HR: 15:10h
AN: H53E-07 INVITED     [Abstracts]
TI: Climate, weathering, transport and sedimentary record during Himalayan erosion
AU: * France-Lanord, C
EM: cfl@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, BP 20, Vandoeuvre les Nancy, 54501 France
AU: Singh, S K
EM: sunil@prl.ernet.in
AF: CRPG-CNRS, BP 20, Vandoeuvre les Nancy, 54501 France
AU: Singh, S K
EM: sunil@prl.ernet.in
AF: Physical Research Laboratory , Navrangpura, Ahmedabad, 380 009 India
AB: The Himalayan system is one where climate and tectonic both play dominant controls on erosion and sedimentary fluxes. Deciphering climatic changes from the sedimentary record are however not straightforward and the study of the modern erosion system is the main source of interpretation. The comparison of the Brahmaputra and Ganga rivers is very interesting as they reflect very contrasted climatic conditions. Sediments of the Brahmaputra show apparent signals of lower weathering than those of the Ganga. Clay assemblages of the Brahmaputra contain no smectite and some kaolinite whereas those of the Ganga are rich in those secondary phases. Similarly, chemical indexes of alteration are lower for the Brahmaputra than for the Ganga. Such weathering indices tend to suggest that weathering hence climate is more active and humid over the Gangetic basin than the Brahmaputra. However, this led to a complete misinterpretation of the reality. The monsoon is much more intense over Eastern than Western Himalaya as shown for instance by a much higher runoff for the Brahmaputra than the Ganga. Based on modern dissolved chemistry of both rivers, chemical erosion of silicates and carbonates of the Brahmaputra is about double of that of the Ganga. It is important to take into account his apparent contradiction between river sediment and dissolved element budget when interpreting the sedimentary record in term of climate conditions. Because Himalaya is a system where physical erosion is by far the dominant process particularly on it's eastern side, the sediments of the Brahmaputra appears less matured than those of the Ganga. For the Ganga, lower precipitation level induces lower physical erosion flux and longer residence time in the floodplain; then relatively arid condition in the floodplain generates smectite. On the contrary, Brahmaputra climate favours fast transport of particles and reduced processing in the floodplain. Sediment transport processes generate a second source of uncertainty. Mineralogical sorting due to the transport dynamics induces geochemical differences. Finer grain sediments appear with higher level of weathering than coarser fractions of the same sedimentary flux. We report this in the river system as well as in the sedimentary record.
DE: 3655 Major element composition
DE: 1815 Erosion and sedimentation
DE: 1045 Low-temperature geochemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting