NC State scientist looked back millions of years to see future ocean ‘dead zones’

A large dead zone existed in the North Atlantic Ocean the last time global temperatures were as high as they could be 100 years from now, according to a new study led by an N.C. State University researcher.

To find a time as consistently warm as temperatures are expected to become, scientists had to look back to the Pliocene era. That was 2.6 million to 5.3 million years ago.

Catherine Davis, an N.C. State assistant professor of marine, earth and atmospheric sciences, worked with a team of researchers to find where oxygen minimum zones existed during that period. Also known as “dead zones,” the areas exist between 200 and 1,000 meters below the ocean’s surface and lack enough oxygen for most fish and other aquatic life to survive.

Right now, those zones are mostly found in the Indian and Pacific Oceans, Davis said. But the peer-reviewed study published in Nature Communications found fewer dead zones in the North Pacific.

“We can also see them in the Atlantic in a warmer world, which is not something that has traditionally been taken into account,” Davis told The News & Observer.

Collaborators on the study included researchers from George Mason University and Yale University.

To determine where dead zones existed in the Pliocene era, scientists looked in global sediment records for fossils from a plankton that thrives in low-oxygen areas. They compared those with computer models that simulated dead zones. Where the two overlapped, scientists could be confident they’d found an ancient dead zone.

“We had both a theoretical simulation tact, as well as a very data-based ‘did this exist in the sediment’ tact,” Davis said.

When oceans warm up — as scientists are already seeing due to climate change caused by humans’ release of greenhouse gases — oxygen levels start to drop.

“In the past couple of decades, we’ve been seeing how the oceans are losing their oxygen. So people are getting concerned about that, how it’s going to continue changing in the future so that we can adapt to it like we want to adapt to anything with climate change,” said Anya Hess, a Ph.D. candidate at Rutgers University who studies paleoceanography.

Studies like Davis’ could be used, Hess said, to understand those changes and potentially adapt to them.

Davis agreed with that, pointing to the fact that dead zones can effectively act as a floor for many fish. That means the creatures can be squeezed into the area between the dead zone and the surface, an idea that is important for fisheries managers to be aware of.

“Without taking into account where these dead zones are, you can see lots of fish for example in a survey and what’s happening is not that you have more fish resources, it’s that all of those fish are vertically compressed to the surface,” Davis said.

A key limitation of the study, Davis said, is that the fossil record is looking at a compressed sample that covers millions of years while the computer model offers snapshots that cover millenniums at a time. By comparison, humans consider events that happen over years, decades or a lifetime.

Oxygen rates are decreasing in the ocean, though, at levels that previously would have taken periods much longer than a human’s lifetime.

“We are seeing things that are happening on the scales of decades or hundreds of years that in geologic records were taking thousands or millions of years,” Davis said.

To better understand those, Davis is interested in next researching how a still-warming world can impact dead zones.

Where temperatures remained largely stable during the Pliocene, scientists expect warming to continue as more greenhouse gases are released over coming decades and centuries. It’s unlikely, Davis said, that the climate will have stabilized a century from now.

“A warming climate is more what we’re experiencing right now or in actuality will be experiencing 100 years from now, most likely,” Davis said.

This story was produced with financial support from 1Earth Fund, in partnership with Journalism Funding Partners, as part of an independent journalism fellowship program. The N&O maintains full editorial control of the work.