Mass Extinctions Due to Sea Level Changes, Study Says

John Roach
for National Geographic News

June 17, 2008

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].)

Sea Changes

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.

The other environment has sediments composed mostly of calcium carbonate, which is produced by organisms living in the water—such as corals—and chemical processes. These waters are clear, as in the Caribbean.

The seas rise and fall as the climate shifts and as the Earth's tectonic plates move. As a result, these two types of marine environments expand and contract, Peters said.

The marine animals in them sometimes disappear when their shallow-water environments vanish or are dramatically altered.

"It is not just a matter of rising and falling sea level, it is the environmental consequences that go along with changes in sea level that I'm measuring in this study," Peters said.

He found that all five mass extinction events, and many of the smaller events in the fossil record, are associated with changes in sea level and sedimentation.

Shrinking Habitat

Wolfgang Kiessling, a paleontologist at Humboldt University in Berlin, Germany, is an expert on mass-extinction events.

While he doubts that changing sea levels alone drive mass die-offs, he said the study is important because it shows that habitat size is relevant to species diversity.

"The bigger habitat you have, the more species you can sustain—and when habitat shrinks, you get increased risk of extinction," said Kiessling, who was not involved in the study.

However, he added, changes in sea level alone are insufficient to explain the magnitude of mass-extinction events such as the one 65 million years ago, which wiped out the dinosaurs. (See "Volcano Theory of Dino Die-Off Gets New Support" [November 5, 2007].)

"There's got to be more," he said, "something like supervolcanoes, asteroid impacts. … "