The iron hypothesis, proposed by oceanographer John Martin in the late 1980s, rests on a simple biological fact. In vast stretches of ocean known as High-Nutrient Low-Chlorophyll regions, which cover up to twenty percent of the ocean surface, phytoplankton starve for lack of iron even though nitrate and phosphate abound. These microscopic plants use iron as a co-factor in the enzymes of photosynthesis and nitrogen fixation, so when iron arrives, blooms follow. Martin argued that during cold, dusty glacial periods, windblown dust delivered extra iron to these waters, stimulating marine productivity and drawing down atmospheric carbon dioxide, which was eighty to one hundred parts per million lower during ice ages than today. Subsequent research confirmed the mechanism but steadily shrank its estimated contribution, with models attributing perhaps twenty parts per million of the glacial decline to iron-driven fertilization.

To test the hypothesis where it should have mattered most, the team turned to the EGRIP ice core, drilled within the Northeast Greenland Ice Stream about 360 kilometers from the Greenland Summit. The ice they analyzed spans the Pleistocene-Holocene transition, from 13,000 to 10,300 years before present, capturing the Younger Dryas cold event, the warmer Bølling-Allerød interval before it, and the Early Holocene after. Crucially, much of the dust that settled on Greenland during the Younger Dryas originated in the Gobi and Taklamakan deserts of Asia and the Sahara, and a large share of it traveled across the North Pacific before reaching the ice sheet. That geography means Greenland ice records changes in atmospheric iron deposition over the iron-starved Subarctic North Pacific, the very region where marine sediment cores show that productivity stubbornly refused to rise during dusty cold periods.

The technical challenge lies in measuring not just iron, but the right kind of iron. The researchers used the Bern Continuous Flow Analysis system, which melts ice at 2.8 centimeters per minute and channels the meltwater from the pristine inner core of each sample to an array of detectors. Two iron fractions emerged from this pipeline. The first, called Fe_ICP, was quantified by single-particle inductively coupled plasma time-of-flight mass spectrometry and captures most of the atmospheric iron pool, excluding only iron locked inside refractory silicate minerals. The second, dubbed labile iron or LFe, was measured by online acidification of the meltwater at a pH of roughly 1.6 followed by spectrophotometric detection. This mild acid treatment releases iron bound in colloids, hydroxides and organic complexes, the forms most easily grabbed by the siderophore molecules that phytoplankton deploy to scavenge iron from seawater. Labile iron is therefore an upper bound on the iron potentially available to marine life.

The contrast between the two fractions is the heart of the discovery. Median Fe_ICP concentrations surged seventeen-fold between the Early Holocene and the Younger Dryas, rising from 1.76 micrograms per liter to 30.16 micrograms per liter, a signal consistent with the known dustiness of the cold period. Yet labile iron climbed only twenty-nine percent, from a median of 0.51 to 0.66 micrograms per liter after excluding volcanic horizons, a statistically robust difference confirmed by a Wilcoxon rank sum test on more than 21,000 data points. As a result, the labile share of total iron collapsed from roughly forty percent during the Holocene to a mere two percent during the Younger Dryas. In other words, the ocean received vastly more dust, but almost none of the extra iron came in a form plankton could use.

The culprit, the researchers argue, was chemistry. Ice-core acidity measurements reveal that Greenland snow was mildly acidic during the Early Holocene, at a median of 1.06 micro-equivalents per liter, but turned distinctly alkaline during the Younger Dryas, at minus 4.93 micro-equivalents per liter, because carbonate-rich dust from the deserts neutralized the aerosol. Iron solubility is exquisitely sensitive to pH: theoretical calculations show that a single unit increase in pH can reduce dissolved iron(III), the thermodynamically stable form, by nearly two orders of magnitude. That swing matches the gap between the seventeen-fold rise in total iron and the modest rise in labile iron almost exactly. The Bølling-Allerød, still slightly alkaline, showed even lower labile iron concentrations than the Holocene, reinforcing the link between alkalinity and iron lock-up.

The hemispheric comparison makes the case even more striking. In Antarctic ice from EPICA Dome C, labile iron rose as much as twelve-fold between the Holocene and the Antarctic Cold Reversal, contributing up to sixty-four percent of the total iron content during that cold interval. Antarctica stayed acidic even during glacial times, likely because abundant biogenic sulfur dioxide emissions prevented complete neutralization of the aerosol, keeping iron dissolved and potentially bioavailable. This chemical divergence helps explain why sediment records show that iron fertilization boosted productivity in the Subantarctic zone of the Southern Ocean during ice ages, while the North Pacific, bathed in alkaline dust, saw no comparable response over the past 800,000 years.

If not iron, what did control North Pacific productivity during the last glacial transition? The study points to sea ice and ocean stratification. Sediment records from the eastern and western Subarctic Pacific and the Bering Sea indicate extensive spring sea-ice cover during the Last Glacial Maximum, precisely when iron fluxes peaked, and sea ice acts as a physical barrier that blocks both light and direct iron deposition. Productivity rose as perennial ice retreated, peaked during the largely ice-free Bølling-Allerød, and dipped again amid variable ice during the Younger Dryas. Meanwhile, nitrogen isotope measurements from foraminifera show that nitrate consumption was more complete during the Younger Dryas despite low productivity, a paradox explained by stronger stratification: cold, fresh surface waters suppressed vertical mixing, cutting off the upwelling of nutrient-rich deep water even as iron arrived from above.

There is one dramatic exception to the iron drought, and it comes from volcanoes. By identifying ten volcanic horizons through spikes in acidity and electrical conductivity, the team found that labile iron soared to as high as 17 micrograms per liter during eruptions, more than twenty-five times background levels. Acidic volcanic gases react with ash surfaces to create soluble iron salts such as iron chlorides, fluorides and sulfate hydrates, and ash also carries phosphorus, silica, zinc, manganese and copper. At least three interhemispheric eruptions, dated to 10,481, 12,917 and 13,028 years before present, could have triggered transient phytoplankton blooms in the North Pacific, though such blooms would last only a few years and leave no trace in sediment cores. The authors also note a modern echo: rising aerosol pH from changing industrial emissions may have locally altered iron availability in recent decades.

The study does carry caveats. Microbial activity in ice can reduce insoluble iron(III) to more soluble iron(II), and post-depositional mineral weathering can form jarosite and other poorly soluble secondary minerals, but the authors judge these effects limited and broadly uniform across the shallow, 150-meter depth interval examined, leaving the relative changes intact. Even so, the implications are significant for climate science and for any future proposals to fertilize the ocean deliberately. The iron hypothesis, the work suggests, is not a simple matter of dumping dust on the sea; whether iron nourishes plankton depends on the acidity of the atmosphere that carries it, the chemistry of the aerosol, and the physical state of the ocean below. During the Younger Dryas, the Northern Hemisphere failed that test on every count, and the North Pacific stayed hungry despite a seventeen-fold feast of dust.

Subject of Research: Atmospheric iron bioavailability and its role in North Pacific marine productivity during the Younger Dryas

Article Title: Limited atmospheric iron availability increase during the Younger Dryas in the Northern Hemisphere

Article References: Limited atmospheric iron availability increase during the Younger Dryas in the Northern Hemisphere. (n.d.). https://doi.org/10.5194/cp-22-1675-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1675-2026

Keywords: iron hypothesis, EGRIP ice core, Younger Dryas, North Pacific, labile iron, aerosol acidity, phytoplankton, HNLC regions, marine productivity, volcanic ash, sea ice, Climate of the Past