Variable Silicon Isotope Fractionation Between Authigenic Phases and Pore Fluids in Marine Sediments

Formation and subsequent burial of authigenic phases have been recognized as a key process removing silicon from the ocean. However, the effect of authigenic phase formation on the isotopic mass balance of silicon in the ocean is not clear. Here, we constrain the apparent silicon isotope (delta Si-30) fractionation associated with early diagenesis by measuring delta Si-30 signatures of pore fluids and authigenic phases in a South Atlantic sediment core. The delta Si-30 offsets between authigenic phases and pore fluids vary between -2.9 and -0.4 parts per thousand, supporting a substantial negative delta Si-30 fractionation during early diagenesis. The variable apparent delta Si-30 fractionation covaries with the amount of sedimentary lithogenic materials and may be attributable to the associated kinetic effects. Overall, our data show that authigenic phases buried along with their precursor phases preferentially remove isotopically light silicon from the ocean, with implications for the isotopic mass balance of the marine silicon cycle.