Ancient, Slowing Ocean Currents May Have Triggered Longer Ice Ages

About 900,000 years ago, ice-age cycles became longer and more intense, switching from 41,000-year cycles to 100,000-year cycles. For years, scientists have been puzzled exactly why this was the case, but now they may have uncovered the answer. Deep ocean currents around the globe may be responsible for the increased longevity of ice ages.

Deep ocean currents span the globe, bringing heat to locations that would otherwise be frigid. In order to learn a bit more about these currents, the researchers actually reconstructed the past strength of the Earth's system of deep-ocean currents by sampling deep-sea sediments off of the coast of South Africa. There, powerful currents originating in the North Atlantic Ocean pass by on their way to Antarctica. By measuring the isotope ratios of the elements neodymium, the researchers could infer how much North Atlantic water made it that far in the past.

"The oceans started storing more carbon dioxide for a longer period of time," said Leopoldo Pena, the lead author of the new study, in a news release. "Our evidence shows that the oceans played a major role in slowing the pace of ice ages and making them more severe."

In fact, the researchers showed that over the last 1.2 million years, the currents strengthened during warm periods and weakened during ice ages. In addition, they found that about 950,000 years ago, ocean circulation weakened significantly and stayed week for 100,000 years. After this turning point, the Earth entered a new phase of longer ice age cycles.

So what slowed down the current in the first place? It's possible that advancing glaciers in North America stripped away soils in Canada. This caused thicker, longer-lasting ice to build up on the remaining bedrock. The advancing ice could have then trigged the slowdown of the currents, leading oceans to vent less carbon dioxide which, in turn, suppressed the interglacial period.

"The ice sheets must have reached a critical state that switched the ocean circulation system into a weaker mode," said Steven Goldstein, one of the researchers, in a news release.

The findings are published in the journal Science.