Underwater Sediments Yield Clues To Past Climates
UniSci
December 2, 1999
Jonathan Overpeck, the new director of the University of Arizona Institute for the Study of Planet Earth, wrings information on past climates out of ocean and lake sediments, as in the 825-year proxy record of North Atlantic trade wind strength that he and several colleagues reproduced in a Nov. 26
Science article.
The article sheds light on the amount of variability in the North Atlantic region, back well before any instrumental record. It also considers how this variability factors into climate change beyond the North Atlantic -- for example, over North America.
"This study says, 'Hey, there's a lot of decadal variability.' We didn't know it was the dominant natural mode going back 800 years," Overpeck noted.
In particular, they found trade wind strength varied in cycles of roughly 12 to 13 years duration, as well as over century time scales. The century-scale cycle seems to be linked to solar activity, Overpeck said.
There are hints of other connections as well.
"One of the most intriguing findings of our study is the possible link between major drought in the United States and Atlantic variability," Overpeck said.
Their proxy record was drawn from an analysis of sediment cores from the Cariaco Basin near Venezuela. The stagnant bottom waters of the basin preserve the annual record of shells, clay and silt settling to the ocean floor.
"The Cariaco Basin is like a tropical version of ice cores," Overpeck explained. Scientists have used ice cores to reconstruct a spectacular variety of past conditions, including estimates of carbon dioxide levels for the past 400,000 years.
Working with ocean cores does have its messy moments, though, as Overpeck and colleagues discovered when they tried to harvest the lowest-lying sediment. The underwater ooze contained bubbles of hydrogen sulfide and methane gas.
"The cores from the deepest part of the basin have the most gas," he recalled. "When you bring the cores up on deck, they more or less explode."
The team moved to shallower waters for the cores they eventually analyzed. They found an abundance of fossil Globigerina bulloides, a microscopic marine plankton whose abundance through time is closely linked to changes in trade wind strength.
Basically, G. bulloides populations tend to dwindle with the trade winds when the North Atlantic is warm, and thrive when the Atlantic is cold and trade winds are strong. That's because they feast upon the upwelling of nutrients from the deep ocean that comes with high winds.
The robust correlation for the period of instrumental records gave them the confidence to use G. Bulloides fossils to create a proxy record of trade wind strength back to the middle of the 12th century. The record also works well for sea surface temperatures, apparently because the top layer of ocean water warms when the trade winds die down.
These circulation patterns may well relate to drought patterns in the United States: Tree-ring records and lake sediment data indicate the climate was shifting between floods and droughts more dramatically for a couple of centuries before 1400 AD, at about the same time G. bulloides record shows more dramatic swings in population.
Overpeck was co-chief scientist on the Caribbean cruise to collect sediments from the Cariaco Basin. This is just one of many hats he has worn since receiving his doctorate in geological sciences from Brown University in Rhode Island in 1985.
Before becoming the director of the Institute for the Study of Planet Earth and a professor with the UA geosciences department this fall, Overpeck served as director of the World Data Center-A for Paleoclimatology and head of the National Oceanic and Atmospheric Administration's Paleoclimatology Program, both based in Boulder, Colo.
He has also served as a research scientist for Lamont-Doherty Geological Observatory in New York and a fellow with the Institute for Arctic and Alpine Research in Colorado.
Throughout his career, he has been involved in reconstructing elements of climatic conditions at various points in the past 20,000 years, using a variety of tools including ice cores, pollen and tree rings as well as sediment cores.
Now he wants to be involved in providing information on climate variability in a form that the general public can understand. This goal is partly what attracted him to direct ISPE, he says.
"In the next 10 years, we need to connect our basic research on environmental variability with information our society can use," he said.
"It has been estimated that a third of our economy has interaction with climate variability. We want to provide information to help people make decisions to help their business, to help their lives."
Although ISPE is currently housed on the second floor of the Campus Christian Center, Overpeck looks forward to the day when its offices will be located in the second Environmental and Natural Resources Building alongside other departments and government agencies with similar goals.