Scientists testing a popular climate theory in Antarctica found that melting glaciers deliver far less iron to the ocean than previously believed.

Most of the iron feeding carbon-hungry algae actually comes from deep water and sediments, not from the ice itself.

A Climate Hope in the Southern Ocean

For years, researchers studying the Southern Ocean have pointed to one possible bright spot in the climate crisis. The idea, known as iron fertilization, suggested that as Antarctica warms and glaciers melt, iron locked inside the ice would be released into nearby waters. That iron would fuel blooms of microscopic algae, which absorb heat-trapping carbon dioxide as they grow.

There is just one issue. The evidence now suggests this climate benefit may be far smaller than believed.

In what scientists describe as the most precise measurement yet of iron flowing from an Antarctic glacier, a team from Rutgers University-New Brunswick found that meltwater from an ice shelf contributes much less iron to surrounding ocean waters than previously estimated.

The study, published today (February 26) in Communications Earth and Environment, raises new questions about where iron in the Southern Ocean actually comes from. The findings could affect how scientists project and model future climate change.

“It has been widely assumed that glacial melting underneath ice shelves contributes considerable bioavailable iron to these shelf waters, in a process of natural glacier-driven iron fertilization,” said Rob Sherrell, a professor in the Department of Marine and Coastal Sciences at the Rutgers School of Environmental and Biological Sciences and the study’s principal investigator.

Sherrell explained that the new data revise those assumptions. The amount of iron carried by meltwater is several times lower than earlier estimates. In addition, most of that iron appears to originate from a different type of meltwater than the kind produced directly by melting ice shelves.

Why Iron Matters in Antarctica

Although Antarctic waters are plunged into darkness for months each year, the Southern Ocean remains one of the planet’s most biologically productive marine regions. Phytoplankton thrive there and serve as a critical food source for krill, which in turn support penguins, seals, and whales. As phytoplankton grow, they pull large amounts of carbon dioxide from the atmosphere through photosynthesis, making the region the world’s largest oceanic sink for the climate-warming gas.

Much of what scientists previously understood about iron in the Southern Ocean came from simulations and computer models. Sherrell and colleagues from Rutgers and partner institutions in the United States and the United Kingdom decided to gather direct measurements instead.

In 2022, the team boarded the now-decommissioned U.S. icebreaker, the Nathaniel B. Palmer, and traveled to the Dotson Ice Shelf in the Amundsen Sea of West Antarctica. The Amundsen Sea accounts for most of the sea level rise driven by Antarctic melting. Their goal was to collect glacial meltwater directly at its source.

Inside the Ice Shelf Cavity

In the Amundsen Sea, meltwater forms beneath floating ice shelves, which extend from land-based glaciers into the ocean. The melting occurs mainly because relatively warm deep ocean water flows into cavities beneath the ice.

At the Dotson Ice Shelf, the researchers pinpointed where seawater enters one of these cavities and where it exits after mixing with meltwater. They collected samples at both locations.

Back in New Jersey, Venkatesh Chinni, a postdoctoral scholar and the study’s lead author, measured iron levels in the samples, examining both dissolved iron and iron contained in suspended particles. Collaborators Jessica Fitzsimmons and Janelle Steffen at Texas A&M University analyzed isotopic ratios to “fingerprint” the iron and determine its origin. Steffen conducted the initial isotopic measurements in the laboratory of Tim Conway at the University of South Florida.

Using these data, Chinni and the team calculated how much iron was added as water passed through the cavity. Isotopic signatures also helped them identify the type of melting responsible.

Most Iron Comes From Deep Water and Sediments

The results surprised the researchers. Meltwater accounted for only about 10% of the dissolved iron leaving the cavity. In contrast, inflowing deep ocean water supplied 62%, while 28% came from sediments on the continental shelf.