Ebb And Flow Of The Sea Drives World's Big Extinction Events

ScienceDaily 15 Jun 08;

If you are curious about Earth's periodic mass extinction events such as the sudden demise of the dinosaurs 65 million years ago, you might consider crashing asteroids and sky-darkening super volcanoes as culprits.

But a new study, published online June 15, 2008 in the journal Nature, suggests that it is the ocean, and in particular the epic ebbs and flows of sea level and sediment over the course of geologic time, that is the primary cause of the world's periodic mass extinctions during the past 500[sc1] million years.

"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 and the author of the new Nature report.

In short, according to Peters, 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.

Since the advent of life 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 percent of species lost.

For the most part, scientists have been unable to pin down the causes of such dramatic events. In the case of the demise of the dinosaurs, scientists have a smoking gun, an impact crater that suggests dinosaurs were wiped out as the result of a large asteroid crashing into the planet. 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," [sc2] explains 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."

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."

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 explains, 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 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 Wisconsin study, Peters says, 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. 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."

Flux in ocean levels drove mass extinctions: study
Marlowe Hood, Yahoo News 17 Jun 08;

Mass extinctions that wiped out up to 90 percent of Earth's flora and fauna were driven in large part by shifting ocean levels, according to a study published in Nature.

Understanding what made many of the planet's living organism rapidly die out at least five times over the last half billion years remains one of the great challenges in paleontology and biology.

Some theories point an accusing figure at the cooling effect of massive dust shrouds thrown into the atmosphere by volcanoes and asteroids crashing into Earth, or the warming caused by rising levels of carbon dioxide.

Other scientists highlight the role of disease and competition among species for limited resources.

But the new study suggests that it was the ebb and flow of sea levels and sediment over geologic time, rather than cataclysmic events, that doomed tens of thousands of species to extinction.

"The expansions and contractions of those environments have pretty profound effects on life on Earth," said Shanan Peters, a geologist at the University of Wisconsin-Madison and sole author of the study.

Since the beginning of life on Earth, some 3.5 billion years ago, there have been more than 20 mass extinction events, many involving single-cell organisms, say scientists.

And during the last 540 million years that have been five well-documented periods of mass die offs, mainly of marine plants and animals.

With the exception of a colossal asteroid impact some 65 million years ago that left a 180-kilometre (110-mile) wide crater in Mexico, the cause for the other mass extinctions is sharply contested.

Even that one coincided with an abrupt retraction of oceans then covering much of North America and Europe that could have played a major role in the disappearance of dinosaurs, Peters said.

To test his hypothesis, Peters measured to two types of ancient shallow marine environments preserved in the rock record.

One corresponds to typical vacation spots -- white sand beaches, clear blue water -- and is composed mainly of calcium deposits produced by organisms with shells.

The other is characterised by brown or muddy sand, rocky beaches and water that is greenish and cloud. Over time, sediments in these areas accumulate from land erosion.

"I looked at rates of extinction in the fossil record over the last 500 million years," Peters told AFP. "And then I compared them to the environmental changes -- mostly explained by shifts in sea level -- that are encoded in the sedimentary rocks."

What Peters found was a very strong match, showing that the sometimes dramatic rise and fall of oceans levels correlated more consistently with mass extinctions that any other factor.

Sea levels rose more than 80 meters at the end of the last major glacial period some 15,000 years ago, he said.

Climate and the movement of tectonic plates are the key factors that influence the degree to which the continents would flood.

"Most people think of sea level changes in terms of depth of meters or feet. I am looking at a different measure -- the environmental consequences of sea level change, the impact on habitats," he explained.