Sea's Ebb And Flow Drive World's Big Extinction Events

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."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." Sciencedaily-