Sulfate: A time capsule for Earth's O2, O3, and H2O

• Sulfate is the only known carrier for direct O2 or O3 signal in the geological past.
• Sulfate's Δ17O is predicted to have deviated from 0 since the Great Oxidation Event.
• There are many big data gaps on sulfate Δ17O in Earth history.

The stable isotope composition of O2, O3, and H2O in the geological past conveys rich information on history of the Earth system. However, few compounds are known to record O2 or O3 isotope signals directly and reliably. Sulfate (SO42 −), a non-labile oxyanion capable of forming weakly soluble minerals, plays important roles in global sulfur, carbon, and oxygen cycles. Here I review publications on the triple oxygen isotope composition of sulfate which has been recently expanded, demonstrating that in addition to H2O isotope signals, sulfate can carry isotope signatures from O2 and O3. I argue that sulfate, to this point, is the only compound from which direct atmospheric O2 and O3 signals from the distant past can be retrieved. If the current understanding of the Earth's surface oxygenation history holds, we expect to observe little to no measurable deviation from a “normal” triple oxygen isotope composition for sulfate throughout the Archean, but both positive and negative deviations since the early Proterozoic. The full potential of this unique proxy can be achieved by further study on isotope kinetics of sulfur redox cycling and by filling gaps in geological records.