A new study in Nature Geoscience reveals that changes in the West Antarctic Ice Sheet (WAIS) closely tracked marine algae growth in the Southern Ocean over previous glacial cycles, but not in the way scientists expected.
The key factor is iron-rich sediments transported by icebergs from West Antarctica.
Iron acts like fertilizer for algae. But when analyzing a sediment core taken from the Pacific sector of the Southern Ocean in 2001, more than three miles below the water's surface, researchers were surprised to find that a high iron supply did not accelerate marine 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.
The team traced the mismatch to the chemistry of the iceberg-delivered sediment. Their analyses suggest the minerals were highly "weathered," meaning that much of the iron that reached the ocean during past warm spells, when more West Antarctic ice broke and drifted north, was in this less-soluble form.
Based on these results, the research team concludes that climate change may reduce carbon dioxide uptake in the Southern Ocean if the West Antarctic Ice Sheet continues to shrink.
Iron is often the limiting nutrient for algae growth in the waters around Antarctica. According to earlier studies, strong winds during glacial periods carried iron-rich dust from the 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 and increase the Southern Ocean's uptake of carbon dioxide. This added carbon uptake helped intensify global cooling as glacial periods began.
The new study zeroes in on a region south of the Antarctic Polar Front. From the sediment core they recovered, the researchers found that iron input peaked during warm intervals rather than glacial periods. The size and composition of the particles in the core also revealed that the dominant iron source did not come from dust, but from 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.
The study also helps clarify how sensitive the West Antarctic Ice Sheet is to climate change, Struve says. Several recent studies indicate that ice in this part of Antarctica retreated on a large scale during the last interglacial period around 130,000 years ago, when temperatures were roughly similar to today.
"Our results also suggest that a lot of ice was lost in West Antarctica at that time," says Struve.
The disintegration of the ice sheet, up to several miles thick in places, created large numbers of icebergs that scraped sediment from the bedrock below and then dropped it as they drifted north and melted. The core suggests particularly large numbers of icebergs were present at the end of glacial periods and during peak interglacial conditions.
"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."
Beneath the West Antarctic Ice Sheet, the researchers say, there is likely a layer of geologically ancient, highly weathered rock. Each time the ice sheet shrank during past interglacial periods and more icebergs broke off, those icebergs carried large quantities of weathered minerals into the adjacent South Pacific, and algae growth remained low.
"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.
Looking ahead, continued shrinking of the West Antarctic Ice Sheet from global warming could create conditions similar to those 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.
Further retreat could accelerate the erosion of weathered rock layers by glaciers and icebergs. That, in turn, could reduce carbon uptake in the Pacific sector of the Southern Ocean compared with today — a feedback that could further amplify climate change.
Based on a press release from the University of Oldenburg
