Si stable isotopes in the Earth's surface: A review

Silicon (Si) is the second most abundant element on Earth after oxygen. Only few studies have attempted to use stable isotopes of Si as proxies for understanding the Si cycle and its variations in the past. By using three different methods (IRMS, MC-ICP-MS and SIMS), the overall measurements show that the isotopic composition (delta(30)Si) of terrestrial samples ranges from -5.7 parts per thousand to +3.4 parts per thousand. Dissolved Si in rivers and seawater is Si-30-enriched (-0.8 parts per thousand < delta(30)Si < +3.4 parts per thousand) compared to Si in endogeneous rocks (-1.1 parts per thousand < delta(30)Si < +0.7 parts per thousand). This global enrichment is counterbalanced by the Si-bearing phases (biogenic silica, clays, quartz) where Si is, in average, Si-30-depleted (-5.7 parts per thousand < delta(30)Si < +2.6 parts per thousand). These values are the result of fractionation which have been measured or estimated from -0.3 parts per thousand to -3.8 parts per thousand. The fractionation is modeled by two types of approaches: the Rayleigh distillation model (closed system) and the steady-state model (open system). These models have been used in the most recent studies to explain the observed delta(30)Si variations in continental environments and in the sub-Antarctic Ocean. (c) 2005 Elsevier B.V. All rights reserved.