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Ocean’s Epic Ebb & Flow Linked to Mass Extinction

The mass extinctions that have exterminated life on Earth over vast stretches of time like the sudden demise of the dinosaurs 65 million years ago were caused by the oceans.

Since the life emerged on Earth 3.5 billion years ago, scientists think there may have been as many as 23 mass extinction events, many involving simple forms of life such as single-celled microorganisms. During the past 540 million years, there have been five well-documented mass extinctions, primarily of marine plants and animals, with as many as 75-95% of species lost.

Rather than volcanic eruptions or flaming asteroids hurling through space, new research links mass extinctions with the epic ebbs and flows of sea level and sediment over the course of geologic time. The research, published online today in the journal Nature, was conducted at the University of Wisconsin-Madison by Shanan Peters, an assistant professor of geology and geophysics at the University. Peters concluded that changes in ocean environments related to sea level exert a driving influence on rates of extinction, species survival rates and the composition of life in the oceans.

Scientists have not yet determined the cause of mass extinctions conclusively. Although compelling evidence links the demise of the dinosaurs with the atmospheric impact of an asteroid that struck the planet, the reasons behing other mass extinction events are still a matter of speculation.

“Paleontologists have been chipping away at the causes of mass extinctions for almost 60 years,” Peters said. “Impacts, for the most part, aren’t associated with most extinctions. There have also been studies of volcanism, and some eruptions correspond to extinction, but many do not.”

Peters measured two principal types of marine shelf environments preserved in the rock record, one where sediments are derived from erosion of land and the other composed primarily of calcium carbonate, which is produced in-place by shelled organisms and by chemical processes.

“The physical differences between [these two types] of marine environments have important biological consequences,” Peters said, noting differences in sediment stability, temperature, and the availability of nutrients and sunlight.

During hundreds of millions of years, the world’s oceans have expanded and contracted in response to the shifting of the Earth’s tectonic plates and to changes in climate. There were periods of the planet’s history when vast areas of the continents were flooded by shallow seas, such as the shark- and mosasaur-infested seaway that neatly split North America during the age of the dinosaurs.

As those epicontinental seas drained, animals such as mosasaurs and giant sharks went extinct, and conditions on the marine shelves where life exhibited its greatest diversity in the form of things like clams and snails changed as well.

The new research contemplates other influences on extinction such as physical events like volcanic eruptions or killer asteroids, or biological influences such as disease and competition among species occuring at different times. On the other hand, the research creates a common link to mass extinction events over a significant stretch of Earth history.

The research was funded by the National Science Foundation.