HR: 1340h
AN: H43C-0388    [Abstracts]
TI: The Dissolved Ca Isotope Composition of Himalayan-Tibetan Waters
AU: * Tipper, E T
EM: ett20@esc.cam.ac.uk
AF: University of Cambridge, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Galy, A
EM: albert00@esc.cam.ac.uk
AF: University of Cambridge, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Bickle, M J
EM: mb72@esc.cam.ac.uk
AF: University of Cambridge, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AB: Determining the relative proportions of carbonate versus silicate weathering in the Himalaya is important for understanding the long-term atmospheric CO$_2$ budget and the marine Sr isotope record. $^{87}$Sr/$^{86}$Sr is not a straightforward proxy of carbonate to silicate weathering in the Himalaya and up to 50% of the dissolved Ca may be removed by the precipitation of secondary calcite. Ca isotopes have the potential to constrain the relative inputs of carbonates to silicates and incongruent dissolution processes in the weathering environment. Ca is the major cation carried by rivers. Thirty four Himalayan rock and water samples from the Nepal Himalaya and Tibet have been analysed for $^{44/42}$Ca and $^{43/42}$Ca on a Nu-Instruments Multiple Collector -ICP-MS. Unlike the $^{44/40}$Ca ratio the $^{44/42}$Ca is not susceptible to excess $^{40}$Ca production from the decay of K. All samples lie on a single mass fractionation line. There is a total range of 0.4 \permil variation in \delta$^{44}$Ca with values from 0.63 \permil - 0.21 \permil relative to the SRM915a standard. This is comparable to that already reported with \delta$^{44/40}$Ca for small catchments and global rivers. Small first order catchments from each of the main lithotectonic units of the Himalaya have been analysed to examine the effect of lithology on dissolved Ca isotopic composition. In agreement with previous studies elsewhere there is little correlation between source rock and dissolved composition for small rivers spanning a range of source rock from limestone to various silicates and covering a vegetation range from temperate semi-desert to jungle. \delta$^{44}$Ca is not correlated with $^{87}$Sr/$^{86}$Sr or Na/Ca ratios confirming that source rock composition is not the dominant control on the observed range in \delta$^{44}$Ca. A time-series has been examined for the Marsyandi River, central Nepal. In spite of significant systematic variations in major element chemistry including Ca concentration and $^{87}$Sr/$^{86}$Sr the variations in \delta$^{44}$Ca are limited to 0.16 \permil. Either there is only a single isotopic source of Ca or the \delta$^{44}$Ca is controlled by incongruent dissolution processes. The most important incongruent process to affect the Ca budget is the precipitation of pedogenic carbonate. Such incongruent processes should be detectable in the Ca-isotope budget.
DE: 1886 Weathering (1625)
DE: 1040 Isotopic composition/chemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 0330 Geochemical cycles
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting