UC Santa Cruz

The triple oxygen isotope composition (Δ'¹⁷O) of sulfate minerals is widely used to constrain ancient atmospheric pO₂/pCO₂ and rates of gross primary production. The utility of this tool is based on a model that sulfate oxygen carries an isotope fingerprint of tropospheric O₂ incorporated through oxidative weathering of reduced sulfur minerals, particularly pyrite. Work to date has targeted Proterozoic environments (2.5 billion to 0.542 billion years ago) where large isotope anomalies persist; younger timescale records, which would ground ancient environmental interpretation in what we know from modern Earth, are lacking. Here we present a high-resolution record of the [Formula: see text]O and Δ'¹⁷O in marine sulfate for the last 130 million years of Earth history. This record carries a Δ'¹⁷O close to 0o, suggesting that the marine sulfate reservoir is under strict control by biogeochemical cycling (namely, microbial sulfate reduction), as these reactions follow mass-dependent fractionation. We identify no discernible contribution from atmospheric oxygen on this timescale. We interpret a steady fractional contribution of microbial sulfur cycling (terrestrial and marine) over the last 100 million years, even as global weathering rates are thought to vary considerably.