Deep ocean clues to a million-year-old Ice Age puzzle revealed in new study

Woods Hole, Mass. (Nov. 8, 2025) - A recently published study in Science challenges theories regarding the origins of a significant transition through the Earth’s ice ages. Led by an international team of researchers from the Woods Hole Oceanographic Institution (WHOI), the Lamont-Doherty Earth Observatory, the Scripps Institution of Oceanography, and Cardiff University, this research provides fresh insights into the ocean’s role in climate during the Mid-Pleistocene Transition, an enigmatic interval of change in climate cycles that began about one million years ago.

Many theories have been proposed for the Mid-Pleistocene Transition, and an important one is linked to a significant weakening of the Atlantic Meridional Overturning Circulation (AMOC). However, the new findings suggest an equally important but much more nuanced role for the deep ocean.

Using climate records spanning the past 1.2 million years, the team reconstructed deep ocean properties that are crucial for understanding the ocean’s flow and carbon sequestration capabilities. “The deep ocean is enormous, especially when considering its capacity to store carbon dioxide (CO₂) compared to the atmosphere,” said lead author Dr. Sophie Hines, an Assistant Scientist at WHOI. “Even a modest change in ocean circulation could significantly impact global climate.”

The researchers analyzed sediment core samples collected during the International Ocean Discovery Program (IODP) Expedition 361 near Cape Town, South Africa. By studying carbon and oxygen from fossils of single-celled organisms called foraminifera and isotopes of neodymium, the team uncovered details about the changes in deep ocean temperature and salinity, as well as the mixing histories of waters originating in both the northern and southern hemispheres.

Dr. Sidney Hemming, the Arthur D. Storke Memorial Professor of Earth and Environmental Sciences at the Lamont-Doherty Earth Observatory and co-chief scientist on the expedition, said, “Crucially, we show that shifts in different deep ocean properties are not always coincident. With our more highly resolved multi-proxy record that includes transitional intervals, we find that ice age intensification was influenced primarily by changes around Antarctica.”

It is suggested that as the Antarctic Ice Sheet expanded, it enhanced the ocean's capacity to store carbon, leading to lower atmospheric CO₂ levels, colder climates, and prolonged ice age cycles.

Dr. Hines added, “Our research sheds light on the intricate interplay between ocean dynamics and climate change, underscoring the significance of the Southern Ocean in understanding our planet's climate history.”

Recent studies stress the urgency of anthropogenic climate change, particularly in relation to reductions in the AMOC. As the Southern Ocean continues to warm at an alarming rate, understanding its dynamics is critical. The Southern Ocean plays a pivotal role in regulating global climate patterns, and its changes could have significant implications for ecosystems and weather systems worldwide.

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About Woods Hole Oceanographic Institution

Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the oceans’ role in the changing global environment. For more information, please visit www.whoi.edu.