Linking atmospheric and marine redox evolution during the Great Oxidation Event — Isotopic insights from the Duitschland and Rooihoogte formations (South Africa)

The correlative Duitschland and Rooihoogte formations (Transvaal Basin) record a critical juncture in Earth’s history since both formations archive a pronounced shift from mass-independent fractionation of sulfur (MIF-S) to mass-dependant fractionation of sulfur, indicative of a slightly oxygenated atmosphere. Surprisingly, however, while multiple sulphur isotope systematics has fundamentally shaped our understanding of atmospheric oxygenation, the evolution experienced within the marine realm has received comparatively little attention. Here we present geochemical transects of four drill-cores intersecting the two formations, affording a three-dimensional, yet somewhat counterintuitive, insight into marine response to atmospheric oxygenation. For instance, rather than signalling more oxygenated conditions, redox-sensitive trace elements disclose persistently suboxic–oxic marine conditions that, in fact, appear to worsen up-section. Interestingly, succeeding the loss of S-MIF, preliminary Mo-isotope data document a c.1‰ shift toward heavier δ98MoNIST+0.25‰ values. This additionally correlates with a negative shift in δ34S to approximately -25‰, a range common for sulfate reducing bacteria, along with an overall increase in both sulfur content and Fe/Mn. This stratigraphic evolution reconciles with a deterioration of local redox conditions by Fe-rich (Mn-poor) sedimentation and/or microbially induced H2S production, which would lower the Mo isotopic offset between δ98Mo in shales and its seawater precursor. The marine redox potential is thus lowered from somewhere between oxic-suboxic to suboxic-anoxic conditions after the loss of MIF-S. Assimilated, at least within the Transvaal Basin, our data expose a geobiological feedback-driven causality between the earliest oxygenation of the atmosphere and a decreased redox potential of the marine realm.