With no meteorite to blame for the Eocene mass extinction, scientists focused on climate change. In this case, global cooling killed off many species, researchers think.
Here's how they can tell: By measuring isotopes of oxygen, carbon and other elements in Eocene-age rocks, researchers can estimate Earth's past temperature and greenhouse-gas levels. (Isotopes are elements with different numbers of neutrons in their nuclei.) The signal from the Eocene shows the epoch started off extremely warm and then swung toward colder, drier conditions before the big extinction event. However, a sharp spike in these climate signals at the end of the Eocene hints at short-lived but extreme global cooling, followed by a rebound to warmer temperatures.
"The age of the crater matches perfectly with that [short-term] global change," Wielicki said.
Wielicki thinks the Popigai impact created a global icehouse, similar to the climate disasters seen after enormous volcanic eruptions or the dinosaur-killing impact. The meteoritic crash could have pumped massive amounts of sunlight-reflecting sulfur droplets into the atmosphere, he said. The planet's "quick" recovery, in geologic time, set plants and animals on an evolutionary path to modern species.
The end of the Eocene was the last big mass extinction in Earth's history. More than 90 percent of snails disappeared, sea urchins were hard hit and the earliest toothed whales died off, which eventually would be replaced by modern whales. The dramatic shift of European mammals, called the "Grand Coupure," occurred soon afterward, following Eocene-Oligocene transition.
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