A team of researchers from the University of Exeter has shed new light on the causes of Ocean Anoxic Event 2, which led to severe global warming and ocean acidification on Earth approximately 94 million years ago. The study was published on: Nature Communication.

This event, called Ocean Anoxic Event 2, marked one of the most significant climatic disasters in the geological record, causing extinctions in the oceans and on land.

Anoxic events are long periods during which large areas of the Earth’s oceans are deprived of dissolved oxygen, creating toxic waters and leading to mass extinctions and habitat loss. The cause of this anoxic phenomenon, which lasted more than 500,000 years, has been a matter of debate among experts.

But the research team showed that the likely trigger was massive volcanism, mapping the source on a vast oceanic plateau beneath the remote Kerguelen Islands. For the new study, researchers analyzed geochemical and micropaleontological data collected by the International Ocean Exploration Program from ocean sediment cores in the Mentella Basin, located off the coast of Western Australia.

They found that sediment cores showed clear evidence of sedimentary mercury, implying a series of significant “pulses” of intense massive volcanism before and during the main phase of the anoxic event. Additionally, radiogenic isotopes of neodymium and strontium indicate that the Kerguelen Plateau, much closer to the Mentelle Basin, was uplifted by volcanic activity during the Cretaceous period.

The research team suggests that powerful volcanic events will result in an explosion of carbon dioxide, which will contribute to the warming of the climate and cause the acidification of the oceans. They believe that this phenomenon was the main cause of the anoxic event and caused the Earth to approach the turning point of the transition to the “greenhouse world” at that time.

Chloe Walker-Trivett, who oversaw the research during her PhD. The research, conducted at Camborne School of Mines, University of Exeter Penrhyn Campus, said: “Although OAE 2 has been studied really well, most research so far has focused on the northern hemisphere, leading to a one-sided view of the event.

“Our Southern Hemisphere research site near southwestern Australia was at a high southern latitude (~60 degrees south) in the mid-Cretaceous when OAE 2 occurred, giving us a completely new perspective and pointing to Kerguelen volcanism as a possible trigger.”

Seve Kender, from the University of Exeter and co-author of the study, added: “The timing of mass volcanism in the geological past is difficult to determine, but is crucial if we are to use past rapid global warming events as an analogue of volcanism that could possibly change future climate.”

“Our key innovation was the combination of a new mercury proxy for volcanic eruptions and radiogenic neodymium and strontium isotopes that identify sources of eroded rocks in the ocean basin.

“Increasing amounts of eroded material from a volcanic source indicated that the nearby Kerguelen Major Igneous Province was uplifted during active volcanism, rather than some other volcanic regions previously thought to be responsible.”