U. ROCHESTER (US)—A tropical turtle fossil discovered high in the Canadian Arctic suggests that a rapid spike in carbon dioxide some 90 million years ago created a super-greenhouse effect, raising polar temperatures rather dramatically. The find strongly suggests that animals migrated from Asia to North America not around Alaska, as once thought, but directly across a freshwater sea floating atop the warm, salty Arctic Ocean.
“We’re talking about extremely warm, ice-free conditions in the Arctic region, allowing migrations across the pole,” says John Tarduno, professor of geophysics at the University of Rochester. Tarduno was leading an expedition into the isolated region to search for clues to past changes in the Earth’s magnetic field when one of his students noticed the turtle’s well-preserved outline in the volcanic rocks along the fjord where they were working.
Tarduno and collaborator Donald Brinkman of the Royal Tyrrell Museum of Canada named the fossil Aurorachelys, or aurora turtle. It strongly resembles a freshwater Mongolian species, which raises obvious questions about how it came to rest in the marine waters of the North American Arctic.
At the time the aurora turtle lived, the Arctic Ocean was probably even more separated from the global oceanic circulation system than it is today. Numerous rivers from the adjacent continents would have poured fresh water into the ancient Arctic sea. Since fresh water is lighter than marine water, Tarduno thinks it may have rested on top of the salty ocean water, allowing a freshwater animal such as the aurora turtle to migrate with relative ease.
Tarduno believes that the same volcanic events that produced massive lava flows that cover some of the High Arctic islands also could have produced a series of islands along a low underwater mountain range in the Arctic Ocean called Alpha Ridge. If the ridge did indeed poke above the surface of the water at one time, it would have given the turtles—and countless other species—the ability to island-hop all the way from ancient Russia to Canada. Tarduno says it’s possible that the same volcanic rock may not have allowed only the turtle’s migration, but also would have contributed to creating the climate in which the turtle thrived.
“We found this turtle right on top of the last flood basalts—a large stretch of lava from a series of giant volcanic eruptions,” says Tarduno. “That leads us to believe that the warming may have been caused by volcanoes pumping tremendous amounts of carbon dioxide into the Earth’s atmosphere. There’s evidence that this volcanic activity happened all around the planet—not just the Arctic. If it all happened on a short enough timescale, it could cause a super-greenhouse effect.”
He notes that current changes in the recent Arctic climate have affected his field studies. “It is difficult to separate short-term climate trends from a longer-term pattern, but our last few field seasons in the High Arctic have been extraordinarily warm,” says Tarduno. “Sometimes students exchange parkas for short-sleeve shirts.”
Tarduno plans to return to the Arctic to look for places where other fossils might be located. He says the site he’s found is incredibly rich, already yielding fossils he and his team are still analyzing in an effort to paint a more complete picture of the time when the Arctic was warm.
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