MASS EXTINCTIONS DUE TO SEA LEVEL CHANGES, STUDY SAYS
By John Roach
National Geographic News
June 17, 2008

Original Link

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.

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.

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

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WHAT CAUSES ANIMAL DIE-OFFS?
National Geographic

Original Link

More than 90 percent of all organisms that have ever lived on Earth are extinct. As new species evolve to fit ever changing ecological niches, older species fade away. But the rate of extinction is far from constant. At least a handful of times in the last 500 million years, 50 to more than 90 percent of all species on Earth have disappeared in a geological blink of the eye.

Though these mass extinctions are deadly events, they open up the planet for new life-forms to emerge. Dinosaurs appeared after one of the biggest mass extinction events on Earth, the Permian-Triassic extinction about 250 million years ago. The most studied mass extinction, between the Cretaceous and Paleogene periods about 65 million years ago, killed off the dinosaurs and made room for mammals to rapidly diversify and evolve.

The causes of these mass extinction events are unsolved mysteries, though volcanic eruptions and the impacts of large asteroids or comets are prime suspects in many of the cases. Both would eject tons of debris into the atmosphere, darkening the skies for at least months on end. Starved of sunlight, plants and plant-eating creatures would quickly die. Space rocks and volcanoes could also unleash toxic and heat-trapping gases that -- once the dust settled -- enable runaway global warming.

An extraterrestrial impact is most closely linked to the Cretaceous extinction event. A huge crater off Mexico's Yucatán Peninsula is dated to about 65 million years ago, coinciding with the extinction. Global warming fueled by volcanic eruptions at the Deccan Flats in India may also have aggravated the event. Whatever the cause, dinosaurs, as well as about half of all species on the planet, went extinct.

Massive floods of lava erupting from the central Atlantic magmatic province about 200 million years ago may explain the Triassic-Jurassic extinction. About 20 percent of all marine families went extinct, as well as most mammal-like creatures, many large amphibians, and all non-dinosaur archosaurs. An asteroid impact is another possible cause of the extinction, though a telltale crater has yet to be found.

Largest Ever Die-Off

The Permian-Triassic extinction event about 250 million years ago was the deadliest: More than 90 percent of all species perished. Many scientists believe an asteroid or comet triggered the massive die-off, but, again, no crater has been found. Another strong contender is flood volcanism from the Siberian Traps, a large igneous province in Russia. Impact-triggered volcanism is yet another possibility.

Starting about 360 million years ago, a drawn-out event eliminated about 70 percent of all marine species from Earth over a span of perhaps 20 million years. Pulses, each lasting 100,000 to 300,000 years, are noted within the larger late Devonian extinction. Insects, plants, and the first proto-amphibians were on land by then, though the extinctions dealt landlubbers a severe setback.

The Ordovician-Silurian extinction, about 440 million years ago, involved massive glaciations that locked up much of the world's water as ice and caused sea levels to drop precipitously. The event took its hardest toll on marine organisms such as shelled brachiopods, eel-like conodonts, and the trilobites.

Happening Now?

Today, many scientists think the evidence indicates a sixth mass extinction is under way. The blame for this one, perhaps the fastest in Earth's history, falls firmly on the shoulders of humans. By the year 2100, human activities such as pollution, land clearing, and overfishing may have driven more than half of the world's marine and land species to extinction.

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