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Melting icebergs fertilize ocean

Efforts to remove climate-warming carbon dioxide from Earth’s atmosphere appear to be getting a helping hand from a surprising source: the iron in meltwater from Antarctic icebergs.

Icebergs calving off of Antarctica are shedding substantial iron — the equivalent of a growth-boosting vitamin — into waters starved of the mineral, a new set of studies demonstrates. This iron is fertilizing the growth of microscopic plants and algae, transforming the waters adjacent to ice floes into teeming communities of everything from tiny shrimplike krill to fish, birds and sometimes mammals.

To grow, these plants and animals use carbon drawn into the water from carbon dioxide in the atmosphere. Some share of this carbon will eventually be excreted as wastes that fall to the ocean floor, essentially removing it as a near-term climate risk.

“Icebergs should be considered by climate modelers, because the more icebergs that develop [from the breakup of glaciers], the more carbon dioxide you’ll draw out of the atmosphere,” says Ken Smith of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif.

Smith and colleagues first fingered icebergs as hotspots of biological and chemical activity in a 2007 study published in Science. New data from Antarctic cruises in 2008 and 2009 by Smith and other scientists from nine research institutions now appear as 20 papers in the June Deep Sea Research Part II

Researchers refer to icebergs’ carbon removal as an export. “And the amount of carbon being exported near icebergs is twice as high as in areas away from them,” Smith says.

Counterbalancing icebergs’ carbon removal: No one views the sea-level rise accompanying massive ice melting as a good thing. The rate of iceberg calving — and ice loss — in recent years has increased there, as elsewhere, in response to warming of Earth’s atmosphere.

Prior to the new studies, “we didn’t know the nature of the biological communities associated with icebergs and we certainly didn’t know their direct relationship to carbon exports,” says chemist Timothy Shaw of the University of South Carolina in Columbia, who coauthored several of the new reports.

One surprise: The proliferation of phytoplankton — tiny plants at the base of the marine food chain — that were witnessed in the waters around ice floes “could only account for about half of the increased carbon export we measured,” Shaw says. His team now attributes the other half to changes in the chemistry of iron and carbon use by phytoplankton living next to and under the icebergs.

Benjamin Twining of the Bigelow Laboratory for Ocean Sciences in West Boothbay Harbor, Maine, points to another big surprise: Icebergs’ iron enrichment of southern waters could vary by a factor of 100 from one iceberg to another, or even along walls of a given berg. This patchy enrichment reflects differences in chemical reactions triggered by various organisms and to the unexpectedly complicated turbulence associated with water melting from the floes.

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