Polar-equatorial climate link reported.

Polar-equatorial climate link reported

The more scientists delve into the history of the earth and its climate, the more they appreciate how intimately connected different parts of the planet can be. During the recent Leg 108 of the Ocean Drilling Program (ODP), researchers discovered evidence for a strong climatic link over the last few million years between the polar regions and lower-latitude areas. If this polar-equatorial connection holds, says the leg's co-chief scientist William Ruddiman, it will be a finding of major importance.

Between February and April, ODP's research ship the JOIDES Resolution drilled at 12 sites off the coast of northwest Africa. The result was 3,841 meters of core--the most core ever retrieved on a drilling leg and the most continuous record of sediments ever obtained in this region. This core material revealed that the natural fluctuations between glacial and warmer periods became more pronounced starting about 3 million to 2.5 million years ago and lating until about 500,000 years ago. During that time, increases in the content of organic carbon, diatoms (algae with silicified skeletons) and cold-water planktonic fossils mark a progressive intensification of the trade-wind-driven upwelling of nutrient-rich colder waters in the region. The increases may also reflect greater movement of cool ocean water toward the equator from higher latitudes.

In recent climate history, 2.5 million years ago is something of a magic number because that is about the time when the northern ice sheets began to grow with increasing vigor. And 2.7 to 2.6 million years ago is also the time when scientists think the cyclical northward expansion of the Antarctic polar front, the boundary of very cold water and sea ice, began to intensify. So the Leg 108 shipboard scientists think they have found a clear link between the polar and equatorial climates, but what causes this link is still obscure.

Researchers believe that the cyclic changes in the earth's orbit cause climate changes known as Milankovitch cycles. Orbital changes have been implicated in the growth of ice sheets in the Northern Hemisphere. Leg 108 scientists suspect that changes in the polar regions affect the equatorial areas. But they are not yet certain if, through winds and surface currents, the northern ice sheets directly cool the equatorial regions or instead first cool Antarctica, which then cools the equator.

"Our guests right now," says Ruddiman, a paleoclimatologist at Lamont-Doherty Geological Observatory in Palisades, N.Y., "is that the ice sheets in the Northern Hemisphere are controlling the upwelling off of west Africa," whereas the Antarctic Sea ice controls the behavior of the equatorial ocean to the south.

Leg 108 scientists think they see evidence for Milankovitch cycles with periods of 20,000 to 40,000 years. But "it will take a couple of years to find the real answers about linkages as we look at the rhythms [in the record] in much greater detail," says Ruddiman.

The JOIDES Resolution, now in the middle of Leg 109, is currently deepening a hole started at the Mid-Atlantic Ridge during Leg 106 last fall. Scientists are excited about this site because it may hold important clues to how new seafloor is created.

The Ocean Drilling Program has also recently celebrated the addition of a new member. A consortium of 12 countries (Belgium, Denmark, Finland, Greece, Iceland, Italy, the Netherlands, Norway, Spain, Sweden, Switzerland and Turkey) represented by the European Science Foundation signed up on April 29. The newly added nations join Canada, France, West Germany, Japan, the United Kingdom and the United States.