https://thenextweb.com/syndication/2019/10/04/study-ocean-ecosystems-take-2-million-years-to-recover-after-mass-extinction/

Around 66 million years ago, a giant asteroid struck the Earth, causing the extinction of the dinosaurs, ammonites, and many other species.

The asteroid was equally devastating at a microscopic level, driving ocean plankton to near-extinction. This crippled the base of the marine food chain and shut down important ocean functions, such as the absorption and delivery of carbon dioxide from the atmosphere to the ocean floor.