Features
FOSSIL FIND: His team’s discovery of a fossil of an ancient freshwater turtle in Arctic Canada indicates the region may have been influenced by a “super-greenhouse” effect 90 million years ago, says Rochester geophysicist John Tarduno. (Photo: Paleomagnetic Research Group/Courtesy of Tom Whitley)As geophysicist John Tarduno and his six undergraduates stood in a flat valley of Arctic Canada’s Axel Heiberg Island, the Rochester professor of earth and environmental sciences told his students to keep an eye out for fossils.
Tarduno, who had led several expeditions to the region north of the Arctic Circle supported by the U.S. National Science Foundation, knew that the area’s rocks were rich in fossils. That 2006 trip, though, would feature a surprise.
In the wall of a ravine worn away by melt water, Stephanie Mason ’08, a member of Tarduno’s Paleomagnetism Research Group, discovered an amazingly well-preserved shell of a turtle. A tropical, freshwater, Asian turtle.
The obvious question? How would such an animal end up at 79 degrees north latitude?
“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,” says Tarduno, the leader of the expedition. “We’re talking about extremely warm, ice-free conditions in the Arctic region, allowing migrations across the pole.”
Working with his collaborator Donald Brinkman of the Royal Tyrrell Museum of Canada, Tarduno later named the fossil Aurorachelys, or aurora turtle, a previously undiscovered turtle that resembles a freshwater Mongolian species.
Publishing the findings in the February issue of the journal Geology, Tarduno says the aurora turtle adds to the evidence that a rapid influx of carbon dioxide some 90 million years ago likely caused a “super-greenhouse” effect that created extraordinary polar heat.
Tarduno had led the 2006 expedition to study paleomagnetism—the Earth’s magnetic field in the distant past. His expertise, which allows him to ascertain when points on earth’s crust were at specific locations, allows him to rule out the possibility that millions of years of tectonic activity had brought the fossil from southern climes. The turtle was clearly a native of the area.
As to how a freshwater turtle migrated across a salty ocean, Tarduno points to the results of drilling by the Integrated Ocean Drilling Program, an internationally organized marine research initiative to explore the planet’s geological history and structure. The program’s Arctic Coring Expedition had demonstrated that the past Arctic Ocean had seen episodes of unusually fresh surface waters.
Tarduno and his students had been studying massive lava flows that cover some of the High Arctic islands, and he believes 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 poked above the surface of the water at one time, it would have given the turtles—and other species—the ability to island-hop all the way from ancient Russia to Canada.
At the time that the aurora turtle lived, the Arctic Ocean was probably more isolated 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 a layer of fresh water may have rested on top of the salty ocean water, allowing an animal such as the aurora turtle to migrate with relative ease.
Tarduno believes the same volcanic rock may have allowed not only the turtle’s migration, but also may 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 has already yielded fossils he and his team are still analyzing. He says he hopes to paint a more complete picture of the time when the Arctic was warm enough to be a place to take a vacation instead of an expedition.
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.”
Jonathan Sherwood ’04 (MA) is a senior science writer for University Communications.
