Flux in ocean levels drove mass extinctions: study

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.

Copyright AFP (Agence France-Presse), 2008