Red medusa found at the bottom of the deep sea in Alaska. Credit: Hidden Ocean 2005/NOAA
Continental movement could stifle oxygen in the sea.
A factor previously overlooked – the location of the continents – helps fill Earth’s oceans with life-supporting oxygen. Continental movement could ultimately have the opposite effect, killing most deep-ocean creatures.
“The continental drift seems so slow, as if nothing drastic could come from it, but when the ocean is ready, even a seemingly small event can trigger widespread death of marine life,” said Andy Ridgwell of the University of California, Riverside. Ridgwell is co-author of a new study on the forces affecting ocean oxygen.
As the water on the ocean’s surface approaches the north or south pole, it gets colder and denser and then sinks. When water sinks, it carries oxygen drawn from Earth’s atmosphere to the ocean floor.
Fish on a deep reef at Pearl and Hermes Atoll at Papahānaumokuākea Marine National Monument near Hawaii. Credit: Greg McFall, NOAA
Eventually, a return flow brings nutrients released from submerged organic matter back to the ocean surface, where it feeds the growth of plankton. Today’s oceans have an incredible diversity of fish and other animals, powered by both an uninterrupted supply of oxygen to lower depths and organic matter produced at the surface.
New research has found that this circulation of oxygen and nutrients can come to an abrupt end. Using sophisticated computer models, the scientists investigated whether the positions of continental plates affect how the ocean moves oxygen. They were surprised to see that it was.
This finding, led by UC Riverside-based researchers, is detailed in the journal Nature. Nature. Released today (August 17, 2022).
A puffer fish is resting near the Florida Keys. Credits: OAR/National Submarine Research Program (NURP); University of Maine
“Millions of years ago, very shortly after animal life began in the ocean, the entire global ocean circulation seemed to shut down periodically,” Ridgwell said. “We didn’t expect to find that the movement of the continents could stop the sinking of surface waters and oxygen, possibly significantly affecting the evolution of life on Earth.”
Until now, the models used to study the evolution of marine oxygen over the last 540 million years have been relatively simple and did not take ocean circulation into account. In these models, ocean anoxia — times when ocean oxygen was lost — meant a drop in atmospheric oxygen concentrations.
“Scientists previously assumed that changing oxygen levels in the ocean mostly reflected similar fluctuations in the atmosphere,” said Alexandre Pohl, first author of the study and former UCR paleoclimate modeler, now at the Université Bourgogne Franche-Comté in France.
Diorama of ancient Ediacaran period marine life on display at the Smithsonian Institution. Credit: Smithsonian
For the first time in this study, a model in which the ocean is represented in three dimensions and ocean currents are taken into account is used. According to the results, the collapse in global water circulation causes a sharp distinction to be made between oxygen levels at the upper and lower depths.
This separation meant that the entire seafloor, with the exception of shallows near the shore, completely lost oxygen over tens of millions of years, up until about 440 million years ago, at the beginning of the Silurian period.
“Collapse of circulation would be a death sentence for anything that couldn’t float closer to the surface and still had life-giving oxygen in the atmosphere,” Ridgwell said. Said. Creatures of the deep include strange-looking fish, giant worms and crustaceans, squid, sponges, and more.
The article does not address whether or when Earth might expect a similar event in the future. In fact, it is difficult to determine when a collapse may occur or what triggered it. However, current climate models confirm that increased global warming will weaken ocean circulation, and some models even predict the eventual collapse of the circulation branch that began in the North Atlantic.
“We need a higher resolution climate model to predict a mass extinction event,” Ridgwell said. Said. “However, we already have concerns about water circulation in the North Atlantic today, and there is evidence of reduced water flow to depth.”
In theory, an unusually hot summer or erosion of a cliff could trigger a series of processes that ended life as it appears today, Ridgwell said.
“You would think that the ocean surface, where you can surf or sail, is where all the action is. But underneath, the ocean works tirelessly and provides vital oxygen to animals in the dark depths,” said Ridgwell.
“The ocean allows life to evolve, but it can take life again. Nothing excludes that as the continental plates continue to move.”
Reference: Alexandre Pohl, Andy Ridgwell, Richard G. Stockey, Christophe Thomazo, Andrew Keane, Emmanuelle Vennin, and Christopher R. Scotese, “Continental configuration controls ocean oxygenation during the Phanerozoic”, Aug 17, 2022. Nature.
DOI: 10.1038/s41586-022-05018-z
