Probing surface Earth reactive silica cycling using stable Si isotopes: Mass balance, fluxes, and deep time implications.

Geological reservoir δ 30 Si values have increasingly been applied as paleoclimate and paleoproductivity proxies. Many of these applications rely on the assumption that the surface Earth Si isotope budget is in mass balance with bulk silicate Earth, such that trends in δ 30 Si over time can be attributed to changes in flux to or from key silica reservoirs. We compiled δ 30 Si data from modern reservoirs representing the major sources and sinks of surface Earth reactive silica, to which we applied an inverse model to test assumptions about mass balance. We found that δ 30 Si values of reverse weathering products must closely match those of diatoms, conflicting with previous assumptions. Model results also revealed that of the 19 to 21 teramoles per year Si released during silicate mineral weathering, ~10 to 18 teramoles per year is stored in terrestrial silica sinks, consistent with assumptions of incongruent weathering reactions. Our results demonstrated that the modern silica cycle summary is in isotopic mass balance.