Undersea Volcanic Eruptions Linked to Ancient Die-Off

John Roach
for National Geographic News
July 16, 2008
A surge of undersea volcanic activity about 93 million years ago sapped the oceans of oxygen, triggering a mass extinction of marine life, a new study finds.

The catastrophic event buried a thick mat of organic matter—from large clams to single-celled algae—on the seafloor, which today is a major source of oil.

The new findings allow scientists for the first time to directly pinpoint undersea magmatism—the formation of rock from cooling magma—in the dino-era carnage.

The research also opens a window onto the way Earth responds to large inputs of greenhouse gases, experts say.

(Explore an interactive on the greenhouse effect.)

At the time of the mass die-off, Earth's climate was warm and muggy, and ocean circulation was sluggish, according to study co-author Steven Turgeon, an earth scientist at the University of Alberta in Edmonton, Canada.

Palm trees grew on the North Slope of Alaska and carbon dioxide levels were 3 to 12 times higher than today's concentrations. The carbon dioxide spike was mostly a result of increased volcanic activity linked to Earth's rapidly shifting tectonic plates, Turgeon said.

"And now we have this magmatic pulse that happened at that time—a huge one by what we can tell—and that's what caused this chain reaction to drive the oxygen from the ocean and cause this mass extinction," he said.

(Related: "Mass Extinctions Due to Sea Level Changes, Study Says" [June 16, 2008].)

Turgeon and colleague Robert Creaser report their findings in tomorrow's issue of the journal Nature.

Oxygen-Starved Oceans

Scientists have long suspected that undersea volcanism could be responsible for the marine mass extinction, an anoxic—or oxygen-depleting—event known as the OAE2. But until now the evidence was "sketchy," Turgeon noted.

Oceanic anoxic events starve some species of marine life of oxygen, an essential gas. The geologic record suggests that the events occur when carbon dioxide levels are several times higher than current concentrations.

Turgeon and Creaser analyzed rocks in northeastern South America and central Italy that were undersea in the middle of the Cretaceous, which lasted from 145 million to 65 million years ago.

The researchers looked for evidence of the metallic element osmium, hoping that it would yield insight to the trigger of the OAE2.

One type of osmium "signature" is primarily derived from river debris that drains into the oceans. Another type comes from magmatism and extraterrestrial sources, such as meteorites and space dust, Turgeon explained.

In a geological blink of an eye, just before the onset of the OAE2, the osmium signature shifted from largely river-runoff osmium to one mostly derived from magmatism or outer space, the scientists found.

Since there is no evidence of a meteor or comet impact 93 million years ago, a 30- to 50-fold increase in magmatism was implicated as the instigator of the mass extinction.

(Related: "Did Million Year Long Eruption Cause Mass Extinction?" [May 2, 2006].)

Timothy Bralower, a geoscientist at Pennsylvania State University in University Park, wrote an accompanying perspective on the research in Nature. The new study helps paint a more complete picture of the OAE2, he said.

Caribbean Plate

According to Bralower, the OAE2 was most likely tied to bouts of magmatism that created the Caribbean tectonic plate, which today lies underneath Central America and the Caribbean Sea.

The surge in undersea volcanism put massive amounts of metals in the ocean. This encouraged the growth of microscopic marine plants called phytoplankton, which produced excessive organic matter.

"When the plants died, this rain of organic matter fell through the marine water column and stripped it of oxygen," Bralower said. "And this anoxic event in the deep waters led to the extinction of the [flora and fauna] that lived on the seafloor."

Today the layer of organic matter is black shale that makes up nearly a third of present-day recoverable oil reserves, Turgeon said.

Warming Clues

During the OAE2, the excessive production of organic matter sucked vast quantities of the greenhouse gas carbon dioxide from the oceans and atmosphere and buried it in the seafloor, Turgeon said.

Other studies have found it took about 10,000 to 20,000 years for carbon dioxide levels to reach present-day levels.

But the effect was temporary. Following the eruptions, carbon dioxide levels and temperatures returned to the "Cretaceous normal," Turgeon said.

"It is short-lived and it is real, and it provides some insight to how the atmosphere works," he said.

According to Bralower of Penn State, nothing as dramatic as the OAE2 will likely happen in the near future.

"Today we're just at the beginning," he said, "and we have really no way except for models to predict what is going to happen in the future."

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