|Sediment samples from the oceans surrounding Antarctica give scientists an idea of what the climate may have been more than 45 million years ago. At right, wind convergence patterns around Antarctica help determine where different types of sediment will end up on the ocean floor. Fine, medium and large grains are deposited in different areas. Photo and graphic courtesy of the Department of Geological Sciences|
Learning how Antarctica has responded to changes in the past is a key to understanding the global climate changes that concern us today, explains doctoral candidate Leah Joseph. Using novel techniques to investigate Antarcticas 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) in May.
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 teams analysis of sediment samples from a site on the Maud Rise in the Weddell Sea.
Because Antarctica is covered with ice, scientists cannot study its history with the 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 measuresmass accumulation rate, grain size and magnetic fabricfor 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.
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 teams findings argue against that conclusion.
In desert-like conditions, you get those fine, wind-blown grains, and we dont find those in the grain size distribution until much later, Joseph says. 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.
Josephs 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 beenfindings that bear on todays global warming concerns.
The stability of the Antarctic ice sheet is a very big climate question today, and Leahs 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, professor and chair of the Department 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, professor of geological sciences. The feedback is not as simple as warm weather causing ice melt and thats it, he says. Its a more complex relationship involving ocean circulation patterns, and before we can do more useful climate modeling prediction, we have to understand those feedbacks.