What the geological past can tell us about the future of the ocean's twilight zone

Paleontological reconstructions of plankton community structure during warm periods of the Cenozoic (last 66 million years) reveal that deep-dwelling “twilight zone” (200–1000 m) plankton were less abundant and diverse, and lived closer to the surface, than in colder climates. We suggest this results from temperature control on microbial remineralization of sinking organic matter, which accelerates at warmer temperatures. In a warmer ocean, a smaller fraction of organic matter reaches the ocean interior, reducing food supply and dissolved oxygen at depth. Using an Earth system model evaluated against paleo observations, we illustrate how anthropogenic warming may impact future carbon cycling and twilight zone ecology. Our findings indicate significant changes are already underway, and without strong emissions mitigation, widespread ecological disruption in the twilight zone is likely by 2100, with effects persisting for millennia.