for National Geographic News
The rise and fall of the seas may have a more lethal toll on Earth's life than asteroids and supervolcanoes, according to a new study.
Over the past 540 million years, every increase in the rate of extinctions—including the five so-called mass extinctions—has been linked to environmental changes wrought by changing sea levels, the study says.
Only some mass-extinction events, though, have been clearly linked to space-rock impacts and supervolcano eruptions—blasts many times greater than any in recorded times—researchers say.
"To me, that is pretty striking," study leader Shanan Peters, a geologist at the University of Wisconsin, Madison, said.
The research may be especially relevant today, as what some scientists are calling the sixth mass extinction may already be underway, perhaps due to global warming.
(Related: "Ancient Mass Extinctions Caused by Cosmic Radiation, Scientists Say" [April 20, 2007].)
Since the beginning of life on Earth 3.5 billion years ago, scientists estimate there have been as many as 23 major extinction events.
During the past 540 million years, there have been 5 major mass extinctions, primarily of marine plants and animals. Each time, between 75 and 95 percent of all species vanished. (See a prehistoric time line.)
The idea that sea level changes are associated with these mass-extinction events has been around for almost 60 years, Peters noted, but until now scientists have been unable to quantify the environmental consequences of sea level change.
Peters and colleagues examined two types of shallow marine environments preserved in the fossil record for their study, to be published in the journal Nature tomorrow.
In one environment, sediments from land erosion are dumped into the oceans. These waters tend to be murky, filled with particles and algae.
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