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.
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.
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.
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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