Geologists examining shale deposits have determined the likely kill mechanism in several mass extinction events: a loss of oxygen and simultaneous build up of hydrogen sulfide. A rapid rise in sea level from Greenland and/or Antarctic ice sheet destabilization could produce similar conditions.

Geologists Discover a Critical Kill Mechanism Behind a 350 Million-Year-Old Series of Extinctions
A major oil source in North America reveals insights into one of the planet's most devastating mass extinctions. The Bakken Shale Formation, spanning an area of 200,000 square miles (520,000 square kilometers) beneath parts of Canada and North Dakota, has been a prolific source of oil and natural
scitechdaily.com