INTERPRETING SILICON ISOTOPES IN THE CRITICAL ZONE

Metal and metalloid stable isotope ratios have emerged as potentially powerful proxies for weathering, element cycling and export in the Critical Zone. The simplest possible interpretative framework for these isotope ratios has three parameters: (i) the isotope ratio of the parent minerals undergoing weathering, (ii) the partitioning of the element between solute and the new secondary phases, and (iii) the fractionation factors associated with the formation of new secondary phases. Using the example of silicon, we show how all three of these parameters vary along a gradient of erosion rate and regolith residence time defined by three sites located on granitoid bedrock. These sites run from the kinetically limited Rhone Valley in the Central Swiss Alps to the tectonically inactive and supply-limited Sri Lankan high-lands, with the Sierra Nevada mountains as a site of intermediate weathering intensity. At each site, primary mineral specific Si-30/Si-28 ratios span >0.4 parts per thousand. These minerals weather differentially, such that the isotope ratio of silicon solubilised from rock differs at the three sites and is not necessarily equal to bulk bedrock composi-tion. The partitioning of silicon between secondary clay and solute is reflected in the clay mineralogy and chemical composition: more intense weathering produces Si-poor clays. The clay composition thus comprises a first-order mass-balance control on the extent to which any fractionation factor can be expressed. Finally, the Si iso-tope fractionation factor associated with clay formation varies systematically with clay mineralogy: the formation of Si-deplete clay minerals is associated with larger fractio-nation factors. The magnitude of the fractionation may be mechanistically linked to relative aluminium availability. These findings provide the framework needed to use Si isotope ratios as a quantitative proxy to explore Si cycling and reconstruct weathering in the present and past.

Automated summary

Metal and metalloid stable isotope ratios are important proxies for weathering, element cycling, and export in the Critical Zone. By studying sites with varying erosion rates and regolith residence times, it was found that three key parameters influence these isotope ratios, particularly in the case of silicon. The research also highlights how the composition of clay minerals plays a significant role in expressing fractionation factors related to silicon isotope ratios.