A new study published in Nature Geoscience shows that changes in the West Antarctic Ice Sheet (WAIS) closely followed shifts in marine algae growth in the Southern Ocean during past glacial cycles. However, the relationship unfolded in a surprising way that challenges long-standing assumptions.

At the center of the discovery is sediment rich in iron that was carried into the ocean by icebergs breaking away from West Antarctica.

Iron usually acts as a nutrient that supports algae growth. Yet when scientists examined a sediment core collected in 2001 from the Pacific sector of the Southern Ocean, retrieved from more than three miles below the sea surface, they found that higher iron levels did not lead to faster algae growth.

"Normally, an increased supply of iron in the Southern Ocean would stimulate algae growth, which increases the oceanic uptake of carbon dioxide," says lead author Torben Struve of the University of Oldenburg. Struve worked as a visiting postdoctoral research scientist in 2020 at the Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School.

Why More Iron Did Not Boost Algae Growth

The research team traced this unexpected outcome to the chemical properties of the sediment delivered by icebergs. Their analysis indicates that much of the iron was highly "weathered," meaning it had undergone extensive chemical alteration over time. During earlier warm periods, when more ice broke off from West Antarctica and drifted northward, the iron entering the ocean was often in this poorly soluble form.

Because algae cannot easily use this type of iron, increased delivery did not translate into stronger biological growth.

Based on these findings, the researchers conclude that continued loss of the West Antarctic Ice Sheet could reduce the Southern Ocean’s ability to absorb carbon dioxide as the climate warms.

How Iron Normally Fuels Carbon Uptake

In the waters surrounding Antarctica, iron often limits how much algae can grow. Previous studies have shown that during glacial periods, strong winds transported iron-rich dust from continents into the ocean. In regions north of the Antarctic Polar Front – a boundary where cold Antarctic waters meet warmer waters to the north – that dust helped fertilize algae.

As algae populations expanded, the Southern Ocean absorbed more carbon dioxide from the atmosphere. This increased carbon uptake helped strengthen global cooling at the onset of glacial periods.

The new study instead focuses on waters south of the Antarctic Polar Front. There, evidence from the sediment core shows that iron input was highest during warm intervals rather than during glacial periods. The size and makeup of the particles also revealed that the main source of iron was not dust, but icebergs calved from West Antarctica.

"This reminds us that the ocean’s ability to absorb carbon isn’t fixed," says co-author Gisela Winckler, a professor at the Columbia Climate School and a geochemist at the Lamont-Doherty Earth Observatory.

Signs of Major Ice Loss in the Past

The findings also provide insight into how responsive the West Antarctic Ice Sheet is to rising temperatures. Struve notes that several recent studies suggest large-scale retreat occurred in this region during the last interglacial period around 130,000 years ago, when global temperatures were similar to those seen today.

"Our results also suggest that a lot of ice was lost in West Antarctica at that time," says Struve.

As the ice sheet, which reached several miles thick in some areas, broke apart, it produced large numbers of icebergs. These icebergs scraped sediment from the bedrock beneath the ice and released it into the ocean as they drifted north and melted. The sediment record indicates especially high iceberg activity near the end of glacial periods and during peak interglacial conditions.

Why the Form of Iron Matters

"What matters here is not just how much iron enters the ocean, but the chemical form it takes," says Winckler. "These results show that iron delivered by icebergs can be far less bioavailable than previously assumed, fundamentally altering how we think about carbon uptake in the Southern Ocean."

The researchers suggest that beneath the West Antarctic Ice Sheet lies a layer of very old, heavily weathered rock. Each time the ice sheet retreated during earlier interglacial periods, increased iceberg activity carried large amounts of these weathered minerals into the nearby South Pacific. Despite the greater iron input, algae growth remained limited.

"We were very surprised by this finding because in this area of the Southern Ocean the total amount of iron input was not the controlling factor for algae growth," Struve says.

What This Means for Future Climate Change

As global warming continues, further thinning of the West Antarctic Ice Sheet could recreate conditions similar to those seen during the last interglacial period.

"Based on what we know so far, the ice sheet is not likely to collapse in the near future, but we can see that the ice there is already thinning," says Struve.

If retreat continues, glaciers and icebergs could erode weathered rock layers more rapidly. This process could lower carbon uptake in the Pacific sector of the Southern Ocean compared with today, creating a feedback that could further intensify climate change.