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

The stable isotope composition of O 2 , O 3 , and H 2 O in the geological past conveys rich information on history of the Earth system. However, few compounds are known to record O 2 or O 3 isotope signals directly and reliably. Sulfate (SO 4 2 − ), 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 H 2 O isotope signals, sulfate can carry isotope signatures from O 2 and O 3 . I argue that sulfate, to this point, is the only compound from which direct atmospheric O 2 and O 3 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. [ABSTRACT FROM AUTHOR]