Marine anoxia linked to abrupt global warming during Earth's penultimate icehouse

Massive carbon (C) release with abrupt warming has occurred repeatedly during greenhouse states, and these events have driven episodes of ocean deoxygenation and extinction. Records from these paleo events, coupled with biogeochemical modeling, provide clear evidence that with continued warming, the modern oceans will experience substantial deoxygenation. There are, however, few constraints from the geologic record on the effects of rapid warming under icehouse conditions. We document a C-cycle perturbation that occurred under an Earth system state experiencing recurrent glaciation. A suite of proxies suggests increased seafloor anoxia during this event in step with abrupt increase in CO₂ partial pressure and a biodiversity nadir. Warming-mediated increases in marine anoxia may be more pronounced in a glaciated versus unglaciated climate state.