Silicon Isotopes as Tracers of Terrestrial Processes

Abstract

Chemical weathering of silicate rocks consumes carbon dioxide during the breakdown of minerals and liberates ions and silicic acid. Weathering reactions can lead to the fractionation of stable silicon (Si) isotopes, which allows variations in Si isotopic ratios to be used to quantify the global Si cycle. Here, I detail the methods and applications of assessing variations in continental Si stable isotope compositions, and show how such studies can elucidate the processes that affect the terrestrial Si cycle. The global Si cycle and its stable isotope variations are strongly affected by biology, with Si uptake by phytoplankton in oceans and lakes, and biological uptake by terrestrial plants, especially modern crops, on land. This biological uptake and subsequent release often leads to a strong seasonal cycle overprinted onto the underlying weathering reactions. Nevertheless, it can be shown that weathering reactions lead to the fractionation of Si into a light Si reservoir stored in soils and sedimentary basins, and a heavy Si reservoir in rivers, lakes and the oceans.