Paleoclimate theories left in he cold?

It may be difficult to predict even tomorrow's weather, but scientists drilling deep into the ocean floor have discovered what the climate was like above the Arctic Circle millions of years ago. What they have found may surpirse those who study the earth's very ancient history. The new data cast a scientific shadow of doubt on teh generally accepted date for the onset of major glacier buildup in that area of North America, as well as on an established theory of global climate cycles.

Using the drillship JOIDES Resolution, participants in the Ocean Drilling Program (ODP), headquartered at Texas A&M University is College Station, drilled at two locations in the Labrador Sea and one in Baffin Bay (see map). The Baffin Bay site was at the highest latitude ever tested by drilling. Battling icebergs and arctic stomrs, the international group collected more than a mile of sediment samples from a deep as 3,500 feet beneath the seafloor.

Evaluation of core samples from the two-month mission, completed Oct. 27, will take up to two years. But preliminary conclusions, based on examinations of rocks and fossils of microscopic organisms from different sediment layers, shed new light on old crustal movement (plate tectonic) and long-ago climatic changes. Accorddinng to ODP scientists, some results apparently shake the scientific bases for certain theories, while others lend support to previous work. For example, the data have confirmed findings from a previous ODP cruise suggesting that Greenland, Canada and Western Europe were one large landmass until they began separating about 85 million years ago. Thirty million years later, Europe left Greenland behind, requiring a major reorganization of the way nearby seas were opening up.

"That [55 million years ago] is the magical age when everything happened," says Mike Arthur of the University of Rhode Island in Narragansett, co-chief scientist for the voyage. "We've linked the tectonic histories of the Labrador Sea and Baffin Bay. Now we think we know that Baffin Bay really started opening up at 55 million years ago." He says sediment profiles also show "something happened" at 36 million years ago, and Baffin Bay stopped expanding. (Earlier studies by others and had shown the same dates for the Labrador Sea's formation.) By that time, the once-subtropical temperatures of the region had already started cooling.

With the formation of Baffin Bay and the Labrador Sea, cold water from the Arctic Ocean could flow into the Atlantic Ocean, dramatically changing the climate. ODP evaluation of Labrador Sea "drop stones" -- debris released from melting icebergs floating free drom glaciers -- confirms major ice sheets existed in the western North Atlantic region 2.5 million years ago--the date generally recognized by scientists as the beginning of the area's major ice buildup.

But additional ODP conclusions using Baffin Bay data "will be very controversial," says Arthur. "We put the major dropstone event at 2.5 million years, but we think we have evidence for a much earlier time for the beginning of major glaciation." Rock analyses, he says, suggest a "tentative" date of 3.5 million years ago and an "extremely tentative" date of 8 million years ago for glacial onset.

Marine geologist Ali Aksu of the Memorial University of Newfoundland in St. John's has studied Baffin Bay sediments since 1975. A member of the ODP cruise, he says, "I don't doubt for one minute the data [indicating earlier glaciation]. It looks like there is something funny going on." The disparate dates, Aksu suggests, may be related to technical differences in data -- one set based on fossils, another on magnetic field changes.

Paleoclimatology work by the ODP team will also raise questions about the Milankovitcch theory (SN: 10/19/85, p. 251), a long-accepted view of climatic cycles, says Arthur. The theory links global climate over millions of years to the way solar energey is received, changed by slight fluctuations in the tilt of the earth and its mean distance from the sun. Under the theory, tilt is the most important factor in high latituddes. But Arthur reports Baffin Bay data indicate tilt "is not the dominant signal," for reasons not yet understood. "What we got was a regional story," he cautions. "We're trying to incorporate that into a global look."