A sediment core from the Pacific sector of the Southern Ocean has provided a research team from the Alfred Wegener Institute, the University of Oldenburg and the American Lamont-Doherty Earth Observatory with evidence of an unexpected climate feedback in the Southern Ocean region: As the researchers report in the latest issue of Nature Geoscience, there was a close correlation between changes in the West Antarctic Ice Sheet (WAIS) and marine algae growth over previous glacial cycles – but the correlation was not as expected. The results suggest that global warming may lead to reduced uptake of carbon dioxide than at present in the Pacific sector of the Southern Ocean if the WAIS, considered as inherently unstable, continues to retreat.
The sediment core that the team analysed contains deposits dating back to around half a million years, covering four glacial cycles. The core was retrieved back in 2001 during an expedition by the research vessel Polarstern in water depths of just under 5,000 metres between South America and New Zealand - an area south of the so-called Polar Front and therefore part of the Southern Ocean.
The investigated sediments showed an unexpected picture with regard to the element iron: In past cold periods, strong winds blew more iron-rich dust from the continents into the oceans. As a result, algae growth increased and the Southern Ocean north of the polar front absorbed more CO2. This process intensified global cooling at the beginning of each glacial period. “Normally, an increased supply of iron in the Southern Ocean acts like a fertiliser: it stimulates algae growth, which increases the oceanic uptake of carbon dioxide,” explains Dr Torben Struve from the University of Oldenburg and lead author of the study. In their investigations, the researchers found that the iron input at the location where the core was retrieved was particularly high compared to neighbouring marine areas, especially during warm periods. “Surprisingly, however, this did not stimulate oceanic algae growth in this region to increase the uptake of CO2,” explains Dr Frank Lamy, palaeoclimatologist at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) and co-author of the study.
More iron did not mean more algae
The researchers attribute this to the fact that the sediments were heavily weathered. The size of the particles indicates that the iron was transported there by icebergs from the WAIS, which is considered as inherently unstable because large parts of its ice are grounded below sea level. According to the analysis, the iron contained in these mineral grains was in a less soluble form that cannot be easily utilised by living organisms. “Beneath the West Antarctic Ice Sheet there are probably sequences of geologically ancient and highly weathered rocks," explains co-author Dr Johann Klages, marine geologist at the AWI. Each time the ice sheet shrank during previous interglacial periods with numerous icebergs breaking off, they carried large amounts of weathered minerals into the adjacent South Pacific. “This correlation was very surprising for us." Dr Lester Lembke Jene, palaeoceanographer and co-author of the study, adds: "The total amount of iron input was not the controlling factor for algae growth in this sector of the Southern Ocean."
The results show that it cannot be generally assumed, as previously thought, that iron supply always increases the Southern Ocean CO2 uptake. It rather depends on the bioavailability of the iron and therefore also on the chemical composition of the transported minerals. The study thus makes an important contribution towards clarifying the question of how sensitive WAIS is to climatic changes. Several recent research findings indicate that the ice in this part of Antarctica retreated significantly during the last warm period around 130,000 years ago. At that time, temperatures were about as high as they are today. In the future, the WAIS could retreat further as a result of global warming, in which case scenarios similar to the last interglacial period become likely, according to the team.
“Based on what we know so far, WAIS is not likely to collapse in the near future, but we can see that the ice there is already thinning and retreating rapidly,” explains Johann Klages. A further retreat could result in increased iceberg release carrying these weathered rock into the Pacific sector of the Southern Ocean. This could in turn lead to the ocean absorbing less carbon dioxide than it does today – a feedback loop that may further intensify climate change. However, in order to be able to say more about the extent and significance of this phenomenon, more detailed geochemical investigations and further meaningful sediment records from the South Pacific are needed.
Original publication
Struve, T., Lamy, F., Gäng, F. et al. South Pacific carbon uptake controlled by West Antarctic Ice Sheet dynamics. Nat. Geosci. (2026). https://doi.org/10.1038/s41561-025-01911-0