NEW CONSTRAINTS ON ATMOSPHERIC OXYGENATION (Invited Presentation)

The establishment of an oxidising atmosphere on Earth exerts a first order control on when widespread oxidative weathering of certain bioessential nutrients began. Early atmospheric oxygenation is most reliably traced using quadruple sulfur isotope (QSI) measurements in shales. Specifically, the transition from mass-independent (S-MIF) to mass-dependent fractionation of sulfur isotopes (S-MDF) reveals trace oxidation of the atmosphere during the Great Oxidation Event. New, temporally constrained, QSI measurements from the Paleoproterozoic Seidorechka and Polisarka Sedimentary Formations in NW Russia place the S-MIF/S-MDF transition between 2501.5 ± 1.7 Ma and 2434 ± 6.6 Ma, which is compatible with QSI records from other cratons, and prior to a Paleoproterozoic ‘snowball Earth’ event. This refutes the hypothesis that increased nutrient flux during deglaciation caused a sharp rise in marine primary productivity and consequent atmospheric oxygenation. It does, though, allow for an alternative concept: atmospheric oxygenation and resultant collapse of a methane-bearing greenhouse prompted global glaciation. The exact temporal and causal relationships between multiple episodes of glaciation during the Paleoproterozoic and atmospheric oxygenation, however, remain uncertain and require a more robust global stratigraphic framework to be established.