Leg 113: drilling into Antarctica's past.
Scientists know from fossil evidencethat the climate of the entire earth was once temperate and nearly uniform. The winds and ocean circulation were relatively weak because there were no great temperature contrasts between the poles and the equator. Now, of course, with its ice-capped poles and tropical equator, the earth presents a very different climatic picture.
"The question is exactly how and whendid we go from a very homogeneous, warm planet to the [climatically] diverse planet we have today,' says Suzanne O'Connell of Texas A&M University in College Station, the staff scientist on the Ocean Drilling Program's recent Leg 113. "We think the answers to that question lie around Antarctica, because it is considered to be the anchor of climate today.' The temperature difference between the cold southern pole and the hot equator causes the air above the ice and water below it to sink and move northward. This then drives most of the planet's wind patterns and ocean flow.
In spite of Antarctica's importance tothe earth's climate, scientists have had relatively little geologic information on the region. So on Leg 113, the drillship JOIDES Resolution spent most of January and February taking cores in Antarctica's Weddell Sea. These cores, which O'Connell says are the first continuous, high-resolution, hydraulic-piston cores obtained in Antarctica, offer the most detailed record yet of Antarctica's glacial history over the last 70 million years. They also hold some clues about the causes of a mass extinction far in the past, as well as some hints about the effects of "greenhouse' warming on the planet in the future.
Leg 113 confirmed that East Antarcticawas once a relatively warm, ice-free continent. The cored sediments older than about 38 million years contain pollen from beech forests, fern spores, fresh-water diatoms that had been blown to sea from inland lakes, and clays that were formed by chemical weathering. In sediments from about 38 million years ago, however, the appearance of silica-rich sediments, clays made by physical weathering and other clues indicate that the continent was cooling down. By 15 million years ago a stable ice sheet had formed over East Antarctica, says James P. Kennett, Leg 113's co-chief scientist, from the University of Rhode Island in Narragansett.
According to Kenneth, West Antarcticawent through the same process of cooling and ice sheet formation, but these steps occurred later; the West Antarctic ice sheet is thought to have begun to form about 6 million years ago. This is not unexpected, says Kennett, considering that the East Antarctic ice sheet rests on land, whereas the West Antarctic ice sheet lies mainly on seawater. The salt in the seawater lowers the melting point of the overlying ice.
What is surprising, he notes, is thestability of the West Antarctic ice sheet over the last 4.8 million years. Because it's marine-based, scientists had speculated that it would be very vulnerable to temperature changes and that, like the Northern Hemisphere glaciers, it would have gone through cycles of melting and freezing. But, says Kennett, "our geological evidence from the cores suggests very high stability--that it formed as a permanent feature of the earch 4.8 million years ago and it's been quite stable since, including times that have been warmer than the present day.'
"That's very comforting,' commentsO'Connell, "because right now the biggest water reservoir in the world is the Antarctic ice sheets.' Some scientists have worried that future "greenhouse' warming of the planet, due to the burning of fossil fuels, would raise the temperature enough to rapidly melt the Antarctic ice sheets and cause dramatic sea level rises.
"We're not saying that [the ice sheets]don't contribute at all to sea level changes,' says O'Connell, "but by and large they appear to be fairly stable.'
Because the East Antarctic ice sheet island-based and less susceptible to temperature changes, Leg 113 scientists were not surprised to find that it, too, has a very stable history. But this finding contradicts the work of other scientists who recently discovered pieces of wood that they think are traces of a forest that grew close to the South Pole 3 million years ago (SN: 3/8/86, p.148). Forests growing that far inland would imply that at least half of the East Antarctic ice sheet had melted away, says Kennett. So shipboard scientists had their eyes out for evidence of a major oscillation in the ice sheet.
"We found no evidence of that whatsoever,'says Kennett. He doesn't know how to reconcile the two different results, but suggests that the wood is much older than what is claimed.
One of the most powerful tools geoscientistshave in unraveling the temperature and ice history is the ratio of oxygen isotopes in calcarious microfossils. According to O'Connell, Leg 113 will provide the first Antarctic deep-sea cores for oxygen isotope work. These cores, by containing oxygen isotopes from times when there were no ice sheets, will also give scientists a reference point for calculating temperatures and ice volumes around the globe. Because the continuous cores also contain many types of microfossils, she says, they will enable biostratigraphers to precisely date and correlate the core sections.
Not all of the important discoveriesduring Leg 113 are directly related to glaciation and climate. For example, Kennett says the scientific party obtained two "very interesting' cores that are continuous across the sedimentary boundary marking the end of the Cretaceous (K) period and the beginning of the Tertiary (T) period--a transition time in which 75 percent of life on earth perished.
Since iridium, an element found inrelative abundance on asteroids, has been found in high levels at K/T boundaries elsewhere on the globe, some scientists have postulated that an extraterrestrial body caused the mass extinction by slamming into the earth. Scientists have yet to analyze the content of iridium and other elements on Leg 113 cores. But according to Kennett, the researchers did find that there is volcanic material mixed in with the K/T boundary sediments, an observation that may support a competing theory that holds that intense volcanic activity was responsible for the mass extinctions (SN: 4/18/87, p.248).
Leg 113 scientists also discovered intheir Cretaceous sections some of the highest concentrations of organic sediments ever obtained by scientific drilling in the deep sea. This suggests to Kennett and O'Connell that during the early Cretaceous, the Weddell Sea area was an extensive, enclosed and oxygen-depleted basin in which organic matter was able to accumulate. While no oil or hydrocarbons were found, the discovery suggests that the Weddell area may be an important source of hydrocarbons.
O'Connell says another unusual aspectof the cruise was that in addition to the usual microfossils, researchers recovered macrofossils, including ammonites, which look like nautiluses, and squid-like belemnites.
The cruise was a success technically aswell. A big problem with working in the polar regions is icebergs--as the Leg 113 ice observer reportedly joked, the sound of the drill string is a mating call for icebergs. That problem was solved by an accompanying Danish ship, which was contracted to tow the icebergs with ropes or push them away with water spewed from its propellers.
One of the consequences of havingAntarctic ice sheets is that large thermal gradients between Antarctica and latitudes farther north make for some of the worst weather in the world around 50|S. Unfortunately, that's where the JOIDES Resolution is now, on Leg 114, which is supposed to address some of the same questions of Leg 113 but in a slightly warmer climate. According to O'Connell, the Resolution is battling 50- to 80-knot winds and rolling 25|. "It's just horrendous,' she says. "They probably have a whole ship full of sick people.'
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|Title Annotation:||Ocean Drilling Program|
|Date:||May 2, 1987|
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