Ocean Extinction

Bridget O'Brien
January 01, 2000

When the continents were moving together to form the giant landmass Pangaea, over 250 million years ago, more than 500 animal families lived in the ocean. There were rugose and tabulate corals, trilobites, bryozoans, and brachiopods, as well as some fish and amphibians. At the end of the Permian period 95 percent of these marine species were extinct.

"The extinction took millions of years, but had extraordinary consequences for life," explains Roberta Hotinski, a doctoral candidate in geosciences. On land, the once-dominant amphibians gave way to reptiles and the age of dinosaurs. At sea, the extinction cleared the way for new corals, fish, and arthropods in the ocean we know today.

ocean with cliff in distance
Charles Fergus / Nancy Marie Brown

What's the future of the oceans? Some 250 million years ago, 95 percent of marine creatures went extinct. Learning why could help us predict what's to come.

What caused such a cataclysm? A handful of theories exist. Some are similar to those suggested for the later extinction of the dinosaurs, including a large meteor hitting the Earth or massive volcanic eruptions. Both events could have released clouds of dust into the air, changing the climate and altering ecological niches. But no evidence exists for either at the end of the Permian period. Instead, this mass extinction seems to have been caused by stagnation, a reduced circulation of ocean waters. "The geologic record hasn't shown a definitive answer," explains Hotinski, "but this has become the most popular theory because there is some evidence for it."

Two theories explain how stagnation could have caused extinctions. Either it led to anoxia, a lack of oxygen, or to hypercapnia, the buildup of carbon dioxide. "Bacteria use oxygen to decompose organic matter. As they use it up, anoxia occurs, and carbon dioxide is released. It's the opposite of photosynthesis," says Hotinski. "Anoxia in shallow and deep water is recorded in the geologic record. It may have killed the shore life in surface waters. The record also shows that organisms that are more susceptible to carbon dioxide poisoning were preferentially killed, supporting the theory that hypercapnia occurred."

But were they the result of ocean stagnation? To find out, Hotinski used a general circulation model, a computer representation of the ocean to which a scientist can apply wind stress and temperature changes at the surface. The computer model tracks salinity and temperature and then uses physics to describe the ocean circulation. In her model, Hotinski warmed the poles about 12 degrees C as if a warming trend had occurred: the geologic record suggests that the Triassic period was warmer than its Permian predecessor. "During the End-Permian period, the climate was not dramatically different from today," explains Hotinski. "The major difference is that the continents were grouped together in one big land mass."

The model showed that when Hotinski warmed the poles, the circulation of waters did indeed slow down and stop. The period of stagnation was brief, only a few hundred years, but the reduced overturning of waters was enough to trigger a greater zone of low oxygen levels in both the intermediate waters (beginning at about 100 meters from the surface) and on the sea floor.

On the other hand, carbon dioxide did not build up significantly. "I increased the amount of phosphate in the upwelling water to boost productivity and to see if hypercapnia would occur." In her model, phosphate is the main nutrient, and increasing it enlarges the amount of organic matter produced. More organic matter leads to greater carbon dioxide levels when it is decomposed. But even the addition of 50 percent more phosphate did not cause carbon dioxide to build up to the level needed for hypercapnia. Concludes Hotinski, "Ocean stagnation would cause deep water anoxia, but it would not cause the carbon dioxide buildup needed for hypercapnia."

As ocean models become more complex, more details may emerge. Hotinski is now working on a more realistic model of the ocean floor. For example, rather than estimating the levels of sulfide and carbon dioxide based on the amount of oxygen used in decomposition, she is now calculating them directly by putting them into her model.

Understanding the Permian extinction, she says, may shed light on the future. "There are implications for Global Warming," Hotinski says, "but we can't extrapolate too far." The mechanisms for extinction occur over a long time scale, but warming the climate today may gradually alter the circulation and chemistry of the ocean. Understanding past extinctions may influence future predictions of how warming will affect the ocean and the atmosphere over time.

Roberta Hotinski is a doctoral candidate in the College of Earth and Mineral Sciences. Her adviser is Lee Kump, Ph.D., professor of geosciences, 303 Deike Bldg., University Park, PA 16802; 814-863-1274; kump@geosc.psu.edu. The National Science Foundation provided the funding for the research.

Last Updated January 01, 2000