TOWARDS A CONSENSUS ON PHANEROZOIC OXYGEN LEVELS (Invited Presentation)

Free-living animals require oxygen, but the role of oxygen in driving animal evolution over the Phanerozoic remains very uncertain. This is due in part to the lack of consensus on how atmospheric oxygen levels have changed over long timescales. I will present a new effort to review and reconcile the various methods for reconstructing ancient oxygen levels, including a revision of the Isotope Mass Balance method which uses sedimentary carbon and sulfur isotope records to infer the production fluxes of oxygen. It seems likely that the atmosphere contained around 5–10% oxygen in the Cambrian, with this fraction rising to ~15-20% in the Devonian, and rising to a peak of >25% in the Carboniferous-Permian, with a decline towards the present day. Evolutionary radiations in the Cambrian and Ordovician appear to be consistent with an oxygen driver, and the Devonian ‘age of the fishes’ may have been triggered by oxygen levels rising above 15% of the atmosphere – a level that suggests oxygenation of the ocean interior.