During the Cretaceous our mammalian relatives were small and puny. They wandered around and tried not to become a warm snack for the reptiles that ruled the day. Insects began to diversify to eventually become the diverse group we now know and occasionally dip into chocolate. In the seas, rays, sharks, and all the bony fishes began to dominate and replace the way cooler cephalopods. Large marine reptiles still roamed the seas as opposed to just a lake in Scotland. The continents were still relatively close but rock n' roll, drugs, and women were soon to split them. The Atlantic was a mere young whipper snapper barely a few hundred kilometers big.

Two spectacularly awful events occurred in the ocean. Ocean anoxic event 1 (120mya) and 2 (93 mya). Rather unshockingly, the complete lack of oxygen in the oceans led to major extinctions. But what caused the OAE? New evidence strengthens the link between OAE2 and volcanism (but not Vulcans whose large brains are also known to cause major oxygen depletion).

In one scenario, the intense volcanic discharge likely changed the chemistry of the ocean and atmosphere. This may have provided micronutrients to phytoplankton resulting in ocean wide blooms. All of the sinking and rotting plankton eventually decayed on the ocean floor and stripped oxygen in the process.

Or...

Volcanoes disgorged clouds of CO2 to the atmosphere, warming the climate to the extent that Earth's ocean circulation system ground to a near-halt. Beyond the surface layers, water was no longer turned over and anoxia was the result.

Either way volcanoes are evil and an enemy of our salty friends.

Turgeon, S.C., Creaser, R.A. (2008). Cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode. Nature, 454(7202), 323-326. DOI: 10.1038/nature07076