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

Much of this nutrient-rich water eventually sinks and circulates below the surface to the mid-Pacific; the journey takes a century or two. At the equator, the southern water meets with opposing currents from the north and rises, making the nutrients available to near-surface algae. But most of these nutrients pass on by; the mid-Pacific is too far from iron-rich dust sources on land for algae to make much use of them.

In 2012, Lamont scientists conducted a research cruise in this remote region, and took cores from the seabed. Costa and her colleagues analyzed sediments from the cores dating to the last ice age, some 17,000 to 26,000 years ago. As expected, they found two or three times more dust reaching the area compared to today, due to reduced plant cover in the cold, dry climate. Marine plant growth might have been expected to have increased accordingly, but it didn’t. The sediments showed that productivity stayed the same, or even declined. Their conclusion: algae in the southerly latitudes, which also got dusted at the same time, snapped up the iron — along with most of the other nutrients. That left the Pacific algae high and dry.

“This shows how different parts of the system are connected,” said Lamont marine geochemist Jerry McManus, a coauthor on the paper. “If you push hard in one place, the system pushes back somewhere else.” This undercuts the idea that iron fertilization could be a major force in spurring and maintaining ice ages. He said. “That doesn’t mean it’s not an influence, but the global system may be self-regulating and [that] reduces the potential impact of fertilization,” he said. The study does not say so, but McManus adds that it also suggests “we should be very careful about thinking we can use artificial fertilization to combat climate change.”

Recently researchers have done a series of artificial iron fertilization experiments. These have ranged from a 2012 privately sponsored seeding off British Columbia said to have produced a 10,000-square-mile algae bloom, to a similar 2009 German cruise in the southwest Atlantic. Some earlier projects were done in the southern ocean—the place “where you get the most bang for your buck,” said McManus. Because iron is needed only in trace quantities, researchers have calculated that in some areas, each kilogram added could produce 100,000 kilograms of algae, at least locally.

The most recent experiments have sparked protests from environmental groups. Some scientists say iron dumping could alter marine ecosystems in unpredictable and possibly harmful ways. Another problem: for excess carbon to be truly locked away, it must sink to the seafloor when the algae die. Some studies have suggested that while algae may grow quickly when fertilized, much of the carbon they take up remains near the surface for hundreds of years, cycling through other marine creatures, or bleeding directly back into the air.

Phoebe Lam, an oceanographer at the University of California, Santa Cruz, who studies marine cycling of iron and carbon, said the paper “shows there are downstream consequences to anything you do in the ocean. It’s what the geoengineers don’t necessarily think of. It makes the idea of artificial iron fertilization require a discussion of much more subtlety.”

Sylvain Pichat, a marine geochemist at the University of Lyon, said the study “indeed shows that we need to think about the oceans and the climate system as a whole.”

Columbia U earlier this month, British researchers published a study showing how much still remains to be discovered: they observed that big icebergs increasingly calving off Antarctica are releasing vast trails of iron as they melt, triggering algae blooms for hundreds of miles — a possible mechanism that one could speculate might eventually push back against the manmade forces implicated in the calving.

— Read more in K. M. Costa et al., “No iron fertilization in the equatorial Pacific Ocean during the last ice age,” Nature 529 (27 January 2016): 519-22 (DOI: 10.1038/nature16453); and Jeff Tollefson, “Ocean-fertilization project off Canada sparks furore,” Nature (23 October 2012) (DOI: 10.1038/490458a)