Earth has been through many mass extinctions, and the causes of most remain mysterious. A new study suggests the ebbs and flows of ocean levels and sediment could be to blame.
"The expansions and contractions of those environments have pretty profound effects on life on Earth," says Shanan Peters, a University of Wisconsin-Madison assistant professor of geology and geophysics.
Peters argues that changes in ocean environments related to sea level exert a driving influence on rates of extinction, which animals and plants survive or vanish, and generally determine the composition of life in the oceans.
Few smoking guns
Since life sprang forth on Earth about 3.5 billion years ago, scientists think there may have been as many as 23 mass extinction events. 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 percent of species lost.
The dinosaur extinction 65 million years ago has a smoking gun: an impact crater that suggests a space rock was at least partly responsible for the die-off.
But the causes of other mass extinction events have been murky, at best.
"Paleontologists have been chipping away at the causes of mass extinctions for almost 60 years," said Peters, whose work was supported by the National Science Foundation. "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 main 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 explained in a statement, noting differences in sediment stability, temperature, and the availability of nutrients and sunlight.
In the course of 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 huge ontinental 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 idea was put forth by the journal Nature yesterday.
No one cause
Arnold I. Miller, a paleobiologist and professor of geology at the University of Cincinnati, says the new study is striking because it establishes a clear relationship between the tempo of mass extinction events and changes in sea level and sediment:
"Over the years, researchers have become fairly dismissive of the idea that marine mass extinctions like the great extinction of the Late Permian might be linked to sea-level declines, even though these declines are known to have occurred many times throughout the history of life. The clear relationship this study documents will motivate many to rethink their previous views."
The new study does not preclude other influences on extinction such as physical events like volcanic eruptions or killer asteroids, or biological influences such as disease and competition among species, Peters said. But what it does do, he argues, is provide a common link to mass extinction events over a significant stretch of Earth history.
"The major mass extinctions tend to be treated in isolation (by scientists)," Peters says. "This work links them and smaller events in terms of a forcing mechanism, and it also tells us something about who survives and who doesn't across these boundaries. These results argue for a substantial fraction of change in extinction rates being controlled by just one environmental parameter."