Oceans are losing oxygen faster than scientists can predict the consequences. This project turns to the Cretaceous Period—when volcanoes repeatedly triggered global ocean dead zones—to find the exact tipping points that push marine oxygen past a critical threshold. Current climate models cannot tell us precisely how much CO₂ or nutrient pollution it would take to trigger a modern ocean anoxic event (OAE), because past analogues remain poorly quantified. The researcher will calibrate a new technique—boron isotopes in radiolarian silica shells—to reconstruct ancient atmospheric CO₂ and ocean pH with unprecedented accuracy. Combining this with Earth system modelling, the work will test which feedbacks tipped the Cretaceous into anoxia and whether today’s oceans are preconditioned to follow the same path. If successful, the research will pinpoint the specific CO₂ and nutrient loads that could trigger a modern OAE within centuries. This is fundamental science: it will not directly change policy or technology tomorrow. But understanding where the Earth system’s anoxia thresholds lie could eventually inform emissions targets, coastal nutrient management, and marine conservation strategies—systems that quietly underpin fisheries, oxygen supplies, and ecosystem stability.
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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.
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