EAPS DLS - Jordon Hemingway (ETH Zurich)

Interpreting triple-oxygen isotope compositions in the geologic sulfur cycle

Oxidation of the iron-sulfide mineral pyrite (FeS2) during weathering forms sulfate (SO42-), increases atmospheric carbon dioxide levels (pCO2), and decreases oxygen levels (pO2). This process represents a major control on Earth-surface redox state over geologic timescales. In addition to regulating atmospheric composition, the triple-oxygen isotope content (d18O and Δ’17O) of pyrite weathering-derived sulfate is thought to be a direct proxy for pO2/pCO2 due to the incorporation of anomalously 17O-depleted O2 signals. Triple-oxygen isotope measurements of evaporites, barite, and carbonate-associated sulfate are thus becoming a common method to reconstruct pO2/pCO2 throughout Earth history. Despite this importance, the electrochemical mechanism of pyrite oxidation—including its impact on sulfate oxygen isotopes—remains largely unconstrained, hindering our ability to interpret the triple-oxygen isotope compositions of geologically preserved sulfate.

Here, I will discuss recent efforts to better constrain the mechanism and triple-oxygen isotope consequences of pyrite oxidation. Specifically, I will present: (i) sulfoxyanion isotope fractionation factor estimates derived using quantum chemical simulations, (ii) measurements from modern weathering environments draining pyrite-rich lithologies, and (iii) a new seawater sulfate Δ’17O record for the Cenozoic and Cretaceous. Combined, these results imply a pyrite oxidation mechanism that incorporates an anomalously 17O-enriched signal— likely from reactive oxygen species such as hydrogen peroxide—but is overprinted by massdependent processes during fluvial transport and marine recycling. I will discuss how these results update our interpretation of geologic archives, focusing particularly on the (in)ability of sulfate oxygen isotopes to inform past atmospheric pO2 and pCO2 levels.

About this Series:

The Department Lecture Series at EAPS at MIT is a series of Weekly talks given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. For more information and Zoom password please contact Maggie Cedarstrom: maggie84@mit.edu