrometry, I have been able to measure the concentration and isotopic composition of osmium in the oceans, rivers, and hydrothermal fluids. These results are very exciting and show that (1) the mean residence time of osmium in the oceans is about 40,000 years and roughly 80% of Os in the oceans is derived from the continents and the rest from hydrothermal alteration of basalt and dissolution of cosmic dust, (2) contrary to the general belief the Himalayan erosion has not provided sufficient osmium to affect the osmium evolution of the oceans, and (3) there is no net flow of osmium into the oceans via high temperature alteration of oceanic crust. This work has paved the way to examine in detail the geochemistry of Os in natural waters. I am working on the following problems:

(1) Determination of the behavior of Os during sub-marine alteration of basalt: The unradiogenic fraction of Os in seawater is derived from alteration of oceanic crust and via dissolution of cosmic dust/micrometeorites. A detailed understanding of the present-day balance between these sources is necessary to interpret the past variations in seawater Os isotopic composition. My work has demonstrated that the high temperature alteration of oceanic crust does not provide significant amounts of unradiogenic osmium to seawater (Fig. 1). The low temperature alteration, on the other hand, may supply an overwhelming amount of Os. The key issue then is to what extent the low temperature alteration of oceanic crust contributes Os to seawater. This has a direct bearing on the question of the cosmic dust flux to the earth. I will measure Os isotopes in low temperature hydrothermal fluids from several geologically significant settings to investigate the extent to which low temperature alteration of basalts contributes Os to the oceans.

Figure 1. Inverse of 188Os versus 187Os/188Os diagram showing that one sample (Monolith 1991) plots on the mixing line between seawater and a component with 187Os/188Os = 0.11. All other samples define a nearly horizontal array that can be attributed to nearly quantitative precipitation of Os during the ascent and cooling of the hydrothermal fluids. The data when combined with the estimated mass flow of black smoker fluids show that the high temperature alteration of MORB cannot supply unradiogenic fraction of Os dissolved in the oceans. Only the low temperature hydrothermal fluid sample appears to be enriched in Os compared to seawater (Sharma et al., 2000).

(2) Osmium and Iridium transport through estuarine environment: The Os/Ir ratios of meteorites and all major terrestrial reservoirs are close to 1.0. In contrast, enromous fractionation between Os and Ir is observed in the deep oceans (Os/Ir = 65). The Ir data from the Baltic Sea in combination with those of the rivers and the deep oceans appear to suggest substantial scavenging of Ir in the estuary. On the other hand, the concentration of Os in the rivers is quite similar to that of the deep oceans (= 55 femtomol per kg). It follows that the high Os/Ir ratio of the deep oceans is a direct consequence of the different rain out mechanisms of these elements in the estuaries or in the oceans. Studies suggest that, during transport through estuaries, Ir is quantitatively scavenged by Fe-oxyhydroxides and Os removal is closely associated with that of Al and organic matter. If true, the above observations suggest that the process of fractionation of Os and Ir through the estuarine filter may play a dominant role in influencing the Os/Ir ratio of deep oceans. I will investigate the Os and Ir removal in estuaries along a salinity gradient in order to establish the behavior of these elements as the river plume mixes with seawater.

(3) Osmium isotopes of the Arsenic-enriched groundwater of Bangladesh: The occurrence of high arsenic in the alluvial aquifers in the Bengal Delta Plain (BDP) in Bangladesh as well as in the east-Indian state of West Bengal has emerged as an issue of great concern during the past decade because of its impact on the health of a large cross section of the population, reliant on groundwater for potable supplies. The aquifers from which the arsenic-rich groundwater is derived are the Holocene sediments comprising sequences of sand, silt and clay deposited and subsequently reworked by the meandering Ganges-Brahmaputra-Meghna river system. Throughout the Quaternary period, these have carried sediment down gradient from the Bihar-Rajmahal hills and the Himalayas and deposited in the BDP of both Bangladesh and West Bengal. Two principal contradictory hypotheses have been proposed concerning the origin of arsenic in groundwater in the BDP: (1) arsenic is derived from the oxidation of As-rich pyrite in the shallow aquifer as a result of lowering of water table due to overabstraction of groundwater for irrigation, (2) arsenic is derived as a result of reductive desorption from surface reactive mineral phases such as iron oxyhydroxides, present ubiquitously as coatings on the aquifer sediments in response to changes in the redox conditions, caused by return flow from rice fields and/or wetland cultivation as well as due to overabstraction of groundwater. In collaboration with Dr. P. Bhattacharya (Royal Institute of Technology, Sweden) I am investigating the Os isotopes in selected groundwater samples with variable As-enrichment and from different parts of the BDP. This study will be the first of its kind. The objectives of this study are two fold: (1) tracing the sources of Os and possibly As given that the Os derived from arsenopyrite will have a different isotopic composition compared to that derived from iron-oxyhydroxide, (2) deducing the present-day Os flux into the oceans from modern-deltas.

(4) Behavior of Re, Os, and Pt during weathering and soil formation: coupled 187Re-187Os, 190Pt-186Os systematics of laterites: There is, at present, a large uncertainty in our understanding of the behavior of Re, Pt and Os during weathering and soil formation. This issue is intimately related to the 187Os/188Os and 186Os/188Os ratios of the average eroding upper continental crust and, in turn, affects our ability to interpret the long-term variations in seawater Os isotopic record. The basic question in hand is to what extent does the erosion of different landmasses influence the seawater Os isotopic composition. Laterites cover more than one-third of the exposed continental surface with laterite-capped landforms occurring extensively in the humid tropical regions (30° N to 30° S). Given that the tropics supply >60% of the annual water input to the oceans, an understanding of the laterite genesis and evolution is important in evaluating the fluxes of elements from the continents into the oceans. I will continue to investigate the behavior of Re and Os during laterite genesis. My initial investigations have revealed that (1) in contrast to Os, Re is mobile under oxidizing conditions and is released during weathering and consequently the Re/Os ratio of the topsoil is ~0, (2) depletion of Re and enrichment of Os in the topsoil may be an important feature of many lateritic soils (3) precipitation of Fe-Mn-Al oxyhydroxides and interaction with organic acids may control Os enrichment in the topsoil, and (4) weathering and onset of Re leaching occurs very early and consequently ancient lateritic soils may not develop highly radiogenic 187Os/188Os ratios with time. The decoupling between Re and Os can be used to derive model ages for individual soil horizons (Sharma et al., 1998a). In addition to the Re-Os systematics, I will also investigate the extent to which Pt and Os are fractionated from each other during laterite genesis.

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