Bold claim: the long-held idea that melting Antarctic glaciers could fertilize the Southern Ocean with iron—and thus spur phytoplankton blooms to help curb climate warming—faces serious scrutiny. For years, scientists studying this region saw glacier-derived iron as a potential silver bullet in the climate fight. As ice shelves melt with rising temperatures, iron trapped in ice was thought to be released into nearby waters, fueling tiny algae that would draw carbon dioxide from the atmosphere as they thrived.
New findings, however, cast doubt on that mechanism. In what researchers call the most precise measurement yet of iron input from an Antarctic glacier, a team from Rutgers University–New Brunswick showed that meltwater from an Antarctic ice shelf delivers far less iron to surrounding waters than previously believed. The study, published in Communications Earth and Environment, questions the strength and sources of iron inputs in the Southern Ocean near Antarctica and suggests that climate-model projections based on glacier-driven iron fertilization may need revision.
Lead investigator Rob Sherrell, a professor in Rutgers’ Department of Marine and Coastal Sciences, explains that the work shifts the prevailing assumption. The data indicate that the iron content in meltwater is several times lower than earlier estimates, and that most of the iron that does reach the ocean originates from a different kind of meltwater than the ice-shelf meltwater under study.
The Antarctic Southern Ocean remains astonishingly productive despite months of darkness. Its phytoplankton form the base of the food web, supporting krill, penguins, seals, and whales. Phytoplankton blooms also act as a major sink for carbon dioxide, helping to mitigate climate warming on a global scale.
Historically, iron sources in this region were modeled and simulated rather than measured directly. Sherrell and collaborators from Rutgers and several other institutions took a different route. In 2022, they sailed aboard the decommissioned U.S. icebreaker Nathaniel B. Palmer to the Dotson Ice Shelf in West Antarctica’s Amundsen Sea—the area linked to a large portion of sea-level rise. Their mission was to sample glacial meltwater right at its source.
In the Amundsen Sea, meltwater emerges from beneath floating ice shelves—the seaward extensions of glaciers—and its release is driven largely by warm deep-ocean water intruding into cavities beneath the ice. The team pinpointed where seawater enters a subglacial cavity and where it exits after meltwater mixes in. They collected samples at both entry and exit points.
Back in New Jersey, postdoctoral researcher and lead author Venkatesh Chinni analyzed dissolved iron and iron within suspended particles. Collaborators Jessica Fitzsimmons and Janelle Steffen from Texas A&M University measured isotopic ratios to help trace iron sources, with initial isotopic work conducted in the lab of Tim Conway at the University of South Florida.
By comparing entry and exit iron, and leveraging isotopic fingerprints, the researchers inferred the type of melting at work. The results surprised the team: only about 10% of the dissolved iron in the outflowing water came from meltwater itself. The lion’s share—about 62%—came from inflowing deep water, and roughly 28% from sediments on the shelf.
To put it plainly: about 90% of the iron exiting the ice shelf cavity originates from outside the cavity—deep waters and seabed sediments—not from the meltwater alone. Additionally, isotopic signals point to a liquid, low-oxygen meltwater layer beneath the glacier that facilitates the dissolution of solid iron oxides from bedrock, potentially offering a larger iron source than ice-shelf meltwater alone.
Taken together, these findings challenge the conventional view of where Southern Ocean iron comes from in a warming world. The researchers stress that more work is needed to understand the subglacial processes at play, but the central claim is clear: meltwater itself carries very little iron, and most of the iron it does carry appears to come from the erosion and dissolution of bedrock beneath the glacier rather than from the melting ice driving sea-level rise.
Sherrell notes that many colleagues will find this conclusion surprising, underscoring how the climate narrative around iron fertilization may have overestimated the role of glacial meltwater. The study thus invites renewed discussion about how iron enters the Southern Ocean and how these dynamics should be represented in climate predictions.
Source note: This material originated from the researchers and their institutions and is presented here in a summarized, edited form for clarity and readability. For the full context, see the original publication and accompanying press materials.
