Volcanically modulated pyrite burial and ocean–atmosphere oxidation

It is widely assumed that changes in the rate of biological O₂ supply to the atmosphere played little, if any, role in driving the Great Oxidation Event (GOE) because extensive biological O₂ production via oxygenic photosynthesis significantly predates atmospheric oxygenation on Earth. Moreover, the C isotope record seemingly precludes a dramatic increase in O₂-liberating burial of organic C in the late Archean. However, organic C burial is not the only way in which the biosphere may oxidize the Earth's atmosphere. Biologically mediated burial of reduced mineral phases may also yield net oxidation. For example, reducing power can be transferred from organic matter to H₂S via anaerobic respiration and ultimately stored in sedimentary pyrite (FeS₂). Changes in pyrite burial may thus impact Earth's oxidation trajectory. Here, we revisit the earliest recognized episode of sulfide-rich (euxinic), pyrite-precipitating conditions in Earth's ocean, ∼200–300 million years prior to the GOE, as preserved in the ∼2.7–2.6 billion-year-old (Ga) Roy Hill Shale of the Jeerinah Formation. Applying Fe speciation and trace metal proxies to drillcore samples from two sites, we characterize both spatial and temporal heterogeneity in the accumulation of H₂S in the otherwise Fe-rich Hamersley Basin. We argue that extensive pyrite burial in these locally euxinic environments was the consequence of enhanced volcanic emanations of SO₂ to the atmosphere rather than oxidative weathering of crustal sulfides. Because SO₂ reduction and burial as pyrite is chemically analogous to CO₂ reduction and burial as organic matter, we further posit that this alternative oxidation pathway contributed to Earth's protracted oxygenation during the GOE. Our results thus highlight the need for continued study of the interplay between solid-Earth, volcanic, and biological processes and their joint modulation of oceanic and atmospheric composition.