| Summary: | International audience IntroductionRecent progresses in silicon isotopic measurements usingMC-ICP-MS technology have brought hopes among thebiogeochemical scientific community in order to improve ourknowledge on the silicon cycle (Cardinal et al., 2003).Results and DiscussionThe first silicon isotopic dissolved profiles obtained bythis technique have confirmed the biological fractionationeffect (De la Rocha et al., 2000) linked to the siliceousorganism productivity both in marine (Antarctic Ocean) andfresh water (Lake Tanganyika) systems. This isotopicdesequilibrium (δ29Si = 0.94±0.11 in surface and 0.62±0.06 indeep) is controlled by the Rayleigh distillation equation andallows to reconstruct the dissolved silicic acid utilisation basedon the fractionation factor recorded in biogenic opal material.The fractionation observed in our studies between thedissolved Si pool in surface waters and the estimated diatomsignature displays a δ29Si strikingly similar to what wasmeasured on cultured batch (De la Rocha et al., 1997). Thesein-vivo measurements thus confirm the small impact inchanges of temperature, salinity, pH and diatom species in thefractionation process.Spatial and temporal changes of the silicic acid utilisationwill be discussed for the Tanganyika lake and Antarctic watersamples along with the isotopic signature of silicon sources.ConclusionsSeveral questions still need consideration: A betterconstrain of the Si isotopic fractionation factor and theparameters controlling it; Determining the δ29Si for silicic acidsupply to the euphotic zone and its spatial variability;Deepening our perception of this tracer to provide valuableinformation on paleo-diatom rich productivities.
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