Surprise Discovery: Sleeping Giant Could End Deep Ocean Life

Ocean oxygen can be reduced by continental movement.

The positioning of continents, a previously unrecognized element, contributes to the oxygenation of Earth's waters. Moving continents might ultimately have the reverse result, killing most deep-sea animals.

According to Andy Ridgwell, a geologist at the University of California, Riverside, "Continental drift appears so sluggish, like nothing terrible could come from it, but when the ocean is primed, even a seemingly insignificant event might spark the widespread extinction of marine life." A recent study on factors influencing oceanic oxygen is co-authored by Ridgwell.

The ocean's surface water sinks as it gets colder and denser as it gets closer to the north or south pole. As the water sinks, oxygen drawn from the atmosphere of the planet is carried to the ocean floor.

At Pearl and Hermes Atoll in the Papahnaumokukea Marine National Monument close to Hawaii, fish are seen on a deep reef. Thanks to Greg McFall and NOAA

When organic matter sinks, nutrients released from it eventually return to the ocean's surface, feeding plankton development. The enormous variety of fish and other creatures found in today's oceans is sustained by both the continuous flow of oxygen to lower depths and the organic matter created at the surface.

This oxygen and nutrient circulation can abruptly stop, according to recent study. The scientists looked into whether the placement of tectonic plates affected how the ocean circulates oxygen using sophisticated computer models. They were taken aback to learn that it does.

The journal Nature describes this discovery made by scientists at UC Riverside. It was released on today (August 17, 2022).

"The entire global ocean circulation appears to occasionally shut down many millions of years ago, not long after animal life in the ocean got started," said Ridgwell. We did not anticipate discovering that the migration of continents may cause surface waters and oxygen to stop sinking, potentially having a significant impact on how life developed on Earth.

Up until recently, very straightforward models that did not take into consideration ocean circulation had been used to look into how marine oxygen has changed over the last 540 million years. These models assumed a decline in atmospheric oxygen concentrations during periods of ocean anoxia, or the absence of oceanic oxygen.

According to Alexandre Pohl, the study's first author and a former paleoclimate modeler at UCR who is now at Université Bourgogne Franche-Comté in France, "scientists previously considered that shifting oxygen levels in the water primarily reflected similar oscillations in the atmosphere."

The Smithsonian Institution has a diorama depicting aquatic life from the Ediacaran era. Thanks to Smithsonian

This study used a model for the first time that included an ocean in three dimensions and took ocean currents into consideration. The findings show that a sharp divergence between oxygen concentrations in the top and lower depths is caused by a collapse in global water circulation.

This separation caused the seafloor to completely lose oxygen, with the exception of shallow areas near the coast, for many tens of millions of years, until the beginning of the Silurian period about 440 million years ago.

Anything that couldn't swim closer to the surface and the oxygen that was still there in the atmosphere would have perished if circulation had collapsed, according to Ridgwell. Fish with strange appearances, enormous worms and crustaceans, squid, sponges, and other creatures inhabit the deep.

If and when Earth might anticipate a comparable event in the future are not addressed in the research. In fact, it might be challenging to pinpoint the exact moment or cause of a collapse. Existing climate models, however, show that rising temperatures would impair ocean circulation, and some of them even show that the branch of the circulation that begins in the North Atlantic will eventually collapse.

To forecast a catastrophic extinction catastrophe, Ridgwell stated that a better resolution climate model would be required. "Having said that, there are indications that the flow of water to depth is decreasing, and we do have worries about water circulation in the North Atlantic today."

Theoretically, Ridgwell said, a summer that is abnormally warm or a cliff's erosion may set off a chain of events that upends life as we know it.

"You'd assume the action would be on the ocean's surface, where you could go sailing or surfing. The ocean, however, is continuously at work beneath the surface, giving organisms in the shadowy depths essential oxygen, according to Ridgwell.

The ocean promotes life, but it also has the power to end it. As the continental plates continue to shift, nothing rules that out.

Ocean oxygenation during the Phanerozoic is controlled by continental configuration, according to a study published in Nature on August 17, 2022, by Alexandre Pohl, Andy Ridgwell, Richard G. Stockey, Christophe Thomazo, Andrew Keane, Emmanuelle Vennin, and Christopher R. Scotese.                                                                                                                                                          By UNIVERSITY OF CALIFORNIA - RIVERSIDE