Continental drift has the ability to block ocean oxygen.
A previously overlooked factor – the location of the continents – is helping to fill Earth’s oceans with life-supporting oxygen. Continental drift may eventually have the opposite effect, killing most deep-sea species.
“Continental drift seems too slow for anything drastic to come of it, but the ocean is the primary ocean, where even a seemingly small event can lead to the death of marine life on a large scale,” said Riverside geologist Andy Ridgewell. Ridgewell is a co-author of a new study on the forces that affect ocean oxygen.
As the ocean’s surface water approaches the North or South Pole, it becomes colder and denser and then sinks. As the water sinks, it carries oxygen that is pulled from the Earth’s atmosphere to the ocean floor.
Fish in the deep reefs of 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 to the ocean’s surface, where they fuel plankton growth. Today’s oceans contain an astonishing diversity of fish and other animals, powered by a continuous supply of oxygen to the shallow depths and organic matter produced at the surface.
New research finds that this cycle of oxygen and nutrients can end abruptly. Using complex computer models, scientists have studied how the location of the continental plates moves ocean oxygen. They were surprised to see that.
The discovery, led by researchers from the University of California Riverside, is detailed in the journal temper nature. It was released today (August 17, 2022).
Puffer fish are located near the Florida Keys. Credit: OAR/National Marine Research Program (NURP); Maine University
“Millions of years ago, soon after animal life began in the ocean, the entire global ocean cycle seemed to stop periodically,” Ridgwell said. “We do not expect continental drift to stop the sinking of surface water and oxygen, and to significantly affect the evolution of life on Earth.”
Until now, models used to investigate the evolution of ocean oxygen over the past 540 million years were relatively simple and did not take into account ocean circulation. In these models, marine hypoxia—the times when ocean oxygen disappears—represents a decrease in atmospheric oxygen concentrations.
“Scientists previously thought that changing oxygen levels in the ocean often reflect similar fluctuations in the atmosphere,” said Alexandre Paul, the study’s first author and designer of the UCR paleoclimate model, now at Bourgogne-Franch-Comte University in France.
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, where ocean currents were calculated. According to the findings, the decline in the global water cycle leads to a stark separation between oxygen levels at the upper and lower depths.
This separation deprived the entire ocean floor of oxygen, except for shallow places near the coast, for several billion years, starting with the Silurian period about 440 million years ago.
“The circulatory collapse was a death sentence for anything that couldn’t swim close to the surface and still had lifelike oxygen in the atmosphere,” Ridgwell said. Among the creatures of the depths are strange-looking fish, worms, giant crustaceans, squid, seaweed, and others.
The paper did not specify when or whether the Earth expects a similar event in the future. In fact, it is difficult to say when a crash occurred or what might lead to it. However, current climate models assert that global warming will impair ocean circulation, and some models predict a final collapse of the circulatory branch that begins in the North Atlantic.
“We need a high-resolution climate model to predict a mass extinction event,” Ridgewell said. “We already have concerns about the water cycle in the North Atlantic today, and there is evidence that water flow to depth is decreasing.”
In theory, an unusually warm summer or cliff erosion could trigger the cascade of life-enhancing processes seen today, Ridgewell said.
“The surface of the ocean, you might think you could surf or go sailing,” Ridgwell said. But the ocean works tirelessly below to provide vital oxygen to animals in the dark depths.”
“The ocean allows life to thrive, but it also gives that life back. Nothing dictates it because the continental plates are constantly moving.”
Note: Alexander Ball, Andy Ridgewell, Richard J. Stocky, Christophe Tommaso, Andrew Kane, Emmanuel Finin, and Christopher R. Scotties, 17 August 2022, “Continental Formation Controls Ocean Oxygenation During Aeons of Life” temper nature.
DOI: 10.1038/s41586-022-05018-z