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The Southern Ocean plays a pivotal role in mitigating atmospheric carbon, absorbing roughly 40 % of human‑made emissions. This capacity is largely driven by phytoplankton blooms that sequester CO₂. Yet the growth of these blooms in the cold Southern Ocean is constrained by iron scarcity. A recent study, however, found that adding iron did not boost phytoplankton as anticipated. The reason? The iron was derived from melting ice and is poorly soluble.
Iron normally enters the ocean through wind‑blown sediment from land during glacial periods. In the northern Southern Ocean, this wind‑transported iron readily fuels phytoplankton. In contrast, the study’s southern transect revealed that the bulk of iron originates from the West Antarctic Ice Sheet. This iron is chemically “mature” and much less bioavailable. The authors suggest it comes from ancient subglacial bedrock, effectively delivering exhausted iron that is difficult for algae to use. This finding challenges the assumption that simply increasing iron supply will enhance carbon uptake.
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The implications extend to climate change. During the last interglacial, about 100,000 years ago, retreat of West Antarctica supplied up to 90 % of the lithic input to the studied Southern Ocean sector. Because temperature, ice volume, and sea level during that interval mirror today’s conditions, this period is often used as a proxy for the present climate.
Consequently, the study suggests that in warmer climates, iron from the West Antarctic Ice Sheet will dominate the Southern Ocean’s nutrient budget. Torben Struve, the lead author, notes that the ice sheet is already thinning, which could accelerate subglacial rock erosion and release even more insoluble iron. The result would be a decline in phytoplankton growth, reduced atmospheric CO₂ uptake, and a potential amplification of climate warming. The unexpected twist highlighted by this research underscores how difficult it may be to predict the full climate feedbacks of ongoing warming.
