A groundbreaking study analyzing sediment cores from the Pacific sector of the Southern Ocean has unveiled unexpected insights into the complex climate feedback mechanisms involving the West Antarctic Ice Sheet (WAIS). Led by Dr. Torben Struve of the University of Oldenburg, the research, published in Nature Geoscience, challenges long-standing assumptions about the interplay between iron fertilization, marine primary productivity, and carbon dioxide uptake in this crucial region of the global climate system.
The study focused on a sediment core extracted in 2001 from nearly 5,000 meters depth, positioned at 116 degrees west and 62 degrees south, nestled south of the Antarctic Polar Front between South America and New Zealand. This sediment archive provides a pristine record covering four glacial cycles, spanning approximately half a million years, making it invaluable for reconstructing past climate-ice-ocean interactions in one of Earth’s most sensitive environments.
Central to the research is iron (Fe), an element widely regarded as a limiting nutrient that stimulates phytoplankton blooms in the ocean. Conventionally, increased iron supply to Southern Ocean waters, often supplied by dust during glacial periods, has been linked to intensified biological productivity and enhanced carbon sequestration via the biological pump. This mechanism has been thought to amplify global cooling during ice ages by facilitating higher atmospheric CO₂ drawdown.
However, the team’s analysis of the Southern Ocean south of the Antarctic Polar Front reveals an anomalous pattern: iron concentrations peaked during warmer interglacial intervals rather than the colder glacial phases. Intriguingly, this iron source was not predominantly aeolian dust, as previously emphasized in Antarctic nutrient studies, but rather sediment-rich debris released from melting icebergs generated by the disintegration of the West Antarctic Ice Sheet. The mineral grains, embedded in the icebergs, were abraded from the subglacial bedrock beneath WAIS, reflecting the dynamic interactions between ice sheet retreat and ocean biogeochemistry.
The West Antarctic Ice Sheet is known for its unique vulnerability due to extensive grounding below sea level, making it prone to rapid disintegration during warming phases. Geological evidence, bolstered by this study, indicates a substantial retreat of the WAIS about 130,000 years ago during the last interglacial period, at temperature levels comparable to today’s warming trend. This massive ice loss released vast quantities of iron-laden sediment via drifting icebergs, profoundly influencing nutrient supply dynamics in the adjacent Southern Ocean sector.
Unexpectedly, despite the increase in iron supply from these icebergs, the researchers documented only weak or no stimulation of phytoplankton growth, contradicting classical fertilization paradigms. Dr. Frank Lamy from the Alfred Wegener Institute highlights that this diminished biological response led to a paradoxical reduction in CO₂ uptake—a critical feedback weakening the ocean’s role as a carbon sink during warm intervals.
This counterintuitive effect arises from the geochemical nature of the transported sediment. Detailed mineralogical and chemical analyses revealed that the iron within these weathered grains was predominantly in less soluble forms, severely limiting its bioavailability to marine microorganisms. Unlike freshly supplied, bioavailable iron in dust particles, the weathered sediments carried by icebergs failed to effectively fertilize phytoplankton communities, decoupling iron input from carbon drawdown capacity.
These findings fundamentally alter previous assumptions regarding the Southern Ocean carbon cycle. The study suggests that in this region, total iron input alone does not control marine productivity or carbon sequestration. Instead, the bioavailability of iron, governed by mineralogical composition and chemical weathering state, is the decisive factor shaping phytoplankton responses and thus the efficiency of the biological carbon pump.
Dr. Struve emphasizes the importance of subglacial geology in mediating this feedback: beneath the WAIS lies a layer of ancient, highly weathered bedrock that supplies iron-poor mineral material during ice sheet melting episodes. As the ice sheet thins and calves icebergs, these sediments are transported to ocean waters where biological uptake is suppressed despite elevated iron concentrations.
Looking toward the future, the consequences of continued WAIS shrinkage amidst anthropogenic warming are alarming. The past interglacial analogue suggests a risk of diminished carbon uptake in the South Pacific sector of the Southern Ocean, potentially exacerbating atmospheric CO₂ accumulation and climate warming. This negative feedback loop underscores the complexity of ice-ocean-atmosphere interactions and the challenges in predicting ice sheet contributions to global climate trajectories.
While the ice sheet is not expected to collapse imminently, ongoing observations document substantial thinning and retreat. The study advocates for intensified research efforts focusing on sediment core analyses across multiple locations in the Southern Ocean to refine understanding of these feedbacks. Advanced geochemical profiling and sediment provenance studies will be vital to elucidate the extent and timing of iron bioavailability variations and their ecological impacts.
Overall, this research redefines the narrative around Southern Ocean iron fertilization and carbon cycling, challenging oversimplified models and highlighting the nuanced interdependencies among ice sheet dynamics, sediment transport, and marine ecosystems. It provides a critical foundation for integrating geological and biogeochemical perspectives to improve predictions of future climate-carbon feedbacks in one of Earth’s most climatically sensitive regions.
Subject of Research: Not applicable
Article Title: South Pacific carbon uptake controlled by West Antarctic Ice Sheet dynamics
News Publication Date: 2-Feb-2026
Web References: DOI: 10.1038/s41561-025-01911-0
Image Credits: Johann P. Klages / Alfred Wegener Institut
Keywords: West Antarctic Ice Sheet, Southern Ocean, iron fertilization, climate feedback, carbon uptake, phytoplankton, sediment core, icebergs, interglacial period, bioavailability, geochemistry, global warming
