A FIRST LOOK AT THE DISTRIBUTION OF THE STABLE ISOTOPES OF SILICON IN NATURAL WATERS

Abstract

The first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported. Both marine and riverine δ30Si are more positive than δ30Si of igneous rocks, suggesting isotopic fractionation during weathering and clay formation and/or biomineralization. The δ30Si value for dissolved silicic acid from several ocean basins is +1.1 ± 0.3‰ (n = 69). More positive values are found in surface waters, due to discrimination against 30Si during biogenic opal formation by diatoms. The deep Pacific Ocean is depleted in 30Si relative to the deep Atlantic by roughly 0.4‰. δ30Si of riverine dissolved silicon averages +0.8 ± 0.3 (n = 8). The biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea. The average δ30Si value of inputs of silicon to the ocean most likely falls between +0.3 and +0.9‰, and that of the outputs falls between +0.9 to +1.9‰, probably in the neighborhood of +1.0‰.