Shell dates from warm period 90 million years ago

Geologists from the University of Rochester have uncovered a 90 million-year-old fossil of a tropical freshwater Asian turtle in Nunavut's High Arctic.

The fossil was found in a slab of ancient basalt on Axel Heiberg Island.

The turtle, dubbed Aurorachelys, or aurora turtle, lived in the region 90 million years ago when polar temperatures averaged above 14 C, similar to those found in today's northern Florida.

A paper on the fossil, published in the Feb. 1 journal Geology, suggests that carbon dioxide pouring into the atmosphere from volcanic activity during that period may have caused a "super-greenhouse" effect, boosting temperatures in the polar region.

"We're talking about extremely warm, ice-free conditions in the Arctic region, allowing migrations across the pole," said John Tarduno, professor of geophysics at the University of Rochester and leader of the team that found the turtle fossil, in a news release on the find.

The turtle resembles a kind of freshwater turtle found in Mongolia, so its presence in the High Arctic suggests that it may have migrated from Asia to North America by floating on a freshwater layer on top of the then-warm, salty Arctic Ocean.

Tarduno's team found the fossil in 2006 when they went to the Arctic to study paleo-magnetism- that is, the Earth's magnetic field in the far distant past.

Tarduno said study of the magnetism in rocks where the fossil was found rules out the possibility that the fossil came from southern waters. The turtle was clearly a native of the area, Tarduno noted.

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, Tarduno said, and rivers would have poured fresh water into the ancient sea.

Fresh water is lighter than sea water, so Tarduno thinks fresh water may have rested on top of the salty water, allowing a freshwater animal such as the aurora turtle to migrate.

According to the news release, Tarduno also believes volcanoes could have produced a series of islands along an underwater mountain range in the Arctic Ocean called the Alpha Ridge.

If the ridge poked out 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, Tarduno said.

In recent years, Tarduno has uncovered fossils of other warm-water species in the High Arctic, such as crocodile-like beasts, which once thrived there 90 million years ago.