Dynamique sédimentaire, érosion physique et altération chimique dans le système himalayen

Abstract : Chemical weathering of the earth crust supplies the essentiel elements for numerous biogeochemical cycles. Physical erosion of large orogens, such as the Himalayan range, is accompanied by significant weathering fluxes possibly affecting the global environment. The objective of this PhD is to understand how surface processes affect river sediment properties in order to asses current erosion and weathering rates but also to decipher their past variations. To answer this question we studied the transport dynamics, the physical and the geochemical characteristics of the sediments in the Ganga basin. This study suggests that about 10% of the flux eroded in the Himalayas is currently stored in the Ganga floodplain. By comparing this flux to the weathering flux of the Himalayan range, we show that River sediments are progressively depleted in the most mobile elements, as weathering proceeds during transfer in the floodplain. By comparing this flux to the weathering flux of the Himalayan range, we show that floodplain weathering is predominant in weathering Himalayan sediments. Cation exchange occuring when Ganga and Brahmaputra (G&B) sediments enter the marine environment are limited and enhances the long term carbon storage, linked to silicate weathering by only ca. 20%. Finally, the Bay of Bengal sedimentary record, which documents the last 20 000 years of Himalayan erosion shows that the sediments exported during the last glacial maximum (LGM) were significantly less weathered compared to the sediments currently exported. The Himalayan system is thus not buffered towards the high frequency climate forcing changes of the Quaternary and modern weathering rates cannot easily be extrapolated over the past.
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Maarten Lupker. Dynamique sédimentaire, érosion physique et altération chimique dans le système himalayen. Sciences de la Terre. Institut National Polytechnique de Lorraine, 2011. Français. ⟨NNT : 2011INPL038N⟩. ⟨tel-01749558⟩

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