Award: OCE-1821976

Atmospheric O2 and CO2 levels inform us of the changes in chemical and biological environments, yet the history of atmospheric compositions, and pO2 in particular, is not well-constrained. The triple oxygen isotope (16,17,18O) composition of marine SO2−4 has been proposed to directly record the ratio pO2/pCO2 in the contemporaneous atmosphere. To resolve this atmospheric signal, both a precise measurement of the 17O composition of sulfate (seawater, evaporites, barite) and a model with which to interpret the measurement are needed. In this project we developed a precise measurement procedure of the triple oxygen isotope composition of modern marine sulfate and a novel sulfur cycle model that includes four main fluxes: weathering (Jriv), sulfate reduction (Jreduc) , pyrite burial (Jpy), and reoxidation processes (Jbio, Jox). The atmospheric anomaly passes to ocean sulfate via Jriv. Subsequent microbial and abiotic reactions (Jox, Jbio ) reset the O isotope composition of sulfate. The model deconvolves the potential atmospheric and microbial inputs to this signal allowing assesment of biological activity. Our interpretation of marine sulfate oxygen isotope composition provides a framework for calculating atmospheric composition, relative rates of biogeochemical activity, and can be applied to geologic records of marine sulfate to constrain the pO2/pCO2 ratio over time. Last Modified: 07/31/2020 Submitted by: Adina Paytan