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A mega-flood path across North America caused a period known as he Big Freeze about 13,000 years ago. A research team logs sediments in a cliff exposure at Liverpool Bay in the Northwest Territory in Arctic Canada, where some of the last of the winter ice remains. Graphic depicts a mapped extent of the Cordillerian and Laurentide ice sheets at around 14,750 years ago.

U. SHEFFIELD (UK)—The main cause of a rapid global cooling period, known as the Big Freeze or Younger Dryas, that occurred nearly 13,000 years ago was a mega-flood path across North America which channeled melt-water from a giant ice sheet into the oceans triggering the cold snap.

Mark Bateman from the University of Sheffield´s Department of Geography, and colleagues discovered that a mega-flood, caused by the melting of the Laurentide ice sheet, was routed up into Canada, mixing huge amounts of fresh water 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.

Details of the research are published in the journal Nature.

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Last of winter sea ice in Liverpool Bay and exposed cliff sediment at the MacKenzie Delta in Arctic Canada

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 1,400 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.

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Research team logging sediments in a cliff exposure at Liverpool Bay, Northwest Territory, Arctic Canada

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.

The research team examined the sediments in a large number of cliff sections along the Mackenzie Delta and found that many of the cliff sections showed evidence of sediment erosion.

This evidence spanned over a large region at several altitudes, which could only be explained by a mega-flood from the over-spilling of Lake Agassiz, which was at times bigger than the U.K., at the front of the Laurentide Ice-sheet rather than a normal flood of the river.

Bateman, who has been researching past environmental changes both in the U.K. and elsewhere in the world for almost 20 years, runs the luminescence dating lab at Sheffield.

The lab took the MacKenzie Delta sediment samples from above and below the mega-flood deposits, to find out when the mega-flood occurred, enabling its occurrence to be attributed to the start of the Younger Dryas.

Researchers say the study will help shed light on the implications of fresh water input into the North Atlantic today. There are current concerns that changes in the salinity of the ocean today, could cause another shut down of the Gulf Stream.

Current climate changes, including global warming, may be altering the planetary system which regulates evaporation and precipitation, and moves fresh water around the globe.

The findings, which show the cause, location, timing and magnitude of the mega-flood, will enable scientists to better understand how sensitive both oceans and climates are to fresh-water inputs and the potential climate changes which may ensue if the North Atlantic continues to alter.

“The findings of this paper through the combination of luminescence dating, landscape elevation models and sedimentary evidence allows an insight into what must have been one of the most catastrophic geological events in recent earth´s history,” Bateman says.

“They also show how events within the Earth-climate system in North America had huge impacts in Europe.”