WASHINGTON, D.C. --- How have past environmental changes affected Antarctica? The elusive answer is of more than academic interest. Located at the point where the Pacific, Atlantic and Indian oceans converge and mix, the icy continent not only is profoundly affected by global climate, it also exerts effects that can be felt worldwide.
Learning how Antarctica has responded to changes in the past is a key to understanding the global climate changes that concern us today, explains University of Michigan doctoral candidate Leah Joseph. Using novel techniques to investigate Antarctica's ancient environment, Joseph and colleagues have raised new questions about the critical link between global climate and ocean circulation. They presented part of their work at the spring meeting of the American Geophysical Union (AGU) today.
Sarah L. Jacobson, a senior from Chicago who began working on the project as a National Science Foundation Summer Scholar and continued through the academic year, described the U-M team's analysis of sediment samples from a site on the Maud Rise in the Weddell Sea. Because Antarctica is covered with ice, scientists can not study its history with usual surface geology techniques, such as inspecting layers in rock outcrops. Instead, they study it indirectly by examining materials that have eroded from the continent in the past and settled into the surrounding oceans. The U-M researchers look at three measures: mass accumulation rate, grain size, and "magnetic fabric" for clues to past climate changes.
In warmer, wetter times, soil and rock erode into rivers, which carry sediment into the sea. If ice sheets are present and mobile, the masses of moving ice drag sediment along, and the meltwaters from receding ice sheets help transport sediment from the continent to the sea. These processes result in a higher mass accumulation rate (a measure of the amount of sediment deposited in a given time) than when the ice is very cold and stable, Joseph explains.
Grain size hints at where sedimentary material came from and how it was deposited. Very fine dust grains were most likely swept off from dry, desert-like areas by the wind and slowly settled at random on the sea floor, uninfluenced by ocean currents. Medium-sized grains represent mud that flowed out in plumes from rivers or mud that was piled into drifts by currents that flow along the bottom of the ocean floor. Large grains probably were deposited by very fast currents, possibly those that quickly bring material down to the ocean bottom from the continental shelf.
Magnetic fabric analysis rounds out the picture by determining the orientation of grains in a sample, which is related to the velocity of ocean currents at the time and place they were deposited, Joseph explains. The faster the current, the more the grains are aligned with one another.
In the study reported at AGU, the researchers focused on a relatively dry time span between the late Cretaceous Period and the middle Eocene Epoch, 70 million to 45 million years ago. Previous studies had suggested desert-like conditions on Antarctica during this period, but the U-M team's findings argue against that conclusion.
"In desert-like conditions, you get those fine, wind-blown grains, and we don't find those in the grain size distribution until much later," says Joseph. "So we think the climate was dry, but maybe not as dry as others were saying." The data also indicate a gradual trend from dry conditions toward a warmer, wetter Antarctica, which agrees with other investigators' research.
Joseph's studies of sediment from other sites in the oceans around Antarctica help fill gaps in understanding what happened before the Antarctic ice formed, when it formed and how stable it has been---findings that bear on today's global warming concerns.
"The stability of the Antarctic ice sheet is a very big climate question today, and Leah's data are the first useful data to give us a several-million-year picture of when the ice sheet might have been more mobile," says David Rea, U-M professor and chair of geological sciences and a co-author of the study presented at AGU. The data also suggest that more than simple temperature changes dictate what happens to Antarctic ice, adds study co-author Ben van der Pluijm, U-M professor of geological sciences. "The feedback is not as simple as warm weather causing ice melt and that's it," he says. "It's a more complex relationship involving ocean circulation patterns, and before we can do more useful climate modeling prediction, we have to understand those feedbacks."
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