Why ice ages became longer and more intense

Researchers say they have confirmed the crucial role of the Antarctic Ocean during periods of climate change. Over the past million years, less frequent mixing of deep and surface waters may have influenced the transition to longer, more intense ice ages. 

During the mid-Pleistocene transition period, which began one million years ago, ice ages extended and became more powerful; the frequency of their cycles increased from 40,000 years to 100,000 years.

One of the keys to this phenomenon lies in the deep waters of the Southern Ocean surrounding Antarctica, according to research published in the journal Science, carried out by a team led by professor Samuel Jaccard from the University of Bern.

Oceans contain 60 times more carbon than the atmosphere. As a result, small variations in carbon dioxide (CO2) concentrations play a major role in climate transitions.

To better understand this phenomenon, the researchers studied the evolution of mixing between deep and surface waters in the Southern, or Antarctic, Ocean. The mixing action brings oceanic CO2 to the surface, where it escapes into the atmosphere.

Analysing marine sediments collected from beneath the ocean floor at a depth of 2,800 metres, the team found that the mixing was significantly reduced at the end of the Mid-Pleistocene period, about 600,000 years ago. They claim the reduced mixing lowered the amount of CO2 released by the ocean, which in turn reduced the greenhouse effect and intensified ice ages.

“Mixing plays a very important role in this case, because it brings the dissolved CO2 from the deep waters to the surface, from where it is transferred to the atmosphere and contributes to the greenhouse effect,” said Jaccard in a statement. “A better understanding of these phenomena is crucial, because they are also a factor in present-day global warming.”

The scientists looked in details at the difference in salinity and temperature between the surface and deep waters, which determine the intensity of the mixing.  During the transition to longer ice ages the surface waters became simultaneously colder and less salty. Consequently, the mixing of water layers decreased considerably during ice ages. 

This study was begun by Jaccard at the Federal Institute of Technology in Zurich (ETH Zurich) and continued at the University of Bern with the support of the Swiss National Science Foundation. It was carried out in collaboration with the Max Planck Institute for Chemistry in Mainz (Germany), Princeton University (United States), the University of Cambridge (United Kingdom), the University of Bergen (Norway) and the British Antarctic Survey. 

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