HR: 1340h
AN: H43C-0387    [Abstracts]
TI: Sr Fluxes from the Himalayas: Calculation of Carbonate and Silicate Inputs
AU: * Bickle, M
EM: mb72@esc.cam.ac.uk
AF: Dept. Earth Sciences, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Chapman, H
EM: HJC1000@esc.cam.ac.uk
AF: Dept. Earth Sciences, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Bunbury, J
EM: jmb21@esc.cam.ac.uk
AF: Dept. Earth Sciences, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Harris, N
EM: N.B.W.Harris@open.ac.uk
AF: Dept. Earth Sciences, Open University,, Milton Keynes, MK7 6AA United Kingdom
AU: Fairchild, I
EM: i.j.fairchild@bham.ac.uk
AF: School of Geography, Dept. Earth & Environmental Sciences, Egbaston,, Birmingham, B15 2TT United Kingdom
AU: Ahmad, T
EM: tahmad001@yahoo.co.in
AF: Dept. Geology, University of Delhi, Charta Marg, Dehli, 110007 India
AU: Pomi\`{e}s, C
EM: cpom01@esc.cam.ac.uk
AF: Dept. Earth Sciences, Downing Street, Cambridge, CB2 3EQ United Kingdom
AB: The marked increase in seawater $^{87}$Sr/$^{86}$Sr ratio since 40 Ma suggests that enhanced physical erosion associated with the Himalayan-Tibetan orogeny impacted on global chemical weathering fluxes. Whether this reflects weathering of high $^{87}$Sr/$^{86}$Sr ratio silicates, high $^{87}$Sr/$^{86}$Sr ratio carbonates or increased silicate chemical weathering is controversial. To resolve this we need to determine the proportions of Sr and $^{87}$Sr which are derived from carbonate and silicate minerals. Here we present a new method for doing this which uses arrays of tributary compositions to define the cation ratios of silicate and carbonate inputs. This reveals the importance of incongruent precipitation and dissolution reactions in controlling water chemistry. The use of end-members defined by the water compositions to calculate Sr sources (but not the partition of major cations) is robust against many of the processes which perturb water compositions. The method is applied to the headwaters and flood plain of the Ganges. In the headwaters we estimate that 50% of the Sr flux and 57 ñ 14% of the "excess" $^{87}$Sr flux, which forces changes in seawater Sr-isotopic composition, is derived from silicate. In the flood plain 33 ñ 5% of the Sr flux is silicate derived. Overall the 55% of the "excess" $^{87}$Sr flux from the Ganges is silicate-derived. About 60% of the impact of S.E. Asian rivers on seawater $^{87}$Sr/$^{86}$Sr is due to high $^{87}$Sr/$^{86}$Sr ratio silicate or carbonate sources, the remaining ~ 40% being due to the higher than average silicate Sr fluxes.
DE: 1806 Chemistry of fresh water
DE: 1886 Weathering (1625)
DE: 1030 Geochemical cycles (0330)
DE: 1050 Marine geochemistry (4835, 4850)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting