Sampling rosette with gray sampling bottles at left, the ship’s rail at lower right, and the face of the ice shelf in the background. (Credit: Robert Sherrell)
In A Nutshell
- Scientists assumed melting Antarctic glaciers were a major iron source feeding ocean life in the Southern Ocean. New measurements show meltwater contributes only about 10 percent of that iron.
- The bulk of the iron comes from deep ocean currents and seafloor sediments, not the ice itself. Meltwater’s real role is acting as a buoyancy pump that lifts that iron toward the surface.
- Microbes living in ancient, oxygen-starved water trapped beneath the glacier appear to be the dominant source of whatever iron meltwater does contribute, a finding that surprised the research team.
- Climate models that project how Southern Ocean ecosystems will respond to accelerating Antarctic melt have been working from iron estimates that are off by a factor of ten or more in some cases, and thousands in others.
For decades, climate models used to predict the future of Earth’s most important carbon sink have been built on assumptions that now appear to overestimate meltwater’s iron contribution. A new study has found that the assumed iron contribution from Antarctic glacial meltwater, baked into simulations used by researchers worldwide, may be off by a factor of ten or more. In at least one widely cited model, the number was off by a factor of thousands.
Iron is to ocean life what sunlight is to a garden. Without it, phytoplankton, the microscopic plants at the base of marine food webs, cannot grow. In the Southern Ocean, iron is so scarce that even small amounts can trigger massive blooms of these organisms, which pull carbon dioxide out of the atmosphere as they grow. Scientists describe the Southern Ocean as the largest oceanic sink for atmospheric CO2 on Earth. For years, climate models assumed that glacial meltwater pouring off West Antarctica delivered much of the iron sustaining that system. A new study published in Communications Earth & Environment found that meltwater accounted for just about 10 percent of the dissolved iron leaving one of Antarctica’s most studied ice shelves in 2022.
So where is the iron actually coming from? Mostly from deep, warm ocean currents already loaded with iron long before they touched the ice, plus contributions from seafloor sediments those currents drag across on the way in. The distinction has direct consequences for how scientists model the future of Southern Ocean productivity.
How Researchers Traced Antarctic Ice Shelf Iron
Scientists from Rutgers University, Texas A&M, and several other institutions sailed to the front of the Dotson Ice Shelf in West Antarctica’s Amundsen Sea in January 2022, as part of an expedition called ARTEMIS. Using acoustic instruments to map underwater currents flowing in and out of the cavity beneath the ice shelf, they collected water samples directly from those current cores.
That precision mattered. Warm, salty water flows along the seafloor into the cavity and drives melting. A mixture of that deep water and glacial meltwater then rises and flows back out. By measuring iron concentrations and chemical tracers in both the inflowing and outflowing water, the team could calculate how much iron the cavity was adding and where it came from.
Meltwater from the ice shelf accounted for only about 10 percent of the dissolved iron in the outflowing water. Deep ocean water contributed 62 percent, and iron dissolving out of seafloor sediments accounted for the remaining 28 percent. Those numbers contradict what many models have assumed, and the gap between model assumptions and measured reality has direct consequences for any projection that treats meltwater as a primary iron delivery mechanism.
The Iron Is Coming From Beneath Antarctica’s Glaciers
Perhaps the most unexpected finding involved where the small meltwater-derived iron fraction was actually originating. Different processes leave different chemical fingerprints on iron, and those fingerprints can reveal where the iron has been. Ice shelf meltwater, produced when warm seawater melts the underside of the ice, should leave a fingerprint close to zero, matching ordinary rock.
What the researchers found at the outflow was a consistently negative fingerprint, one that appeared across multiple measurements over four days and matched readings from a previous expedition in 2011. That signature points to iron that formed in an oxygen-starved environment, driven by microbial activity deep underground where no oxygen can reach. Ice shelf cavities are not oxygen-starved, so the team traced the signal to a different source: the hidden network of water flowing beneath the grounded glacier on the continent itself, kilometers inland.
Beneath slow-moving, thick ice, water can sit trapped for extraordinarily long periods. Microbes living in that darkness slowly break down iron minerals in the surrounding rock, releasing a highly concentrated form of dissolved iron bearing that telltale negative fingerprint. Even though this deep subglacial seepage makes up only about half a percent of total meltwater volume, the calculations showed it was sufficient to dominate the meltwater-derived iron budget.
