Seawater sulfate is a major carrier of oxidizing capacity and influences the redox budget of the Earth's surface on geologic timescales. Records of oxygen and sulfur isotopes in seawater sulfate are used to track changes in sulfate cycling through geologic time. Interpretations of these records are typically based on models that describe the seawater sulfate reservoir as a mass balance between microbial, riverine and sedimentary sulfate fluxes. Here, we investigate the influence of hydrothermal sulfate cycling, which remains an unconstrained but potentially significant additional flux in this mass balance. We find that anhydrite (CaSO4) from eight submarine hydrothermal vent fields is consistently offset from seawater sulfate in δ18O but not Δ’17O or δ34S. Experiments at hydrothermal pressure–temperature conditions indicate that this δ18O offset is driven by oxygen isotope exchange between sulfate and hydrothermal H2O at high temperature. An updated isotope mass balance model shows that a flux of hydrothermal sulfate into seawater, derived from retrograde dissolution of hydrothermal anhydrite, could cause high-temperature oxygen isotope exchange to buffer seawater sulfate δ18O and Δ’17O by as much as 25%. Hydrothermal sulfate preserved in the Troodos Ophiolite (Cretaceous) also records a δ18O and Δ’17O signal of high-temperature oxygen isotope exchange, supporting the conclusion that geologic records of seawater sulfate oxygen isotopes may include a hydrothermal component.
|Original language||English (US)|
|Number of pages||17|
|Journal||Geochimica et Cosmochimica Acta|
|State||Published - Mar 1 2023|
Bibliographical noteFunding Information:
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. We acknowledge the research cruise leaders and funding agencies that made it possible to collect the samples used in this study. See the Supplementary Materials for details. Members of the Johnston Lab are thanked for their helpful comments. B. Passey and N. Ellis are thanked for their help with triple-oxygen isotope analysis of the experimental H2O. J. Alt, B. Killingsworth and one anonymous reviewer are thanked for their thoughtful reviews which significantly improved this manuscript.