HR: 17:00h
AN: V54B-05    [Abstracts]
TI: Behavior of Boron Isotopes During Chemical Weathering: a Global Approach
AU: * GAILLARDET, J
EM: gaillardet@ipgp.jussieu.fr
AF: Institut de Phyisique du Globe de Paris, 4 place Jussieu, Paris, 75252 France, Metropolitan
AU: CHETELAT, B
EM: chetelat@ipgp.jussieu.fr
AF: Institut de Phyisique du Globe de Paris, 4 place Jussieu, Paris, 75252 France, Metropolitan
AB: Boron has two isotopes ($^{10}$B and $^{11}$B) that fractionate largely during Earth surface processes. The major fractionating step takes place during low temperature water-rock interactions. Up to 20-30 \permil difference in $\delta$$^{11}$B units are shown to occur during the adsorption of boron onto surfaces or its precipitation into solids. Light boron has a much greater affinity for neoformed solids while the residual solution is enriched in heavy boron. For example, seawater has a boron isotopic composition of 40 \permil, mainly due to the preferential removal of $^{10}$B during oceanic seafloor weathering, adsorption onto fluvial sediments and chemical weathering reactions occurring in soils. The high sensitivity of boron isotopes fractionation to water rock interactions make it a valuable tool to constrain chemical weathering processes. To have a global picture of the behavior of boron isotopes during chemical weathering of rocks at the surface of the continents, we analyzed the largest rivers for both the dissolved and suspended load. Dissolved boron isotopic compositions were published earlier ([Lemarchand et al., 2000]) and we here focus on the results for the suspended load and for the bottom sands. Boron clearly appears as a mobile element when its abundance in suspended sediments is normalized to upper continental material. Boron depletion in suspended solids is climate dependent, with higher depletion is warm climates. On average, more than 50 % of boron is transported to the ocean in a solid form. While the dissolved load of boron is clearly enriched in $^{11}$B (0 \permil) with respect to the mean upper continental crust (-10 \permil), isotopic composition of the suspended load of major rivers does not differ significantly from that of the continental crust. This result indicates that suspended material is not significantly fractionated with respect to the continental crust by chemical weathering processes or that the mass budget of boron partitioning between solids and water does not allow the residual solids to be significantly different from bedrock. This is supported by a Rayleigh distillation model. Our boron data, both in concentration and isotopic composition, give strong support to the idea that shale erosion is a major source of suspended sediments in large river system, making thus boron isotopes a good tracer of intra-continental recycling. Lemarchand et al., 2000, Nature, vol 408, pp 951-954.
DE: 1886 Weathering (1625)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting