Scientists studying ancient ocean sediments have uncovered an unexpected climate feedback involving the melting of Antarctica’s ice and the Southern Ocean’s role in absorbing carbon dioxide. Contrary to long-standing assumptions, the loss of ice from West Antarctica during warmer periods did not stimulate marine algae growth, despite delivering large amounts of iron into the ocean.
The findings come from a new study published on February 2 in Nature Geoscience, led by researchers from the Columbia Climate School and international collaborators. The research focuses on how changes in the West Antarctic Ice Sheet, or WAIS, influenced biological activity in the Southern Ocean during past ice ages.
Ancient Sediments Reveal a Climate Surprise
To understand how Antarctic ice loss affected the ocean in the past, scientists analyzed a sediment core collected in 2001 from the Pacific sector of the Southern Ocean, at a depth of more than three miles below the sea surface. The core preserves a long record of environmental changes, including variations in iron supply and marine algae growth.
Iron is typically a key nutrient that fuels algae growth in Antarctic waters. As algae grow, they absorb carbon dioxide from the atmosphere, helping regulate Earth’s climate. However, the sediment record revealed a surprising disconnect. Even during periods when iron levels were high due to increased iceberg activity from West Antarctica algae productivity did not rise.
“Normally, an increased supply of iron in the Southern Ocean would stimulate algae growth and enhance the ocean’s uptake of carbon dioxide,” said lead author Torben Struve of the University of Oldenburg, who previously worked as a visiting postdoctoral researcher at Columbia University’s Lamont-Doherty Earth Observatory. “That’s not what we observed here.”
Why Iceberg Iron Failed to Feed Algae
The researchers traced the unexpected pattern to the chemical nature of the iron released by melting icebergs. Their analysis showed that much of the iron carried into the ocean was highly weathered, meaning it had undergone extensive chemical alteration over long periods beneath the ice sheet.
During earlier warm phases, when large portions of the West Antarctic Ice Sheet retreated and icebergs drifted northward, this weathered iron entered the ocean in a poorly soluble form. Because marine algae struggle to use this type of iron, the additional supply failed to boost biological productivity.
This finding challenges the idea that more iron automatically leads to more algae growth. “What matters is not just how much iron enters the ocean, but the chemical form it takes,” said co-author Gisela Winckler, a geochemist at the Columbia Climate School. “Iron delivered by icebergs can be far less bioavailable than previously assumed.”
Implications for Past and Future Climate Change
The study also sheds light on the sensitivity of the West Antarctic Ice Sheet to warming. Evidence from the sediment core suggests especially high iceberg activity during the end of glacial periods and peak interglacial conditions, including the last interglacial period around 130,000 years ago, when global temperatures were similar to today.
As the ice sheet broke apart, icebergs scraped sediment from ancient, weathered rock beneath the ice and released it into the Southern Ocean as they melted. Despite the influx of iron-rich material, algae growth remained limited.
“This reminds us that the ocean’s ability to absorb carbon isn’t fixed,” Winckler said.
Looking ahead, the researchers warn that continued thinning of the West Antarctic Ice Sheet due to modern climate warming could recreate similar conditions. While a complete collapse of the ice sheet is not expected in the near future, ongoing retreat could increase the delivery of poorly soluble iron to the ocean, reducing carbon uptake in parts of the Southern Ocean.
Such a feedback could weaken one of Earth’s natural climate buffers and potentially amplify future warming.