HR: 13:55h
AN: V53B-02    [Abstracts]
TI: Silicon Isotopes in River Waters: A First Look into Variability in the Continents
AU: * Georg, R B
EM: georg@erdw.ethz.ch
AF: IGMR, ETH Zrich, ETH-Zentrum Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AU: Reynolds, B C
EM: reynolds@erdw.ethz.ch
AF: IGMR, ETH Zrich, ETH-Zentrum Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AU: Frank, M
EM: frank@erdw.ethz.ch
AF: IGMR, ETH Zrich, ETH-Zentrum Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AU: Halliday, A N
EM: halliday@erdw.ethz.ch
AF: IGMR, ETH Zrich, ETH-Zentrum Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AU: Halliday, A N
EM: halliday@erdw.ethz.ch
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AB: The isotopic fractionation during biological uptake of silicon has been studied as a proxy for silicic acid utilization in the marine environment (De La Rocha, et al 1997, Varela et al. 2004). These studies have shown that the Si isotope distribution in the ocean has been strongly affected by marine productivity. To what extent continental weathering also contributes to variations in the Si isotope composition of the ocean is unknown, because the inputs into the oceans have not been reliably quantified. We have measured the Si isotope compositions of river waters draining different lithologies over a seasonal cycle in order to constrain continental variability and contributions to the marine Si isotope budget. For our study we sampled four different rivers draining different catchment lithologies in Switzerland in March, May, August and October 2004. The average annual runoff of these rivers ranges from 10m$^{3}$/s to 110m$^{3}$/s, and the concentration of dissolved Si shows strong seasonal variation. Si isotopes were analysed using the high-resolution capacity of the Nu1700 (Nu Instruments, UK) multi-collector ICP-MS at ETH Zurich. Si isotope data are reported as $\delta ^{30}$Si and $\delta ^{29}$Si relative to the NIST NBS28 standard. The Si isotope composition of the river samples can be reproduced with two different chemical preparation methods with an average reproducibility of 0.15$\permil$ (2 $\sigma$). Preliminary results demonstrate that the Si isotope compositions of the rivers undergo seasonal variations and reveal clear differences between lithologically distinct catchments. The difference in $\delta ^{30}$Si between the rivers studied so far is up to 0.4$\pm$ 0.1$\permil$ and seasonal variations can be as high as 0.3$\pm$ 0.1$\permil$. Reference: De La Rocha, et al. 1997, G.C.A. 61 p5051-5056, Varela, et al. 2004, G.B.C. 18 GB1047.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting