Researchers used fossils like this of Early Triassic conodonts, an eel-like marine animal, for oxygen isotope measurement and past temperature reconstruction. Image by Yadong Sun.
Some 250 million years ago, equatorial oceans teemed with mollusks, clams, blue-green algae and microsnails. Fierce, stifling heat proved lethal to most other marine animals and plants, and the land -- then lumped into the giant supercontinent Pangea -- was bleak and largely lifeless. For nearly 5 million years, the Earth remained in this state, following the most severe extinction event in the planet's history, triggered by powerful volcanic eruptions and tremendous flows of thick basalt lava covering an area as vast as the continental United States.
Scientists believe that event, known as the end-Permian mass extinction, snuffed out about 90 percent of living animals. But possibly more puzzling has been why it took so incredibly long for the Earth to recover.
A team of researchers led by Yadong Sun of the China University of Geosciences and the University of Leeds believe it was extreme heat that made the planet hostile to most life. Temperature readings they've collected from that time reveal sea surface temperatures as high as 104 degrees Fahrenheit, according to a study released on Thursday in the journal Science.
The team calculated these temperatures by analyzing the tiny microfossils of toothy, eel-like marine animals called conodonts, which are now extinct. They collected and lugged two tons of limestone rock containing these fossils from South China into the lab for analysis. And they studied the isotopic composition of oxygen molecules trapped in the conodont shells to reconstruct the seawater temperatures. Their finding is that sea surface temperatures in the equatorial ocean may have exceeded 104 degrees Fahrenheit. (To put that in perspective, sea surface temperatures near the equator now range from 77 to 86 degrees Fahrenheit.)
"On land, temperature extremes must have been even higher," said Paul Wignall, a professor and geologist at the University of Leeds and an author of the study. "Pangean summers must have been unliveably hot."
He suspects temperatures on land ranged from 120 to 140 degrees Fahrenheit. Researchers also noted an abundance of tiny organisms like snails and clams in oceans near the equator where it was hottest. And fossils of larger Triassic animals like fish were absent in that area, but found in cooler, high-latitude waters. Also noticeably absent were coal-forming peat swamps.
"There's no early Triassic coal known from anywhere in the world," Wignall said. That's a weird thing.
The idea that it got hot in the early Triassic is not a new idea, Wignall added. But this study backs that theory up with numbers.
"We're showing that it was lethally hot -- so hot that it's killing things," he said. "It limited photosynthesis; it limited the ability of animals to survive.
Their theory for why it stayed so hot for so long goes like this: The volcanic eruptions released mass amounts of sulfur dioxide and carbon dioxide into the atmosphere, which triggered a severe spike in temperatures, causing extinctions. The extinctions upset the normal carbon cycle, particularly the mass die off of plants, which play a key role through photosynthesis.
"If you kill off plant life, you lose a way of taking CO2 out of the atmosphere," Wignall said. "It keeps getting hotter and hotter, and there's no break on it."
Doug Erwin of the Smithsonian Institution characterized the study as an extension of work this group and others have done, but said this study contains the strongest temperature data yet, said.
Still, he cautioned that some may challenge the reliability of that data. The technique used involves comparing the isotopes of two different kinds of oxygen to determine temperature. He calls it a "paleothermometer." That technique, he said, assumes two things: that the same relationship that we see between these isotopes today held true 250 million years ago and that the temperature hasn't since been reset by chemical properties occurring in the rocks in later years.
"I expect there will be people who dispute this result," Erwin said. "But it's a pretty consistent pattern."
And as to how much insight this gives us into the warming patterns occurring today? We're not yet headed toward the degree of catastrophe seen in the Early Triassic, Wignall said, but added that it's hard to say what the planet will look like in a few thousand years.
"This shows us if current global warming keeps going, maybe in a few centuries, this would be a terrible place to live in," Sun said. "A low carbon life would be good for our descendants."
