HR: 1340h
AN: B23B-1269 INVITED    [Abstracts]
TI: Anthropogenic Increase Of Soil Erosion In The Gangetic Plain Revealed By Geochemical Budget Of Erosion
AU: * Galy, V
EM: vgaly@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, B.P. 20, Vandoeuvre, 54501, France
AU: France-Lanord, C
EM: cfl@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, B.P. 20, Vandoeuvre, 54501, France
AU: Galy, A
EM: albert00@esc.cam.ac.uk
AF: University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: Gaillardet, J
EM: gaillardet@ipgp.jussieu.fr
AF: IPGP, 7, place Jussieu, Paris, 75252, France
AB: Tectonic and climatic factors are the key natural variables controlling the erosion through complex interactions. Nonetheless, over the last few hundred years, human activity also exerts a dominant control in response to extensive land use. The geochemical budget of erosion allows the balance between the different erosion processes to be quantified. The chemical composition of river sediment results from the chemical composition of the source rock modified by (1) weathering reactions occurring during erosion and (2) physical segregation during transport. If erosion is at steady state, the difference between the chemical composition of source rocks and that of river sediments must therefore be counterbalanced by the dissolved flux. However, climatic variations or anthropic impact can induce changes in the erosion distribution in a given basin resulting in non steady state erosion. Using a mass balance approach, the comparison of detailed geochemical data on river sediments with the current flux of dissolved elements allows the steady state hypothesis to be tested. In this study, we present a geochemical budget of weathering for the Ganga basin, one of the most densely populated basin in the world, based on detailed sampling of Himalayan rivers and of the Ganga in the delta. Sampling includes depth profile in the river, to assess the variability generated by transport processes. Himalayan river sediments are described by the dilution of an aluminous component (micas + clays + feldspars) by quartz. Ganga sediments on the other hand correspond to the mixing of bedload, similar to coarse Himalayan sediments, with an aluminous component highly depleted in alkaline elements. Compared with the dissolved flux, the depletion of alkaline elements in Ganga sediments shows that the alkaline weathering budget is imbalanced. This imbalance results from an overabundance of fine soil material in the Ganga sediment relative to other less weathered material directly derived from Himalaya. Based on the average composition of the suspended load and of floodplain soils, we estimate that 250x10⁶ t/yr i.e. 5 t/ha/yr is eroded from soil surfaces of the Ganga floodplain. This enhanced soil erosion is likely triggered by intense deforestation and change in land use due to increasing human activity in the basin.
DE: 1030 Geochemical cycles (0330)
DE: 1039 Alteration and weathering processes (3617)
DE: 1065 Major and trace element geochemistry
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1815 Erosion
SC: Biogeosciences [B]
MN: 2007 Fall Meeting