Pinpointing Earth-System Thresholds for Anoxia with new Reconstructions of the Cretaceous Hothouse

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Oxygen levels in Earth's oceans are dropping fast due to anthropogenic nutrient input and CO2 release, and the consequences of this for marine ecosystems are difficult to predict. The Cretaceous Period (66-145 million years ago) witnessed numerous extreme ocean anoxic events (OAEs) - some regional, some global in scale - that are thought to have been caused by pulses of volcanism and CO2 release. If we knew the exact mechanisms by which volcanism triggered these Cretaceous OAEs, and the role of feedbacks and boundary conditions, they could provide vital information as to where potential tipping points in the Earth system lie. Presently, however, our estimates of climate, atmospheric CO2 and carbon cycling in the Cretaceous are qualitative at best, preventing OAEs from being useful analogues. With PETRARCH, I will rectify this, by combining both new and proven geochemical proxy archives with cutting-edge Earth system modelling. Specifically, I will: a) calibrate boron isotopes in the silica shells of radiolarians as a new proxy archive for ocean pH and atmospheric CO2, that will open up new frontiers for palaeoclimatology, b) combine this new radiolarian data with new measurements of carbonate microfossils to reconstruct the Cretaceous carbon cycle, and c) use Earth System modelling, tuned to these data, to test what exact biogeochemical feedbacks and climate forcings tipped the Cretaceous Earth into profound, and sometimes global, ocean anoxia. By probing the sensitivity to continental configuration and starting atmospheric O2 and CO2 levels, I will determine to what extent anoxia is an inevitable consequence of CO2 rise in a greenhouse world, and to what extent the Cretaceous was preconditioned to anoxia. Finally, I will synthesise these lessons from the geological past to pinpoint exactly how much we would have to perturb today's Earth system - its climate, marine ecosystem structure and geochemical cycles - to cause an OAE in the coming centuries.