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Tropical Arctic? Scientists say it was hot 55 million years ago
Scripps Howard News Service


May 31, 2006

Sediment samples taken from a unique drilling expedition in the center of the Arctic Ocean appear to have solved a major mystery about Earth's climate over the last 50 million years.

Scientists report in a series of new papers published Thursday in the journal Nature that the Arctic went through an extreme hot spell during that time, but also began forming glaciers about 14 million years earlier than had been thought, about the same time that the ice cap over Antarctica started to develop.




Researchers studying ancient climate changes have been puzzled for some time by evidence that seemed to show the Arctic lagged behind the southern continent in becoming icy starting by about 43 million years ago, when Earth shifted from a "greenhouse" climate to an "icehouse" climate in a dramatic spate of cooling.

Ice cores taken in Antarctica gave a clear timetable for ice formation there, but the sea ice of the Arctic offers no similar permanent record, and cores taken from glaciers in Greenland and other spots around the rim of the polar region provided an incomplete picture. Glacial deposits from icebergs in the Atlantic seemed to indicate the northern freeze may have been delayed by 15 million years or longer compared to the other pole.

The Arctic Coring Expedition, which battled sea ice as much as 16 feet thick to drill in the same spot near the North Pole for nine days in the fall of 2004, came up with clear evidence that glaciers were dropping pea-sized pebbles into the ocean some 45 million years ago.

"In the past, scientists thought shifting tectonic plates and changes in ocean circulation patterns in the Southern Hemisphere may have promoted ice to form earlier in Antarctica,'' said Steven Clemens, an associate professor of geological sciences at Brown University in Providence, R.I., and a co-author of one of the papers.

"But there was other evidence that contradicted this theory. Now much of what we know about the evolution of ice on Earth makes more sense. And the evidence underscores the importance of greenhouse gases in driving climate change,'' Clemens added.

The sediment core, 430 meters long, shows that the Arctic 55 million years ago was completely free of ice and as warm as 64 degrees Fahrenheit. Then, about 49 million years ago, the region, and the rest of the world, got even hotter, possibly from volcanoes or earthquakes releasing even more carbon into the atmosphere.

Arctic waters got as warm as 73 degrees, at least in the summer, and sprouted a layer of ferns and both freshwater and saltwater algae. The researchers believe that over the next few million years, the basin was in transition, with a layer of fresh water sitting above the denser ocean water and setting the stage for abrupt cooling.

Sediment from the most recent 25 million years or so seems to indicate a steady ice pack cover over the region, with clay, coarse sand and small pebbles blown by the wind from dry land or carried by icebergs that deposited the debris on the sea floor. And the core contains plankton that demonstrates the environment was mostly salt water.

The middle part of the core, dated from 44 million to 25 million years ago, leaves a bit of mystery, however, because it's made up of condensed silt that contains no apparent signs of biological life, possibly the result of the bottom being starved of oxygen during this time.

But the finding that the two ends of the Earth chilled in sync is particularly important, Clemens said, because it may help bolster the study of "bipolar symmetry'' in today's climate, as scientists report signs of rapid melting both in parts of Antarctica and many of Greenland's glaciers, as well as reduced sea ice cover.


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Ketchikan, Alaska