ENVIRONMENTAL HISTORY
Ancient sea ice loss spurred Antarctic cold reversal 15,000 years ago

Researchers collected ancient ice samples from a blue ice area in Western Antarctica's Patriot Hills. Photo by Chris Turney

June 22 (UPI) -- A mysterious period of climate change, known as the Antarctic cold reversal, was triggered by the rapid loss of sea ice nearly 15,000 years ago, according to a study published Monday in the journal Nature Geoscience.

At the end of the last ice age, some 18,000 years ago, atmospheric carbon levels began to rise, Earth's glaciers started receding and the world steadily warmed. But this period of warming didn't proceed uninterrupted. It happened in fits and starts.

One fit, beginning 14,600 years ago, was particularly pronounced: the Antarctic cold reversal. After a period of greenhouse warming, atmospheric CO2 levels plateaued -- remaining at 240 parts per million for 1,900 years.

Scientists weren't sure what caused the plateau, but researchers recently found evidence of increased biological activity during the reversal period.

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"We found that in sediment cores located in the sea-ice zone of the Southern Ocean biological productivity increased during this critical period, whereas it decreased farther north, outside of the sea-ice zone," Michael Weber, researcher at the Institute for Geosciences at the University of Bonn in Switzerland, said in a news release. "It was now important to find out how climate records on the Antarctic continent depict this critical time period."

To better understand how changes in ice patterns influenced the region's biological activity and the Antarctic carbon cycle, an international team of researchers headed to Western Antarctica's Patriot Hills Blue Ice Area in search of marine biomarkers trapped in ancient ice layers.

"The cause of this long plateau in global atmospheric CO2 levels may be fundamental to understanding the potential of the Southern Ocean to moderate atmospheric CO2," said lead researcher Chris Fogwill.

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"Whilst recent reductions in emissions due to the Covid-19 pandemic have shown that we can reduce CO2, we need to understand the ways in which CO2 levels have been stabilized by natural processes, as they may be key to the responsible development of geoengineering approaches and remain fundamental to achieving our commitment to the Paris Agreement," said Fogwill, professor of glaciology and palaeoclimatology at Keele University in Britain.

Blue ice areas are formed when high winds push snow into large embankments. The combination of wind-drive snow transport, ice flow and sublimation leaves older, smoother and bluer ice exposed.

Many blue ice areas feature especially ancient ice, and some contain ice as much as 2.5 million years old, researchers say.

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"Instead of drilling kilometers into the ice, we can simply walk across a blue ice area to travel back through time," said researcher Chris Turney, a professor of climate change at the University of New South Wales. "This provides the opportunity to sample large volumes of ice necessary for studying new organic biomarkers and DNA that were blown from the Southern Ocean onto Antarctica and preserved in the blue ice."

Analysis of the ice samples collected from the Patriot Hills revealed a growing abundance of marine organisms during the 1,900-year Antarctic cold reversal, researchers reported.

When scientists ran climate models fueled by paleoclimate data from the time period, their simulations showed the rise in biological activity coincided with dramatic seasonal changes in sea ice extent.

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The research suggests sea ice losses triggered an increase in biological activity, which helped pull CO2 from the atmosphere and sequester it in the ocean.

In future studies, scientists said they hope to use their findings to improve climate change prediction models for the Southern Ocean and Antarctica.

"Our results highlight the role Antarctic sea ice plays in controlling global CO2, and demonstrate the need to incorporate such feedbacks into climate-carbon models," researchers wrote in the study.