Constraining the oxygen history of Earth’s middle years using redox sensitive tracers

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: Sedimentary rocks, if interpreted correctly, can provide a timeline of the tumultuous oceanic and atmospheric conditions on the evolving Earth. The first long-lived accumulation of atmospheric oxygen began ~2.4 billion years ago (Ga), classically known as the Great Oxidation Event. Although this event has been documented for some time, mounting evidence points to more dynamic oxygen trends than previously thought. High oxygen levels at this time are implied by several lines of evidence, including high, modern-like concentrations of elements sensitive to oxidation and reduction (redox) reactions and high organic carbon in marine shales. Recent research shows a reversal of these oxygenation trends after ~2.05 Ga—contrary to a long-held view that oxygen increased with time. This de-oxygenation event is poorly-resolved, but might provide important links to understanding how surface conditions were inadequate for complex biological evolution. It could also provide analogues for rapid changes in ocean chemistry reminiscent of that attendant with modern climate change.