U-M geologists may be close to determining the cause of an abrupt global cold snap that occurred about 10,000 years ago as the climate was warming and glaciers were retreating across North America at the close of the last Ice Age.
Geologists Theodore C. Moore Jr. and David K. Rea believe extreme fluctuations in Great Lakes levels, which at times reduced many of the lakes to mere puddles, are related to this 500-year-long global freeze.
Once their analysis of Lake Huron sediment core samples and seismic tracings is complete, Moore and Reatogether with several Canadian colleaguesbelieve they will be able to eliminate one of two competing theories on what happened between 10,000 and 11,000 years ago to chill an otherwise warming climate.
Advocates of one theory maintain that a massive surge of glacial meltwater flowing into the North Atlantic lowered ocean and atmospheric temperatures enough to affect global climate. Other scientists believe ice water from melting glaciers became trapped in the Great Lakes region and that this intense local cooling was enough to trigger widespread climate changes.
Studying what happened at the end of the last Ice Age will help scientists predict the potential effects of rapid changes, produced by different factors, which are currently affecting Earths climate, according to Moore and Rea.
They will present their research results at the annual meeting of the Geological Society of America in Cincinnati this week.
We found evidence for at least four, and possibly six or more, episodes of exposed and eroded lake floor sediments indicating water levels in the interior of Lake Huron 30 to over 100 meters (about 90 to 300 feet) lower than current levels, says Moore, who is director of the Center for Great Lakes and Aquatic Studies and professor of geological sciences.
Moore explains that the U-M study confirms prior research on Great Lakes fluctuations, but shows that water level changes were more frequent and complex than previously believed.
The most significant fluctuations in lake levels occurred between 11,000 and 8,000 years ago, says Rea, professor of geological sciences and a research scientist at the Center for Great Lakes and Aquatic Studies. They were probably caused by shifts in the flow of glacial meltwater and topographic changes that periodically blocked the flow of meltwater to and from the Great Lakes.
Rea and Moore analyzed core samples and high-resolution seismic reflection studies of Lake Huron sediment layers, which they obtained during a 1991 expedition on the U-M research vessel Laurentian.
Using radiocarbon dating of wood samples and shells found in the sediment layers, Moore and Rea have dated four incidents of extremely low lake levels. The most recent occurred between 8,100 and 8,200 years ago. The second took place between 8,900 and 9,000 years ago. Lake levels fell again about 9,300 years ago. The lowest levels documented to date occurred between 9,700 and 9,900 years ago.
We dont know yet if this last fluctuation represents the maximum lowering time, Moore says. Our seismic records indicate older lake sediments with erosional features associated with lake level lowering.
Rea says that the next phase of the research will focus on isotope analysis of shells found in the Lake Huron sediments. Glacial ice has a very different ratio of certain oxygen isotopes than rainwater, so we should be able to determine whether Lake Huron was full of glacial meltwater from the north or runoff from local creeks and rivers during this period of time, he explains.
Canadian scientists collaborating on the project plan to analyze pollen samples found in lake sediments to determine the type of vegetation growing in the area during the transition from Ice Age to temperate climate.
Other researchers participating in the project include C.F. Michael Lewis, T.W. Anderson and S.M. Blasco of the Geological Survey of Canada; L.A. Mayer of the University of New Brunswick; and U-M graduate student David Dobson. The research is funded by the National Science Foundation.