Referring to research done by a team led by University of Edinburgh researchers and published in Science journal (behind a paywall), the author writes:
The biggest extinction event in planetary history was driven by the rapid acidification of our oceans, a new study concludes. So much carbon was released into the atmosphere, and the oceans absorbed so much of it so quickly, that marine life simply died off, from the bottom of the food chain up.
That doesn’t bode well for the present, given the disturbingly similar rate that our seas are acidifying right now.