Emerging threatsSeeding iron on the Pacific’s floor may not pull carbon from air as thought

Scientists plumbing the depths of the central equatorial Pacific Ocean have found ancient sediments suggesting that one proposed way to mitigate climate warming — fertilizing the oceans with iron to produce more carbon-eating algae — may not necessarily work as envisioned.

Plants need trace amounts of iron to perform photosynthesis, but certain parts of the oceans lack it, and thus algae are scarce. Recent shipboard experiments have shown that when researchers dump iron particles into such areas, it can boost growth. The algae draw the greenhouse gas carbon dioxide from the air to help build their bodies, so fertilization on a large scale could, theoretically, reduce atmospheric CO2. Seafloor sediments show that during past ice ages, more iron-rich dust blew from chilly, barren landmasses into the oceans, apparently producing more algae in these areas and, presumably, a natural cooling effect. Some scientists believe that iron fertilization and a corresponding drop in CO2 is one reason why ice ages become icy and remain so.

Columbia U reports that the researchers in the study say that increased algae growth in one area can inhibit growth elsewhere. This is because ocean waters are always on the move, and algae also need other nutrients, such as nitrates and phosphates. Given heavy doses of iron, algae in one region may suck up all those other nutrients; by the time the water circulates elsewhere, it has little more to offer, and adding iron doesn’t do anything. The study appears today in the leading journal Nature .

“There’s only a limited amount of total nutrients in the oceans. So if there’s greater use in one area, it seems you’d have lesser concentrations in other areas,” said lead author Kassandra Costa, a doctoral student at Columbia University’s Lamont-Doherty Earth Observatory who led the analysis. “The basic message is, if you add to one place, you may subtract from another.”

Much of the equatorial Pacific’s near-surface water comes from the Southern Ocean. In the southerly latitudes, powerful winds circle Antarctica. This stirs things like a giant ladle, dredging large amounts of nitrates, phosphates and other nutrients from bottom waters where they tend to settle — so much so, that the nutrients can’t all be used up by resident algae. This makes the Southern Ocean an attractive place for potential artificial fertilization; experiments have shown that adding iron there does cause more algae to grow.