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 (δ30Si) of terrestrial samples ranges from − 5.7‰ to + 3.4‰. Dissolved Si in rivers and seawater is 30Si-enriched (− 0.8‰ < δ30Si < + 3.4‰) compared to Si in endogeneous rocks (− 1.1‰ < δ30Si < + 0.7‰). This global enrichment is counterbalanced by the Si-bearing phases (biogenic silica, clays, quartz) where Si is, in average, 30Si-depleted (− 5.7‰ < δ30Si < + 2.6‰). These values are the result of fractionation which have been measured or estimated from − 0.3‰ to − 3.8‰. 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 δ30Si variations in continental environments and in the sub-Antarctic Ocean.