A new study published in Nature Geoscience reveals that changes in the West Antarctic Ice Sheet (WAIS) closely tracked the growth of marine algae in the Southern Ocean during past ice ages, but not in the way scientists expected.

The key is iron-rich sediment transported by icebergs from West Antarctica.

Iron acts like a fertilizer for algae. But when researchers analyzed sediment cores taken more than three miles below the surface on the Pacific side of the Southern Ocean in 2001, they were surprised to find that high iron supplies did not promote the growth of marine algae.

“Normally, an increase in iron supply in the Southern Ocean stimulates algae growth, which increases the uptake of carbon dioxide in the ocean,” says lead author Torben Struve from Oldenburg University. Struve served as a visiting postdoctoral fellow at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School, in 2020.

The researchers traced the discrepancy to the chemistry of the sediment carried by the iceberg. Their analysis suggests that the minerals were highly “weathered,” meaning that much of the iron that reached the ocean during past warm periods, when more West Antarctic ice broke off and drifted north, was in this less soluble form.

Based on these results, the researchers concluded that if the West Antarctic Ice Sheet continues to shrink, climate change could reduce the Southern Ocean’s carbon uptake.

Iron is often the nutrient that limits algae growth in waters around Antarctica. Previous research suggests that strong winds during the ice age carried iron-rich dust from the continents into the ocean. In areas north of the Antarctic Polar Front (the boundary where the cold waters of Antarctica meet the warm waters of the north), the dust feeds algae and helps increase the Southern Ocean’s carbon dioxide uptake. This additional carbon uptake accelerated global cooling as the Ice Ages began.

The new study focuses on the region south of the Antarctic polar front. From the sediment cores they recovered, the researchers found that iron uptake peaked during warm periods, not ice ages. The size and composition of the particles in the core also revealed that the main source of iron does not come from dust, but from icebergs carved from West Antarctica.

“This is a reminder that the ocean’s carbon absorption capacity is not fixed,” say co-authors Gisela Winklera professor at the Columbia Climate School and a geochemist at the Lamont-Doherty Earth Observatory.

Struve said the study also helps shed light on how sensitive the West Antarctic ice sheet is to climate change. Several recent studies have shown that ice in this region of Antarctica underwent a major retreat during the last interglacial period about 130,000 years ago, when temperatures were about the same as today.

“Our results also suggest that a large amount of ice was lost in West Antarctica at that time,” Struve said.

The collapse of the ice sheet, which was several miles thick in some places, formed numerous icebergs that scraped sediment from the bedrock beneath them and drifted north, melting and falling. This core suggests that particularly large numbers of icebergs were present at the end of the ice age and at the peak of the interglacial period.

“The important question here is not only how much iron gets into the ocean, but also what chemical form it takes,” Winkler said. “These results show that the bioavailability of iron carried by icebergs may be much lower than previously assumed, fundamentally changing the way we think about carbon uptake in the Southern Ocean.”

Researchers say that beneath the West Antarctic ice sheet there is likely a layer of geologically old, highly weathered rock. As the ice sheet shrank and more icebergs broke up during past interglacial periods, those icebergs carried large amounts of weathered minerals into the adjacent South Pacific Ocean, and algal growth remained low.

“We were very surprised by this finding because in this region of the Southern Ocean, total iron input is not a factor controlling algae growth,” Struve said.

In the future, the West Antarctic ice sheet may continue to shrink due to global warming, creating a situation similar to the last interglacial.

“Based on what we know so far, it is unlikely that the ice sheet will collapse in the near future, but we know that the ice there is already thinning,” Struve said.

Further retreat could accelerate erosion of weathered rock layers by glaciers and icebergs. As a result, carbon intake in the Antarctic and Pacific region may decrease compared to current levels, and this feedback could further amplify climate change.

Based on a press release from Oldenburg University