Photo courtesy University of Rochester
A fossil of a tropical, freshwater, Asian turtle suggests that animals migrated from Asia to North America directly across a freshwater sea floating atop the warm, salty Arctic Ocean, scientists announced today in the journal Geology.
The finding (in the photo above) also suggests that a rapid influx of carbon dioxide some 90 million years ago was the likely cause of a super-greenhouse effect that created extraordinary polar heat.
“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, New York, and leader of the expedition that found the fossil. Tarduno’s work was funded in part by the National Geographic Society’s Committee for Research and Exploration.
“We’ve known there’s been an interchange of animals between Asia and North America in the late Cretaceous period, but this is the first example we have of a fossil in the High Arctic region showing how this migration may have taken place,” he says in a news release about the research.
Photo of Arctic site where turtle fossil was found courtesy University of Rochester
Tarduno led the 2006 expedition to the Arctic to study paleomagnetism — the Earth’s magnetic field in the distant past.
“Knowing from previous expeditions to the area that the rocks were rich with fossils, Tarduno kept an eye out for them and was rewarded when one of his undergraduate students uncovered the amazingly well preserved shell of a turtle,” the University of Rochester news release said.
“Together with collaborator Donald Brinkman of the Royal Tyrrell Museum of Canada, they later named the fossil Aurorachelys, or aurora turtle. The turtle strongly resembles a freshwater Mongolian species, which raised obvious questions about how it came to be in the marine waters of the North American Arctic,” the release said.
Paleomagnetic data ruled out the possibility that millions of years of movement of the Earth’s crust had brought the fossil from southern climes, the university said. “The turtle was clearly a native of the area.”
Tarduno’s study of massive lava flows that cover some of the High Arctic islands has led him to believe that the same volcanic events that produced those igneous rocks 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,” the release explained.
“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 also believes 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,” Tarduno says. “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.”
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,” says the university statement. “He hopes to paint a more complete picture of the time when the Arctic was warm.”