Rapid sea level rise triggered by ice saddle collapse
12 July 2012, by Tamera Jones
Two of the largest rises in sea level in the last 20,000 years may have been caused by the collapse of huge valleys of ice linking massive North American ice sheets, say researchers.
Melting of these so-called ice saddles led to enormous surges in water being discharged into the surrounding oceans, raising sea levels dramatically, and may have led to a change in the Earth's climate.
The findings will help scientists improve their understanding about how such rapid increases in sea level are possible.
Dr Lauren Gregoire and colleagues from the University of Bristol used climate and ice sheet models to simulate the separation of two massive ice sheets – the Laurentide and the Cordilleran – that once covered North America. Their simulation showed that the ice saddle between them collapsed around 11,600 years ago, which would've led to a rise in sea level of nine metres in just 500 years.
'The events produced by our models coincide with real events, which gives us confidence that our models are accurate.'
Dr Lauren Gregoire from the University of Bristol
The researchers think this corresponds with a real event 14,600 years ago, which raised sea levels by as much as 18 metres.
'The meltwater pulse produced by the saddle collapse can explain more than half of the sea level jump observed around 14,000 years ago. The rest came from the progressive melting of ice sheets in Europe and Antarctica,' explains Gregoire.
In another experiment designed to replicate the separation of ice domes around Hudson Bay 8200 years ago, they found that melt-water from a collapsed saddle linking the domes would've led to a 2.5-metre rise in sea level.
The second pulse revealed by the researchers' model corresponds with the separation of three massive ice domes around Hudson Bay 8200 years ago. This melt is thought to have caused the most abrupt climate event of the last 9000 years. It disturbed ocean circulation and cooled the regions around the North Atlantic for several decades.
'The events produced by our models coincide with real events, which gives us confidence that our models are accurate,' says Gregoire.
The researchers found that ice saddles linking domed ice sheets started melting when the Earth's orbit around the Sun changed and the climate warmed, in what are known as Milankovitch cycles. Mammoths and early humans would have experienced the full effect of these changes.
The North American ice sheet during the Saddle Collapse, 14,600 years ago.
'One ice sheet covered the Rockies, while another covered the plains of Canada. They were linked by an ice saddle which was thinner than the two ice sheets; a pass made of ice between two mountains of ice. When the climate warmed, this saddle, which was at a lower altitude, was the first to melt,' explains Gregoire.
Ultimately, this mechanism would have separated large domes of ice up to three kilometres thick, creating an ice-free corridor. The ice sheets, on the other hand, remained at a lofty altitude and so took much longer to finally melt.
There's plenty of evidence for this recorded in ocean floor sediments and ancient coral reefs. And while it's clear that these pulses of meltwater must have come from retreating ice sheets, until now, no-one was sure about the exact geographical source.
'We've unearthed a relatively straightforward mechanism that we hadn't been able to investigate before. Our novel methodology that relied on supercomputers made this discovery possible,' she adds.
Lauren J. Gregoire, Antony J. Payne and Paul J. Valde, Deglacial rapid sea level rises caused by ice sheet saddle collapses, Nature, 487, 219-222 (12 July 2012), doi:10.1038/nature11257
Sea level change,
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