New Article in Nature Geoscience | by Helen Coxall et al.

A saltier North Atlantic kick-started circulation at the end of the greenhouse world

A drastic change in ocean circulation patterns over 34 million years ago occurred because surface waters in the far North Atlantic became salty enough to sink. This start-up of Northern deep-water formation purged stagnant waters sourced from the Arctic resulting in a release of carbon dioxide into our atmosphere.

Some 35 million year old benthic foraminifera shells of the kind analyzed in the study

This is according to an international team of scientists.

Writing in the journal Nature Geoscience, the team describe how fossils from the ocean floor, including those of ancient fish teeth, were used to reconstruct the conditions and circulation patterns in the north western North Atlantic during this time period – known as the Eocene – and determine the impact of sudden changes.

Around 50 million years ago Earth’s climate was much warmer and wetter than it is today, with hardly any ice on the planet and sea levels that were much higher than the shapes of coastlines we see today. Carbon dioxide levels decreased slowly and the Earth cooled gradually until the Antarctic ice sheet eventually formed quite rapidly approximately 34 million years ago. The reason why Antarctica froze precisely then is unclear but the new findings bring us close to the answer.

“We have found key evidence that under warm greenhouse climate conditions the North Atlantic was not salty enough to produce dense deep water flows, as exist today and drive the circulation system that helps keep Europe warm. Only when the influence of freshwater dwindled could the circulation state change and deep-water start sinking.” Dr. Helen Coxall, Stockholm University.

And this circulation change happened at a critical time, the new study suggests, about one million years before Antarctica became glaciated.

 “The close timing of the increased North Atlantic Circulation just prior to the Antarctic ‘deep-freeze’ was probably not a coincidence and the new data imply that the change was connected”, says first author of the study Dr. Helen Coxall, Stockholm University.

 “The CO2 burp at around 35 million years ago caused by the first pulse of northern deep water could have temporarily delayed the onset of Antarctic ice sheet growth but once the purge was over the stronger North Atlantic Current would have helped cool Antarctica by ‘stealing’ heat from the southern hemisphere.

The Antarctic ice sheet is now the largest single mass of ice on Earth and covers an area of almost 14 million square kilometres. Approximately 61 percent of all fresh water on the Earth is held in the Antarctic ice sheet and, if released, would cause sea levels to rise by up to 60 metres.

 “Today we know that salty conditions in the North Atlantic are needed for the circulation to operate. We have evidence here that in a greenhouse world the North Atlantic deep circulation could not operate because conditions were not salty enough – the source of freshwater was higher rainfall and river inflow in the Arctic and subpolar regions under the warm climate - melting Greenland and other northern ice could cause a similar freshening and disruption of the circulation in the future warming world.”
The team believe the study is a perfect illustration of how the climate, oceans and carbon cycle are intricately linked, and that the results could help to predict future implications of a changing climate under human influence.

“We should not consider the climate system in isolation – the climate, the oceans, and the carbon cycle are all intricately tied up together. When we poke one, as we are doing now by burning fossil fuels, we are potentially poking them all,” continued Professor Lear, a co-author of the study.

Link to the article

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