A first look at the distribution of the stable isotopes of silicon in natural waters

The first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported. Both marine and riverine delta(30)Si are more positive than delta(30)Si of igneous rocks, suggesting isotopic fractionation during weathering and clay formation and/or biomineralization. The delta(30)Si value for dissolved silicic acid from several ocean basins is +1.1 +/- 0.3 parts per thousand (n = 69). More positive values are found in surface waters, due to discrimination against Si-30 during biogenic opal formation by diatoms. The deep Pacific Ocean is depleted in 30Si relative to the deep Atlantic by roughly 0.4 parts per thousand. delta(30)Si 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 delta(30)Si value of inputs of silicon to the ocean most likely falls between +0.3 and +0.9 parts per thousand, and that of the outputs falls between +0.9 to +1.9 parts per thousand, probably in the neighborhood of +1.0 parts per thousand. Copyright (C) 2000 Elsevier Science Ltd.