Melting West Antarctic Ice Fails to Boost Ocean Algae Growth

Scientists have found that ice melting from West Antarctica sends large amounts of iron into the Southern Ocean, but this iron does not help algae grow as much as expected. The iron arrives in a chemical form that marine life struggles to use, which could limit the ocean's ability to take in carbon dioxide and slow global warming. Researchers made this discovery by studying sediment cores and water samples from the region south of the Antarctic Polar Front.

Background

The Southern Ocean plays a big part in controlling Earth's climate by absorbing carbon dioxide from the air. Algae in these cold waters grow and pull in the gas, then sink to the deep sea, trapping it away for centuries. Iron acts as a key nutrient for this algae, much like fertilizer for plants on land. In the past, during cold glacial times, winds blew iron-rich dust from far-off continents into the ocean north of Antarctica, sparking algae blooms that helped cool the planet further.

But the area south of the Antarctic Polar Front, where West Antarctic ice meets the sea, works differently. Here, icebergs and glacier melt carry iron from the continent directly into the water. Over thousands of years, changes in the West Antarctic Ice Sheet have sent varying amounts of this iron seaward. During warm periods in the past, like interglacials, the ice sheet shrank, and more icebergs broke off, hauling sediments northward. Scientists expected this to fuel more algae growth and extra carbon uptake. Instead, records from the ocean floor tell another story.

In 2001, researchers pulled a sediment core from more than three miles below the surface in the Pacific sector of the Southern Ocean. This core holds layers of mud built up over 500,000 years, recording past iron levels, algae activity, and ice sheet behavior. The data shows iron inputs peaked right when the West Antarctic Ice Sheet was smallest, during warm spells. Yet algae growth stayed low, even with all that iron arriving.

Key Details

The team behind the study looked closely at the iron in those sediments. They found it came mostly from icebergs scraping up rocks from under the West Antarctic Ice Sheet. These rocks are old and heavily weathered, meaning chemical reactions over time have changed the iron into a tough form. This form, mostly iron(III), does not dissolve easily in seawater. Algae need iron(II), a more reactive version, to build their cells and grow.

Iron from Glaciers on the West Antarctic Peninsula

Further work on the West Antarctic Peninsula examined particles in meltwater from three bays: near King George Island, Anvers Island, and Adelaide Island. Glacier runoff there carries high loads of particles rich in iron(II), mixed with some iron(III). These particles also contain organic carbon, which seems to keep the iron(II) from quickly turning into the less useful iron(III) in oxygen-rich seawater.

Samples came from surface waters less than a meter deep, right where meltwater floats. At King George Island's Fourcade Glacier, a land-ending glacier, meltwater made up about 5.8 percent of the surface layer on average. Particle aluminum levels, a sign of land input, were three to seven times higher there than farther south. Iron and manganese particles followed the same pattern.

X-ray microscopy on 61 individual particles, each 0.2 to 2.7 micrometers across, confirmed many were rich in iron(II) and coated with carbon-rich material. This suggests glaciers export a pool of iron that could feed algae, at least near shore. But as these particles spread offshore, their usability drops.

Annual meltwater from these sites totals billions of tons: 2 gigatons from King George Island, 8 from Anvers, and 10 from Adelaide. With about 600 glaciers retreating along the peninsula, this flow is growing. Surface water data from summer 2015 across the Palmer Long-Term Ecological Research grid showed these iron particles reaching farther out to sea.

"What matters here is not just how much iron enters the ocean, but the chemical form it takes," said Torben Struve, lead author of one study and a researcher at the University of Oldenburg. "These results show that iron delivered by icebergs can be far less bioavailable than previously assumed."

What This Means

If more West Antarctic ice melts as the planet warms, the ocean might absorb less carbon dioxide, not more. Past warm periods set up a pattern: shrinking ice led to high iron delivery but no algae boom. Today, the ice sheet is thinning, though not collapsing soon. Models predict faster retreat, sending more weathered iron into the sea.

This iron stays locked in particles that algae ignore. Without bigger algae blooms, the Southern Ocean's carbon pump weakens. That pump relies on algae sinking carbon deep, where it stays out of the air. Less effective pumping means more carbon dioxide lingers in the atmosphere, speeding warming.

Other factors add complexity. Mountains poking through thinning ice, called nunataks, weather into iron-rich sediments. Sun warms their dark surfaces above freezing in summer, creating bioavailable iron. Rock falls could feed glaciers more material. But the bulk from the ice sheet base dominates, and its iron proves stubborn.

Sea ice changes nearby also matter. Its decline ties to saltier surface waters, letting warmer deep water rise and melt ice faster. Less sea ice means less fresh water spread, closing a loop that worsens melt. Iron from all sources will interact with these shifts.

Researchers now track how far glacier iron travels and how carbon coatings hold up offshore. Ongoing sampling in glacial bays and across shelves will map real-time changes. As melt ramps up, this iron flux grows key to the global iron cycle and ocean life.

The West Antarctic Peninsula already sees meltwaters lingering on the shelf through summer, carrying particles into open ocean. Elevated iron shows up far from coastlines. With glaciers pulling back, coastal Southern Ocean productivity could shift, rippling to food webs and carbon storage.

This work highlights how ice loss brings surprises. More iron does not always mean more algae or carbon drawdown. Understanding the iron's true role helps predict the ocean's response to warming.