A new study reveals that iron from melting West Antarctic ice does not boost algae growth as expected, potentially reducing the Southern Ocean's ability to absorb carbon dioxide. Researchers found that the iron delivered by icebergs is in a poorly usable form for marine life. This discovery challenges assumptions about how ice loss affects climate change mitigation.
Scientists have uncovered an unexpected dynamic in the Southern Ocean's role as a carbon sink, linked to the West Antarctic Ice Sheet (WAIS). Published in Nature Geoscience, the study analyzed a sediment core collected in 2001 from the Pacific sector of the Southern Ocean, over three miles below the surface. It shows that during past warm periods, icebergs from the WAIS carried iron-rich sediment into the ocean, but higher iron levels did not accelerate algae growth.
Lead author Torben Struve of the University of Oldenburg, who conducted the research as a visiting scientist at Columbia's Lamont-Doherty Earth Observatory, explained: "Normally, an increased supply of iron in the Southern Ocean would stimulate algae growth, which increases the oceanic uptake of carbon dioxide." However, the iron was highly weathered and poorly soluble, making it inaccessible to algae. This form of iron entered the ocean mainly south of the Antarctic Polar Front during interglacial periods, unlike dust-driven fertilization in northern regions during glacial times.
Co-author Gisela Winckler, a geochemist at Lamont-Doherty, noted: "This reminds us that the ocean's ability to absorb carbon isn't fixed." The findings indicate that the WAIS retreated significantly around 130,000 years ago, when global temperatures resembled today's, releasing weathered rock from beneath the ice sheet via calving icebergs. Struve added: "Our results also suggest that a lot of ice was lost in West Antarctica at that time."
As climate warming thins the WAIS, similar processes could recur, potentially lowering carbon uptake in the Southern Ocean's Pacific sector. Struve cautioned: "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." Winckler emphasized: "What matters here is not just how much iron enters the ocean, but the chemical form it takes." This could create a feedback loop intensifying global warming by diminishing a key carbon absorption mechanism.