Similar fingerprints have been found at Blood Falls, a striking iron-rich seep from Taylor Glacier in Antarctica, and in subglacial streams draining the Greenland ice sheet. The Dotson findings suggest this underground iron pathway may be far more widespread beneath the Antarctic continent than anyone has mapped.
Antarctic Ice Shelves Are Exporting Far More Particulate Iron Than Expected
Beyond dissolved iron, the outflowing water carried far more iron locked up in tiny suspended particles than the water flowing in. Total particulate iron at the outflow was about 46 percent higher than at the inflow, and a quarter of those particles were in a form that could potentially dissolve into usable iron over the weeks and months of a phytoplankton bloom.
That particulate iron pool showed up at roughly 100 times the concentration of dissolved iron in the same water. Experiments have shown that particles from glacial sediments can support phytoplankton growth, meaning this overlooked iron reservoir could rival or exceed dissolved iron in its contribution to ocean productivity. The enrichment was consistent between 2022 and 2011 measurements, suggesting the cavity reliably generates this material year after year.
Why Antarctic Meltwater Iron Models Need to Change
West Antarctic ice shelves are melting faster than they were a generation ago. Models predicting how that melt will affect Southern Ocean ecosystems have been built partly on the assumption that more meltwater means more iron delivered to the surface. The Dotson data tell a different story.
Meltwater does not bring iron so much as it lifts iron. By mixing with iron-rich deep water and making it buoyant, meltwater acts as an underwater elevator, carrying dissolved iron from the seafloor toward the surface where phytoplankton can reach it. As melt rates increase, that elevator may run faster and carry more, potentially boosting iron supply to the surface ocean. But the mechanism is indirect, and models that skip that step are projecting ecosystem futures on faulty math.
As the authors put it, future models must account for deep water composition, sediment chemistry, and subglacial water flow, “while acknowledging that ice shelf melting within cavities is a relatively minor source of dissolved Fe.” For a system that buffers the entire planet against rising CO2, that is not a minor correction.
Research Disclaimer: This article is based on a single study conducted at one Antarctic ice shelf during one field season. While the findings are peer-reviewed and consistent with data from a previous expedition, they may not apply uniformly to other ice shelves or regions. Science is an ongoing process, and future research may refine or expand these conclusions.
Paper Notes
Limitations
The study focuses on a single ice shelf, the Dotson Ice Shelf in the Amundsen Sea, and the authors note that iron sources and cavity dynamics will vary among different ice shelves depending on geometry, deep water sources, and subglacial conditions. Sampling took place over a single austral summer in January 2022, though comparisons with 2011 data suggest the general pattern holds over at least a decade. Direct sampling inside the ice shelf cavity has not yet been done, so iron estimates rely on measurements taken at the cavity entrance and exit. The argument for subglacial iron dominance involves assumptions about concentrations and transport behavior that carry some uncertainty. Expeditions to other Antarctic ice shelves would be needed to determine how broadly the findings apply.
Funding and Disclosures
This research was supported by the National Science Foundation under awards NSF-GEO-NERC 1941304, 1941308, 1941327, 1941292, and 1941483, as well as NSF-OCE-2123354. Additional support came from NASA ROSES award 80NSSC21K0746, a Graduate Merit Fellowship from Texas A&M University, and a Texas Sea Grant award. The authors declare no competing interests.
Publication Details
The study was authored by Venkatesh Chinni, Janelle M. Steffen, Sharon E. Stammerjohn, Pierre St-Laurent, Lisa C. Herbert, Patricia L. Yager, Tim M. Conway, Jessica N. Fitzsimmons, and Robert M. Sherrell, representing institutions including Rutgers University, Texas A&M University, the University of Colorado Boulder, Virginia Institute of Marine Science, Florida State University, the University of Georgia, and the University of South Florida. It was published in Communications Earth & Environment, a Nature Portfolio journal, on February 26, 2026, as Volume 7, Article 162. The title is “Iron supply to the Amundsen Sea, Antarctica is dominated by circumpolar deepwater and continental subglacial sources.” DOI: https://doi.org/10.1038/s43247-026-03264-x
