A sleeping giant could end life in the deep ocean

Continental movement is capable of suffocating marine oxygen.

A previously overlooked factor – the location of the continents – helps fill Earth’s oceans with life-supporting oxygen. Continental movement could eventually have the opposite effect, killing the majority of deep-ocean organisms.

“Continental drift seems so slow, nothing drastic can come from it, but when the ocean is primed, a seemingly small event can lead to the death of marine life on a large scale,” said Andy Ridgewell, a Riverside geologist at the University of California. Ridgewell, co-author of a new study on the forces that affect oceanic oxygen.

As the water on the ocean’s surface approaches the North or South Pole, it becomes colder, denser, and then sinks. As the water sinks, it transfers the oxygen pulled from the Earth’s atmosphere to the ocean floor.

Fish on deep reefs in Pearl and Hermes Atoll in Papahanaumokuakea National Monument near Hawaii. Credit: Greg McFall, NOAA

Ultimately, the return flow brings nutrients released from the sinking organic matter to the ocean’s surface, where they fuel plankton growth. Today’s oceans feature an astonishing diversity of fish and other animals supported by both a continuous supply of oxygen to the lower depths and organic matter produced at the surface.

New research has found that the circulation of oxygen and nutrients can end suddenly. Using complex computer models, the scientists investigated whether the positions of the continental plates influence how the ocean moves oxygen. They were surprised to find that.

This finding, led by researchers from the University of California, Riverside, is detailed in the journal temper nature. It was published today (August 17, 2022).

Balloon fish resting near the Florida Keys. Credit: OAR/National Marine Research Program (NURP); University of Maine

“Several million years ago, not long after animal life began in the ocean, the circulation of the entire global ocean seemed to periodically cease,” Ridgwell said. “We weren’t expecting to find that the motion of the continents could cause surface water and oxygen to stop sinking, possibly significantly affecting the way life on Earth evolved.”

Until now, models used to investigate the evolution of marine oxygen over the past 540 million years were relatively simple and did not take into account ocean circulation. In these models, ocean hypoxia—the times when oceanic oxygen has disappeared—indicate a decrease in atmospheric oxygen concentrations.

“Scientists previously assumed that changing oxygen levels in the ocean mostly reflect similar fluctuations in the atmosphere,” said Alexandre Paul, the study’s first author and a paleoclimate model designer at Bourgogne-Franch-Comté University in France.

Dioramas of seal ships from the ancient Ediacaran period on display at the Smithsonian Institution. Credit: Smithsonian

For the first time, this study used a model in which the ocean was represented in three dimensions, and ocean currents were calculated. According to the findings, a breakdown in the global water circulation leads to a stark separation between oxygen levels at the upper and lower depths.

This separation meant that the entire sea floor, with the exception of shallow places near the coast, had completely lost oxygen for tens of millions of years, until about 440 million years ago at the beginning of the Silurian period.

“The circulatory collapse was a death sentence for anything that couldn’t swim close to the surface and still have life-giving oxygen in the atmosphere,” Ridgwell said. Deep creatures include strange-looking fish, worms, giant crustaceans, squid, sponges, and more.

The paper does not address whether or when the Earth expects a similar event in the future. In fact, it’s hard to say when a crash might happen, or what triggers it. However, current climate models assert that increased global warming will impair ocean circulation, and some models predict a final collapse of the branch of circulation that begins in the North Atlantic.

“We will need a high-resolution climate model to predict the mass extinction event,” Ridgwell said. “However, we do have concerns about water circulation in the North Atlantic today, and there is evidence that water flow to depth is declining.”

In theory, an unusually warm summer or cliff erosion could trigger a cascade of processes that reverse life as it appears today, Ridgwell said.

“You would think that the surface of the ocean, the part where you might surf or sail, is where all the action is. But below, the ocean is working tirelessly away, providing vital oxygen to animals in the dark depths,” Ridgewell said.

“The ocean allows life to thrive, but it can take that life away again. Nothing rules that out as the continental plates keep moving.”

Reference: “Continental Formation Controls Ocean Oxygenation During Aeons of Wildlife” by Alexander Ball, Andy Ridgewell, Richard J. Stocky, Christophe Tomazo, Andrew Kane, Emmanuel Finin, Christopher R. temper nature.
DOI: 10.1038 / s41586-022-05018-z