The core contains sediments spanning roughly 500,000 years and four glacial cycles. It was recovered in 2001 from nearly 5,000 meters water depth at 116 degrees west and 62 degrees south, south of the Antarctic Polar Front between South America and New Zealand. This region forms part of the Pacific sector of the Southern Ocean, a key area for global ocean circulation and carbon exchange with the atmosphere.
In the Southern Ocean, iron is generally thought to act as a fertiliser that promotes phytoplankton growth and enhances the ocean's capacity to absorb carbon dioxide. Earlier work suggested that during glacial periods strong winds transported iron-rich dust from continents to the ocean surface, stimulating algae growth north of the Polar Front. That process likely helped draw down atmospheric CO2 and reinforced global cooling at the onset of ice ages.
Analyses of the new core show a different pattern south of the Polar Front. Here, iron input was highest during warmer intervals rather than during peak glacial conditions. Grain size and composition indicate that the material arrived mainly via icebergs rather than wind-blown dust, and its geochemical signature points to a source in West Antarctica, the sector of the continent west of the Antarctic Peninsula where much of the ice sheet rests on bedrock below sea level and is therefore considered unstable.
The findings support other lines of evidence that the West Antarctic Ice Sheet retreated on a large scale during the last interglacial period about 130,000 years ago, when global temperatures were similar to those of today. As the ice sheet, which is several kilometers thick in places, broke up, it produced large numbers of icebergs that scraped sediments from the underlying bedrock and transported them into the South Pacific. According to the core data, iceberg discharge was especially strong near the ends of glacial periods and at the peaks of interglacials.
However, the elevated iron supply during these warm phases did not translate into a corresponding surge in algae growth. "The growth of phytoplankton - microalgae found in the light-flooded upper layers of the ocean - was either not stimulated or only weakly stimulated. This led to a sharp reduction in CO2 absorption," explains Dr Frank Lamy, a palaeoclimatologist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research and co-author of the study. Laboratory analyses showed that the sediments were highly weathered and that the iron they contained was in a less soluble form that organisms can only use to a limited extent.
The researchers infer that beneath the West Antarctic Ice Sheet there is probably a layer of geologically ancient, strongly weathered rock. Whenever the ice sheet shrank during past interglacial periods and large numbers of icebergs calved, these ice masses carried weathered mineral grains into the adjacent South Pacific, yet phytoplankton productivity stayed relatively low. In this part of the Southern Ocean, total iron input was therefore not the controlling factor for algae growth; instead, the bioavailability and chemical form of the iron determined its fertilising effect.
This mechanism challenges the assumption that increased iron delivery to the Southern Ocean automatically enhances biological carbon uptake. The study shows that additional iron can fail to boost CO2 drawdown if it is delivered in a chemically unfavourable state, even when fluxes are high. In the Pacific sector south of the Antarctic Polar Front, high loads of poorly soluble iron from iceberg-borne sediments can coincide with reduced oceanic carbon uptake.
Looking ahead, the team warns that ongoing global warming and continued thinning of the West Antarctic Ice Sheet could create conditions similar to those of the last interglacial. Further retreat of the ice and intensified erosion of weathered bedrock by glaciers and icebergs may increase the supply of low-bioavailability iron to the Southern Ocean. This would likely weaken the regional carbon sink and act as a positive feedback that amplifies climate change rather than damping it.
To assess the strength and wider relevance of this feedback, Struve and colleagues call for more detailed work on additional sediment cores from across the South Pacific. Such records could clarify how broadly this process operates, how it varies over glacial-interglacial cycles and how it interacts with other factors such as ocean circulation changes, sea-ice dynamics and fluctuations in atmospheric dust transport.
Research Report:South Pacific carbon uptake controlled by West Antarctic Ice Sheet dynamics
Related Links
University of Oldenburg
Climate Science News - Modeling, Mitigation Adaptation
| Subscribe Free To Our Daily Newsletters |
| Subscribe Free To Our Daily Newsletters |
