HR: 17:00h
AN: PP34A-05    [Abstracts]
TI: Li Concentration and Isotopic Composition of River Waters in NE France
AU: * Carignan, J
EM: carignan@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, 15 rue Notre-Dame des Pauvres, Vandoeuvre-lŠs-Nancy, 54501 France
AU: Vigier, N
EM: nvigier@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, 15 rue Notre-Dame des Pauvres, Vandoeuvre-lŠs-Nancy, 54501 France
AU: Brenot, A
EM: brenot@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, 15 rue Notre-Dame des Pauvres, Vandoeuvre-lŠs-Nancy, 54501 France
AU: Millot, R
EM: R.millot@brgm.fr
AF: BRGM, 3 avenue C. Guillemin, Orleans, 45060 France
AB: Some studies showed that dissolved and particulate Li in rivers have different δ⁷Li the dissolved phase being enriched in 7Li. The fractionation between these two phases is interpreted as being related to the extent of silicate rock weathering. The input from limestones or other sedimentary rocks are thought to be negligible in mixed lithology basins. This is why Li isotopes might be used as proxies for paleo silicate weathering intensity. In order to better constrain signals from silicate and other rock types, we have studied the composition of dissolved elements in the Moselle river and tributaries (NE France), for which the draining basin is highly contrasted in lithology: silicate rocks upstream and mostly chemical sediments (carbonate-evaporite) downstream. Some rivers draining exclusively carbonate-evaporite rocks have extremely high Li concentration of up to 8.6 micromole/L compared to 0.07 to 0.4 micromole /L for water draining exclusively silicate rocks. Li and Mg concentrations are well correlated in all samples but water draining carbonate-evaporite rocks define an overall lower Li/Mg ratio than water draining silicate rocks. On the silicate part of the basin, low-Li water (Moselotte river) has δ⁷Li (relative to L-SVEC) of 23.6+/-0.5 permil (n=3) whereas high-Li water (Vologne river) has a much lower δ⁷Li of 14.7+/-1.2 permil (n=3) with no dependence with the river flow or the day of sampling. These two rivers also show different isotopic composition for dissolved Sr (87Sr/86Sr of 0.7167 - Moselotte and 0.7144 - Vologne) and different Li/Mg ratios suggesting a different composition of the source rocks. Indeed, lithology of the Moselotte watershed comprise biotite- and hornblend-rich granitoides whereas albite-rich rocks dominate the Vologne watershed. Rivers draining carbonate-evaporite yield δ⁷Li between 19 and 23 permil with lower 87Sr/86Sr of about 0.7082 and lower Li/Mg ratios, reflecting the important input of these sedimentary rocks to the dissolved load. Our data clearly show that weathering of carbonate-evaporite rocks may release a significant amount of dissolved Li in river water but also that this Li has an isotopic composition in the same range of Li released by silicate rocks. In the Moselle river basin, the contribution of carbonate-evaporite rocks to the dissolved Li load is dominant a few tens of km downstream the silicate rock part of the watershed.
DE: 1806 Chemistry of fresh water
DE: 1886 Weathering (0790, 1625)
DE: 4825 Geochemistry
DE: 4924 Geochemical tracers
DE: 5415 Erosion and weathering
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005