Washington, Nov 27: Researchers have discovered that the mass extinction event in oceans at the end of the Permian period about 251 million years ago happened because of a slow, drawn out demise of deep sea creatures caused due to global warming.

The research team came to the conclusion after studying bryozoans, a common type of colonial marine creature, which had began slowly declining in oceans across the world many millions of years before the mass-extinction event.

"Bryozoans form an excellent 'environmental dipstick' for studying the effects of mass extinctions," said Catherine Powers, a paleobiologist from the University of Southern California who co-authored a study on the new finding. "They live in all sorts of marine environments, from tropical reefs to muddy shallow waters to the deep sea," she added.

The most likely culprit for the gradual bryozoan die-offs was the upwelling of toxic water unleashed from the deep ocean by rising global temperatures.

"At the time, there was massive volcanism in Siberia, which threw up lots of carbon dioxide into the atmosphere, thus leading to global warming," said Powers. "The warming of surface water decreased the oceans' ability to absorb oxygen," she added.

This caused the upwelling of toxic hydrogen sulfide, which is produced by deep-ocean bacteria that don't use oxygen to survive.

"So it is likely that toxic fumes of hydrogen sulfide diffused out of the oceans, killing land-based animals, which plummeted by 70 percent in the same mass extinction event," said Powers.

This so-called 'Great Dying event' wiped out about 90 percent of marine species and 70 percent of land species.

The first to go were the deeper-dwelling animals, followed by the shallow-water inhabitants.

"It indicates that something from the deep ocean is coming up," National Geographic News quoted Powers as saying. "Whatever killed these organisms and probably led to this mass extinction is tied to ocean circulation processes," she added. (ANI)