There are a few things scientists know for sure about how Earth grows warmer: For instance, when there’s more carbon dioxide (CO2) in the atmosphere, that CO2 traps heat. This means that during an ice age, less CO2 is present in Earth’s atmosphere.

“One of the fundamental questions in our field was, ‘Where did that CO2 go during ice ages, and where did it come from when the planet warmed?’”

“One of the fundamental questions in our field was, ‘Where did that CO2 go during ice ages, and where did it come from when the planet warmed?’” said Ryan Glaubke, a paleoceanographer and postdoctoral researcher at the University of Arizona.

Scientists had their suspicions: The ocean was the obvious culprit because it’s enormous and is known to exchange CO2 with the atmosphere. But for CO2 to be stored in the ocean for long periods, it would need to be in cold, salty, dense water far beneath the ocean’s surface. Until now, scientists had no way to prove that salinity levels in the deep ocean were linked to changes in atmospheric CO2 over the scale of ice ages.

Now, new research published in Nature Geoscience seems to confirm what many researchers have long thought was the case: A giant “blob” of salty ocean water kept carbon dioxide locked deep in the ocean during the last ice age, and the blob released that CO2 during an upwelling event 18,000 years ago.

Unusual Upwelling

During his graduate studies at Rutgers University, Glaubke and his fellow researchers collected sediment cores from the seafloor. Sediment cores are long, thin cylinders of mud with successive layers that reflect periods in Earth’s history.

Normally, when scientists collect sediment cores, they use them to learn about past conditions near the ocean’s surface. Single-celled creatures called foraminifera (or forams, for short) live and build their shells near the ocean’s surface. When these creatures die and sink to the ocean floor, their shells become part of the seafloor sediment and provide a record of the composition of the upper ocean.

This team, however, gathered sediment cores from an unusual site on the boundary of the Indian and Southern oceans. In this spot, off the coast of Western Australia, waters from deep in the ocean upwell to the surface.

“It’s really hard to look at the bottom of the ocean from the surface,” said Liz Sikes, a paleoceanographer at Rutgers, a coauthor of the paper, and Glaubke’s former Ph.D. adviser. “But the thing is, these planktic forams are in a place in the ocean where the water that’s at the surface has just returned to the surface and it still retains most of its deep-water qualities.”

Gathering sediment cores from this location meant the scientists could gain an understanding not just of how the upper ocean changed in the past but of how the waters that rose from the bottom of the ocean had also changed.

“What we found, rising from the deep ocean to the surface, was not only this geochemical fingerprint for old carbon that remained at the bottom of the ocean, but at the exact same time, we see this increase in upper ocean salinity by around 2 parts per thousand, which is a very large scale change,” Glaubke said. “That is one of the really important contributions of this paper, I think, which is that it provides this support for this ‘salty blob’ kind of retention mechanism.”

From Glacial to Interglacial

Patrick Rafter, a chemical oceanographer who did not contribute to this paper but was involved with measuring the radiocarbon levels in the collected sediment cores, said he was already convinced that salinity must play an important role in the rate of global ocean overturning, so the results were “not surprising” to him. He noted that the study was rigorous and careful, in that the researchers replicated their anomalous findings with multiple planktic species.

“It’s like any kind of mystery: The more evidence you get supporting it, the more likely you are to think maybe it’s real.”

“It’s like any kind of mystery: The more evidence you get supporting it, the more likely you are to think maybe it’s real,” he said. “So far, the evidence that exists suggests this is a solid finding that we should consider when trying to explain glacial-interglacial climate change.”

Furthermore, the upwelling waters of the Southern Ocean help sustain a global conveyor belt of currents, including the Atlantic Meridional Overturning Circulation. During an ice age, these currents tend to be more sluggish. The strengthening of these currents is an important piece in moving the planet out of an ice age.

“We make the argument that not only is this water mass releasing carbon to the atmosphere and kind of warming the planet, but the salt that then gets entrained in the global conveyor belt probably played a really important role in flipping that switch from glacial mode to interglacial mode,” Glaubke said. “So there’s this dual contribution that the salty blob might be making to ending the last ice age.”

—Emily Gardner (@emfurd.bsky.social), Associate Editor

Citation: Gardner, E. (2026), How the rise of a salty blob led to the fall of the last ice age, *Eos, 107, *https://doi.org/10.1029/2026EO260044. Published on 2 February 2026.

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