A sleeping giant could end life in the deep ocean

A red jellyfish was discovered on the bottom of the deep sea in Alaska. Credit: Hidden Ocean 2005/NOAA

Continental drift has the potential to block ocean oxygen.

A previously overlooked factor—the position of the continents—helps fill Earth’s oceans with life-supporting oxygen. Continental drift may ultimately have the opposite effect, killing most deep-sea species.

“Continental drift seems so slow that nothing drastic comes of it, but the ocean is the primary one, where even a seemingly small event can trigger widespread die-offs of marine life,” Riverside geologist Andy Ridgwell said. Ridgwell is co-author of a new study on the forces affecting ocean oxygen.

As the surface water of the ocean approaches the North or South Pole, it becomes colder and denser and then sinks. As water sinks, it carries oxygen that is drawn down from Earth’s atmosphere to the ocean floor.

Deep Reef Fish Papahanamokuakea Marine National Monument

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

Ultimately, the return flow brings nutrients released from submerged organic matter back to the ocean surface, where it fuels the growth of plankton. Today’s oceans contain an incredible diversity of fish and other animals, supported by an uninterrupted supply of oxygen to shallow depths and organic matter produced at the surface.

New research has found that this cycle of oxygen and nutrients can come to an abrupt end. Using complex computer models, scientists studied how the location of continental plates moves ocean oxygen. They were surprised to see it.

The discovery, led by researchers based at UC Riverside, is detailed in the journal Nature. It was released today (August 17, 2022).

A relaxing balloon fish

Puffer fish rest near the Florida Keys. Credit: OAR/National Undersea Research Program (NURP); University of Maine

“Millions of years ago, shortly after animal life began in the ocean, the entire global ocean cycle seemed to shut down periodically,” Ridgwell said. “We don’t expect continental drift to stop the sinking of surface water and oxygen, and dramatically affect the evolution of life on Earth.”

Until now, the models used to investigate the evolution of ocean oxygen over the past 540 million years were relatively simple and did not account for ocean circulation. In these models, marine anoxia—times when ocean oxygen disappears—represents a drop in atmospheric oxygen concentrations.

“Scientists previously thought that changing oxygen levels in the ocean often mirrored similar fluctuations in the atmosphere,” said Alexandre Boll, the study’s first author and former UCR paleoclimate modeler, now at Université Bourgogne-Franche-Comte in France.

A diorama of ancient Ediacaran marine life

A diorama of ancient Ediacaran marine life 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, in which ocean currents were calculated. According to the results, a decline in the global water cycle leads to a stark separation between oxygen levels at the top and bottom depths.

That separation completely deprived the entire ocean floor of oxygen, except in shallow places near the coast, for several billion years, beginning in the Silurian period about 440 million years ago.

“Circulation collapse would have been a death sentence for anything that couldn’t swim close to the surface and still have life-giving oxygen in the atmosphere,” Ridgwell said. Among the creatures of the deep are strange-looking fish, giant worms and crustaceans, squid, seaweeds and more.

The paper did not specify when or if Earth might expect a similar event in the future. In reality, it is difficult to determine when a collapse might occur or what might trigger it. However, existing climate models confirm that global warming will weaken the ocean circulation, and some models even predict an eventual collapse of the branch of the circulation that begins in the North Atlantic.

“We need a high-resolution climate model to predict a mass extinction event,” Ridgwell said. “We already have concerns about the water cycle in the North Atlantic today, and there is evidence that the flow of water to depth is decreasing.”

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

“At the surface of the ocean, you’d think you could surf or sail. But below, the ocean is working tirelessly to provide vital oxygen to animals in the murky depths,” Ridgwell said.

“The ocean allows life to flourish, but it also takes that life back. Nothing dictates it because the continental plates are constantly moving.”

Note: Alexandre Ball, Andy Ridgwell, Richard G. Stuckey, Christophe Tommaso, Andrew Keen, Emmanuel Venin, and Christopher R. Scotties, 17 Aug. 2022, “Continental configuration controls ocean oxygenation during Phanerozoic” Nature.
DOI: 10.1038/s41586-022-05018-z

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