Caliche as a geological repository for atmospheric sulfate

Recent studies demonstrate that important information on sulfur source, oxidation pathway, transport pattern, and reaction kinetics of atmospheric oxidation of sulfur gases can be uncovered by measuring multiple isotope compositions of the oxidation product - secondary atmospheric sulfate ( SAS). SAS from Earth's distant past, however, is rarely preserved in the rock records because of its high solubility and small quantity. We report here from the measurement of 47 modern and fossil caliche samples that caliches contain variable amounts of carbonate-associated sulfate (CAS), and the Delta O-17 values of the CAS range from - 0.12 to + 2.32 parts per thousand with most values higher than the background value of - 0.05 parts per thousand for seawater sulfate, indicating the presence of an SAS component derived from the oxidation of sulfur gases by atmospheric ozone or related oxidants. This study demonstrates that caliche, a pedogenic carbonate common in the geologic record, could serve as a repository for ancient SAS.