Érosion, altération et paléo-altération en Himalaya
Abstract :
Continental weathering and erosion are fundamental components of global geochemical cycles on earth, facilitating the transfer of atmospheric CO2 to sedimentary reservoirs through alteration of silicates and transfer of organic carbon. In the Himalayas, a region characterized by kinetically limited weathering, the interplay between monsoon intensity and erosion significantly influences the intensity of weathering. Thus, a thorough quantitative evaluation of the impact of Himalayan erosion on the carbon cycle is imperative for understanding the role of this vital orogenic region. The two major rivers Ganga and Brahmaputra draining the Himalayas, act as the major contributors of sediments to the Bangladesh delta and the Bengal Fan. These sediments are exported and deposited by turbidity currents from the delta to the deep fan. Their accumulations provide an historical record of the Himalayan paleo-erosion. The chemical composition of these sediments inherits the source rock compositions and effects of weathering and mineralogical sorting during transport and depositional processes. To quantify the Himalayan paleo erosion, this study focuses on establishing a refined quantitative estimate of the Himalayan silicate weathering intensity. We use the Neogene and Quaternary sediment record retrieved by the IODP expedition 354 which drilled a seven site E-W transect at 8º N to address the objective using a source to sink approach.
The geological formations drained by the Ganga and Brahmaputra rivers exhibit varying degrees of crustal maturity and chemical compositions and hence demands meticulous assessment of the sediment sources. We consider the Neodymium isotopes to trace the sediment sources since they are less affected by sorting and demonstrate an efficient tracing of the provenance. In this study, we employ a controlled pretreatment with acetic acid in order to eliminate the carbonate phases of the sediments and analyse the major and trace element composition silicate phases of the sediments.
The compositions of the source rocks are established by a mixture between a Himalayan endmember and a Trans-Himalayan endmember derived from the modern river sediments of the Ganga and Brahmaputra. The chemical difference between the estimated bedrock and the exported sediment are then used to evaluate the weathering loss of critical cations. Normalization rules have been adopted to account for the effect of grain size sorting, which allows comparing sediments ranging from sand to fine silt.
This approach indicates comparable loss of major silicate cations in the Bengal fan sediments compared to those in the modern system during the Plio-Pleistocene period. On the contrary, the Miocene record shows a depletion of 0.1-0.3 mol/Kg of Na and 0.2-0.4mol/kg of Casil, implying an intense plagioclase weathering, reflecting a regime of higher degree of weathering. At the scale of Neogene such a response does not correlate with the available information on the rainfall and erosion rates. Among the probable factors, higher temperature during the Miocene and changes in floodplain extension can explain such a response. The late Pleistocene record from the Bengal Fan shows a response to the climatic changes highlighting an enhancement in the chemical weathering of the silicates from the Last Glacial Maximum to the present.
Along with the enhanced silicate weathering, high proportions of detrital carbonates are also observed during the Miocene which reflects a higher proportion of carbonates in the source rock. This thesis provides a comprehensive review of these findings, based on estimates on the paleoweathering of silicates in the Himalaya, along with an analysis of associated uncertainties.
Keywords: Paleoweathering, Himalaya, Bengal fan
Composition du Jury
Directeurs de Thèse
Julien Charreau, Professeur, CRPG, Université de Lorraine-CNRS, Nancy
Christian France-Lanord, DR, CRPG, Université de Lorraine-CNRS, Nancy
Rapporteurs :
Jérôme Gaillardet, Professeur, Institut de Physique du Globe de Paris-CNRS
Pascale Huyghe, Maître de Conférence (HDR), ISTERRE Université de Grenoble-Alpes-CNRS,
Examinateurs :
Emmanuelle Pucéat, Professeure , Biogéosciences, Université de Bourgogne-CNRS Dijon
Mara Limonta, CR Postdoc, CRPG, Université de Lorraine-CNRS, Nancy
Aude Gébelin, Professeure. , Géoressources, Université de Lorraine-CNRS, Nancy
Louis Derry, Professeur, Cornell University, Ithaca NY USA, IPGP Paris.