Tuesday, November 10, 2015

Growing Antarctic ice sheets almost dried out the ancient Mediterranean


A drop in sea level caused by growing ice sheets in Antarctica and rising land bridge across the Strait of Gibralter left the Mediterranean Sea cut off from the Atlantic Ocean. This meant it quickly evaporated away to leave a vast salty basin (illustrated above), according to new research based on ice cores from Antarctica

They said a dramatic increase in the amount of water locked up in the continent's ice sheet led to a drop in global sea levels.

This led to a land bridge appearing between Gibraltar and North Africa across the mouth of the Mediterranean where it meets the Atlantic Ocean.

As the water in the sea evaporated, the land around the Strait of Gibraltar began rising upwards as the downward pressure from the water was lifted.

Dr Christian Ohneiser, a geologist at the University of Otago in New Zealand, said the growth in the Antarctic Ice also caused changes in the Earth's crust that may also have contributed.

'We found that the Antarctic ice sheet had an uneven effect on the global sea level because its growth resulted in a complex interplay between gravitational and rotational effects and the deformations to the Earth's crust caused by ice advance and retreat,' he said.

The drying up of the Mediterranean during the Miocene is known as the Messinian Salinity Crisis. 

It saw the sea almost complete desiccated and salt layers several miles thick forming on the basin.

It remained this way for almost 270,000 years before the Zanclean flood saw sea water from the Atlantic Ocean rush back into the Mediterranean basin, filling it within a couple of years.

Dr Ohneiser and his colleagues, whose research is published in the journal Nature Communications, examined 60 sedimentary drill cores from around the edge of Antarctica.

Dr Ohneiser said: 'We determined that the continent's ice was indeed growing in the lead up to the Messinian Salinity Crisis, but the timings of key events did not precisely match.'

They then used a computer model of the Earth that simulated growth in the Antarctic ice sheet to see what geophysical impacts this would have aside from generally lowering the sea level.

These simulations revealed that the Earth's crust began to change and deform as the sea level decreased and ice sheets grew.

Growing ice sheets in Antarctica (stock image used) 5.6 million years ago caused global sea levels to drop and led to the land around the Strait of Gibralter to cut off the Mediterranean from the Atlantic Ocean

The Mediterranean (pictured) is connected to the Atlantic Ocean by a narrow strait between southern Spain and North Africa. This cut off the sea from the Atlantic 5.6 million years ago, causing it to evaporate

Dr Ohneiser added: 'This kept the Mediterranean isolated from the Atlantic Ocean until the crust began to relax and sink.

'At the same time, Antarctica began to melt, raising sea levels again.'

By around 5.33 million years ago, the rising sea level was just enough to wash over the thin land bridge at Gibraltar, resulting in a catastrophic flood that refilled the sea, he said.

Dr Ohneiser added that one of the key implications of the study is that changes in global sea-level are uneven when ice sheets expand or retreat.

He said it could have implications for future sea level rise as the ice sheets around the Arctic and Antarctic melt due to rising temperatures.

'Future melting of the large Southern or Northern hemisphere ice masses will result in an uneven rise in sea-level around the world, and this should be factored into future climate change scenarios,' he added.


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