Étude des isotopes de l'osmium dans les eaux souterraines du Bangladesh et les sédiments himalayens : implications et rôle de l'érosion himalayenne sur le budget océanique de l'osmium

Abstract : Using the Himalayan example, this study documents the systematics controlling the osmium composition of sediments during weathering processes, sedimentary transport and sediment deposition in tidal areas. The relationships observed between osmium concentrations and organic carbon contents in sediments of the Himalayan Kali Gandaki catchment underscore the role of black shales from the TSS (mostly unradiogenic) and LH (highly radiogenic) Himalayan units, despite their limited geographic distribution. However, the highly radiogenic composition displayed by Ganges sediments is not coupled to an enrichment in 187Os in these sediments, which on average are comparable to that of typical continental crust. Instead, the Ganges radiogenic signature results from an impoverishment in non-radiogenic osmium, reflecting a strong dilution by erosion products of crystalline rocks of the HHC unit. Moreover, based on the study of rivers in the tidal zone, we document the complex behavior of osmium at the salt/fresh water transition, potentially involving exchange between sediments and dissolved osmium. Analytical development performed concurrently with the sediment studies allowed the first measurements of groundwater osmium compositions. Groundwaters of Bengal plain aquifers have osmium contents significantly higher than those previously documented for river water or seawater. If this result can be generalized to other aquifers a global osmium groundwater flux to the ocean of about 170 kg per year could be expected. This contribution is significant and would require a reevaluation of both the osmium marine budget and the residence time of osmium in the ocean. This result could partially reconcile the diverging marine Os residence times estimated from mass balance and from glacial-interglacial variations in the 187Os/188Os marine record
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Maxence Paul. Étude des isotopes de l'osmium dans les eaux souterraines du Bangladesh et les sédiments himalayens : implications et rôle de l'érosion himalayenne sur le budget océanique de l'osmium. Sciences de la Terre. Institut National Polytechnique de Lorraine, 2008. Français. ⟨NNT : 2008INPL031N⟩. ⟨tel-01753031⟩

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