ScienceDaily (Mar. 31, 2010) — The main cause of a rapid global cooling period, known as the Big Freeze or Younger Dryas -- which occurred nearly 13,000 years ago -- has been identified thanks to the help of an academic at the University of Sheffield.
Modern-day Hubbard Glacier in Seward, Alaska. New research has identified a mega-flood path across North America which channeled melt-water from a giant ice sheet into the oceans and triggering the Younger Dryas cold snap. (Credit: iStockphoto)
The research team, which included Dr Mark Bateman from the University of Sheffield's Department of Geography, discovered that a mega-flood, caused by the melting of the Laurentide ice sheet, which covered much of North America, was routed up into Canada and into the Arctic Ocean.
This resulted in huge amounts of fresh water mixing with the salt water of the Arctic Ocean. As a result, more sea-ice was created which flowed into the North Atlantic, causing the northward continuation of the Gulf Stream to shut down.
Without the heat being brought across the Atlantic by the Gulf Stream, temperatures in Europe plunged from similar to what they are today, back to glacial temperatures with average winter temperatures of -25oC. This cooling event has become known as the Younger Dryas period with cold conditions lasting about 1400 years. The cold of the Younger Dryas affected many places across the continent, including Yorkshire in the Vale of York and North Lincolnshire which became arctic deserts with sand dunes and no vegetation.
Before now, scientists have speculated that the mega-flood was the main cause of the abrupt cooling period, but the path of the flood waters has long been debated and no convincing evidence had been found establishing a route from the ice-sheet to the North Atlantic.
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Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean
Julian B. Murton1, Mark D. Bateman2, Scott R. Dallimore3, James T. Teller4 & Zhirong Yang4
Permafrost Laboratory, Department of Geography, University of Sussex, Brighton BN1 9QJ, UK
Sheffield Centre for International Drylands Research, Department of Geography, Winter Street, University of Sheffield, Sheffield S10 2TN, UK
Geological Survey of Canada, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, Canada
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Correspondence to: Julian B. Murton1 Correspondence and requests for materials should be addressed to J.B.M. (Email: j.b.murton@sussex.ac.uk).
Abstract
The melting Laurentide Ice Sheet discharged thousands of cubic kilometres of fresh water each year into surrounding oceans, at times suppressing the Atlantic meridional overturning circulation and triggering abrupt climate change1, 2, 3, 4. Understanding the physical mechanisms leading to events such as the Younger Dryas cold interval requires identification of the paths and timing of the freshwater discharges. Although Broeckeret al. hypothesized in 1989 that an outburst from glacial Lake Agassiz triggered the Younger Dryas1, specific evidence has so far proved elusive, leading Broecker to conclude in 2006 that “our inability to identify the path taken by the flood is disconcerting”2. Here we identify the missing flood path—evident from gravels and a regional erosion surface—running through the Mackenzie River system in the Canadian Arctic Coastal Plain. Our modelling of the isostatically adjusted surface in the upstream Fort McMurray region, and a slight revision of the ice margin at this time, allows Lake Agassiz to spill into the Mackenzie drainage basin. From optically stimulated luminescence dating we have determined the approximate age of this Mackenzie River flood into the Arctic Ocean to be shortly after 13,000 years ago, near the start of the Younger Dryas. We attribute to this flood a boulder terrace near Fort McMurray with calibrated radiocarbon dates of over 11,500 years ago. A large flood into the Arctic Ocean at the start of the Younger Dryas leads us to reject the widespread view that Agassiz overflow at this time was solely eastward into the North Atlantic Ocean.
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