Antarctic Glaciers Deliver Far Less Climate-Buffering Iron Than Models Have Assumed

For scientists studying the Southern Ocean, one finding had offered a degree of measured reassurance in an otherwise gloomy climate forecast. As Antarctica's glaciers and ice shelves melt in response to warming temperatures, they release iron that has been locked in ice for centuries. That iron fertilizes blooms of phytoplankton - microscopic algae that absorb carbon dioxide as they grow. More melting, the theory went, means more iron, more phytoplankton, more carbon drawn down from the atmosphere. A natural feedback that softens its own cause.

There is just one problem with this theory: the iron numbers don't hold up.

A study published in Communications Earth and Environment by marine scientists from Rutgers University-New Brunswick reports the most precise measurement of iron inputs from an Antarctic glacier yet undertaken, and the results are substantially lower than the estimates embedded in climate models. The finding does not simply refine existing predictions - it challenges the premise that glacial meltwater is a meaningful driver of Southern Ocean iron fertilization.

Measuring what was assumed

Previous estimates of iron inputs from Antarctic ice shelves relied heavily on indirect methods and modeling assumptions rather than direct measurement. The concentrations of dissolved and particulate iron in subglacial meltwater, and the fraction of that iron that phytoplankton can actually use - the bioavailable fraction - were not well-constrained by direct observation.

The Rutgers team, led by Professor Rob Sherrell from the Department of Marine and Coastal Sciences, conducted a detailed field campaign to measure iron concentrations directly in and around the meltwater discharged from an Antarctic ice shelf. They characterized both total iron and bioavailable iron fractions, using methods designed to minimize contamination and capture the actual chemical forms of iron present in the water.

The measurements showed that the meltwater contributes far less bioavailable iron than widely assumed. Sherrell stated the finding plainly: it had been widely assumed that glacial melting underneath ice shelves contributes considerable bioavailable iron to shelf waters, in a process of natural glacier-driven iron fertilization. The study modifies those assumptions fundamentally.

Why bioavailability matters

Not all iron in seawater is equal from a biological standpoint. Iron in forms that phytoplankton can absorb and use is bioavailable. Iron bound tightly to particles or in chemical forms that cells cannot access is not, regardless of how much of it is present in the water. The distinction is crucial because climate models that calculate the fertilization effect of glacial meltwater need to use bioavailable iron concentrations, not total iron concentrations.

If previous estimates used total iron or overestimated the bioavailable fraction, the predicted fertilization effect would be systematically too high. The Rutgers measurements suggest that is precisely what happened - the bioavailable iron in glacial meltwater is considerably lower than models have been assuming.

Implications for climate projections

Climate models that include Southern Ocean biological carbon uptake as a component of future carbon cycle projections will need to be revised in light of these measurements. The magnitude of the revision depends on how widely the findings apply - whether the ice shelf studied is representative of Antarctic glacial systems broadly, or whether it is atypical in ways that limit generalization.

Sherrell acknowledges that these findings raise questions about the sources of iron in the Southern Ocean near Antarctica more broadly. If glacial meltwater is not the dominant source that models assumed, other iron sources - atmospheric dust deposition, sediment resuspension, upwelling from deeper waters - must be contributing more than currently recognized. Understanding the actual iron budget of the Southern Ocean is now a more open question than it appeared to be.

What this does not mean

The study's conclusion is specific: meltwater from the measured ice shelf contributes less bioavailable iron than assumed. It does not demonstrate that phytoplankton blooms near melting Antarctic ice shelves do not occur, or that the Southern Ocean has no capacity to absorb carbon through biological processes. It narrows the estimated contribution of one specific mechanism - glacial meltwater iron fertilization - to the ocean's carbon uptake.

Climate change projections involve many interacting processes, and adjusting one input does not straightforwardly rewrite the entire forecast. But models that have been using inflated iron inputs from glacial meltwater have been overestimating one component of the Southern Ocean's carbon sink, and correcting that overestimate generally means revising carbon uptake projections downward. In climate modeling, that direction of error consistently worsens the projected trajectory.