TORONTO---University of Michigan scientists have discovered evidence, recorded in 12,000-year-old Lake Huron glacial sediments, for brief, sudden episodes of rapid warming at the end of the last Ice Age which melted glaciers and sent water surging through the Great Lakes to the North Atlantic.
According to U-M scientists, these 10- to 20-year warm intervals, which interrupted an otherwise cool, relatively stable Great Lakes climate, could be related to two events that still affect weather today. They are the El Nino Southern Oscillation (ENSO), which brings warm temperatures and rain from the tropical Pacific to western North America every 3.5 to 7 years, and the Quasi-Biennial Oscillation (QBO), which alters wind speeds in the stratosphere every 2 to 2.5 years.
"Our analysis of 264 annual layers of sediment from a northwestern Lake Huron core sample has produced the first evidence to suggest that Great Lakes-area climate was affected by ENSO and QBO more than 12,000 years ago," said David K. Rea, U-M professor of geological sciences.
Rea and his colleague Ted Moore, U-M professor of geological sciences, presented data from their analysis of Lake Huron sediments for the first time today (Oct. 27) at a meeting of the Geological Society of America.
"There is no strong statistically valid link between El Nino events and today's mid-continental climate," Moore said. "But 12,000 years ago, there was a huge pile of ice sitting on top of much of North America. We believe a strong high pressure region developed over the ice, stabilizing the jet stream's path and pulling El Nino's heat and moisture along the southern boundary of the ice sheet. The closer you were to the ice sheet, the stronger the El Nino signal might have been."
The sudden warm spurts in Great Lakes climate are intriguing, according to Moore, because they occurred during the last half of the Younger Dryas---a 1,600-year-long global deep freeze which suddenly developed about 13,200 years ago just as the Earth was warming and ice sheets were starting to melt. For years scientists have speculated on what could have caused average winter temperatures in northern Europe to plunge by up to 10 degrees Celsius (18 degrees Fahrenheit) within a decade at the start of the Younger Dryas and then soar again just as abruptly 1,600 years later.
Rea, Moore and their U-M colleagues do not have the answer yet, but they believe they will find it in layers of glacial sediment called varves deposited on the floor of Lake Huron. U-M scientists have spent five years analyzing these sediments using radiocarbon dating, pollen counts, oxygen isotope analysis and image processing technology. During their analysis, they found:
--A chronology of climate change recorded in Lake Huron varves that correlates with Greenland ice core data (GISP2) for a period of time shortly after the mid-point of the Younger Dryas.
--Seasonal variations in annual varves with coarse gray glacial clays from around Lake Huron washed in with local precipitation during fall/winter and fine red clay sediment from the north washed in with glacial meltwater in spring/summer. Red layers contained more summer pollen and the "light oxygen isotope" signature found in glacial meltwater.
--Evidence that El Nino-associated climate variability was more common in spring/summer during years when the Laurentide Ice Sheet was closest to the Lake Huron basin.
--A 25-year interval of unusually thick varves dated around 12,200 years ago associated with evidence of rapid warming, low lake levels and high concentrations of glacial meltwater in Lake Huron.
"Varve data indicate that water levels in Lake Huron fell during warm periods when the ice sheet was melting rapidly and glacial meltwater was flowing out toward the North Atlantic," Rea explained. "Lake levels were highest during a long cool period in the first half of the Younger Dryas. During this time, ice sheet melting slowed down and little water flowed into the Atlantic."
"The most intriguing question we hope to answer is what was the impact of all that cold, fresh water dumped into the North Atlantic?" Moore added. "And could it have been responsible for triggering the beginning or the end of the Younger Dryas?"
The U-M study is funded by the National Science Foundation and the Geological Survey of Canada. Collaborators include Holly S. Godsey, a former U-M graduate student; C.F. Michael Lewis, Geological Survey of Canada; Linda C.K. Shane, University of Minnesota; Alison J. Smith, Kent State University; and James C.G. Walker, U-M professor of geological sciences.
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