Climate Change Slowing Atlantic Currents Linked to Intense Ice Ages

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Researchers have revealed, through sediment drilling, that a slowdown in North Atlantic Ocean currents around one million years ago played a pivotal role in driving longer and more intense ice ages. Their findings also provide valuable insights for understanding future climate change scenarios.

Bern/Switzerland – For around the last 800,000 years, the Earth's climate has undergone a succession of warm and ice ages, each with a cycle of around 100,000 years. These climate cycles are caused by recurring changes in the Earth's orbit around the sun. In the period 1.2 million years ago and even further back, however, the Earth's climate followed a different rhythm controlled by the tilt of the Earth's axis: a cycle of warm and ice ages lasting around 41,000 years.

The reasons behind this change in climate cycles – called the "Mid-Pleistocene Transition"– still occupy scientists to this day. A team of researchers led by Dr. Iván Hernández-Almeida from the Past Global Changes global research network based at the University of Bern has used sediment drilling to investigate the role played by changes in ocean currents in the North Atlantic - and thus also show how they could influence the Earth's climate in the future. Their study was published today in Nature Communications.

Oxygen depletion in the North Atlantic deep sea

The research team used sediment cores from the International Ocean Drilling Program (IODP) from the region south of Iceland as the basis for their investigations. Over hundreds of thousands of years, particles from the seawater, including microfossils such as plankton, are deposited layer by layer in the sediments. These preserve information about the prevailing conditions in the ocean at that time, such as temperature or oxygen content, and make the sediments a valuable climate archive of the last million years or more.

With the help of geochemical measurements of oxygen-sensitive elements such as manganese and phosphorus as well as the analysis of fossil microorganisms living on the bottom - so-called benthic foraminifera - the researchers reconstructed how well the deep North Atlantic was supplied with oxygen between 1,1 million years to 800,000 years ago.

Weakening ocean current keeps the carbon in the depths

"We were able to show that during this change in climate cycles, the deep North Atlantic repeatedly went through cold phases in which the oxygen content was greatly reduced," says Iván Hernández-Almeida, first author of the study and scientist at Past Global Changes at the University of Bern. "What is particularly surprising is how long these low-oxygen conditions lasted in a region that is now considered to be very well ventilated."

According to the researchers, the reason for this is that large quantities of melted freshwater from ever-growing glaciers and their icebergs reached the North Atlantic during ice ages. This lighter freshwater stabilized the upper layer of the ocean and thus weakened the formation of ocean current that transports oxygen-rich water into the depths. Apart from a lack of oxygen, which had serious consequences for ecosystems in the deep sea, the weakened ocean current led to the accumulation of more dissolved carbon from the microbial decomposition of dead organisms in the deep sea.

"Since less of the CO2 dissolved in the ocean could reach the surface and escape into the atmosphere due to the weakened deep ocean currents in the North Atlantic there was a decrease in atmospheric CO2 concentration, the expansion of polar ice sheets- and thus a cooling of the planet," says Hernández-Almeida. "This made the North Atlantic - along with the Southern Ocean - a central arena for the transition to longer and colder ice ages."

The study thus closes an important gap: Previous work had mainly focused on processes in the Southern Ocean. The new data show that deep circulation and oxygen conditions also changed significantly in the North Atlantic and that both polar regions therefore contributed simultaneously to the change in ice age cycles.

A warning signal for future changes in the Atlantic

The results are also significant with regard to today's climate change. Climate models predict that the Atlantic Ocean current could weaken as a result of global warming and the increasing input of meltwater from Greenland. "Our study shows that even in much colder phases on Earth, changes in the inflow of freshwater were sufficient to disrupt the transport of water into the depths of the North Atlantic - with devastating and irreversible impacts for the climate and biodiversity," says Hernández-Almeida. "This is an important warning signal of how sensitively this system can react even today. Therefore, such a cooling would by no means be good news in the context of human-induced global warming."

In a next step, the Bernese researcher would like to investigate how the deep circulation and oxygen content of the North Atlantic have changed in other warm and cold phases of the Earth's history. The aim is to gain a better understanding of when this balance shifts. "This would also be an important finding with regard to today's climate change," concludes Hernández-Almeida.

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