Large Antarctic icebergs leave huge plumes of life behind them as they voyage through the Southern Ocean. It has previously been suspected that icebergs fertilize nutrient-poor stretches of water, but research to measure the effect finds it is much larger than expected. Increased iceberg calving may play a role, albeit modest, in drawing carbon dioxide from the atmosphere.
Professor Grant Bigg of the University of Sheffield tested the influence of icebergs in the Weddell Sea by analyzing 175 satellite color images of the ocean. When phytoplankton blooms, the Southern Ocean becomes a greener shade of blue.
Bigg studied the influence of 17 icebergs, each at least 18 kilometers (11 miles) long. He reported in Nature Geoscience that not only were there huge blooms in these giant ice islands' wake, but the extra productivity lasted at least a month and could stretch for hundreds of kilometers.
"We detected substantially enhanced chlorophyll levels, typically over a radius of at least 4-10 times the iceberg's length,” Bigg said in a statement. "The evidence suggests that assuming carbon export increases by a factor of 5-10 over the area of influence, up to a fifth of the Southern Ocean's downward carbon flux originates with giant iceberg fertilization."
The Southern Ocean is thought to account for around 10 percent of the carbon dioxide removed from the atmosphere by oceans, but its contribution is limited by a shortage of nutrients, particularly iron. Dust blown off continents resupplies other oceans with a regular source of iron, but the layer of ice on top of Antarctica means the Southern Ocean has no such source.
Glaciers scrape the Antarctic rocks on their way to the sea, and the resulting icebergs carry iron and other nutrients with them. Nevertheless, previous estimates based on smaller icebergs produced much lower estimations of the total effect. “Large icebergs have a bigger impact, relative to their size, than small ones,” Bigg told IFLScience.
“The one previous remote sensing analysis of small icebergs suggests the range of influence in terms of phytoplankton growth is of the order of the size of the iceberg, but we found that for giant icebergs the range of influence is 4-10 times the size of the iceberg.” Bigg attributes this to “the greater volume of meltwater flux spreading sufficient iron over a larger area.”
Most of the carbon drawn from the atmosphere by phytoplankton escapes again fairly rapidly, but some sinks to the seafloor to be stored for geologic ages.
Bigg, who literally wrote the book on icebergs and their interaction with climate change, added: “If giant iceberg calving increases this century as expected, this negative feedback on the carbon cycle may become more important than we previously thought." However, he told IFLScience, “The area of influence of giant icebergs is considerable, and persistent, yet has a relatively small impact on the carbon cycle, especially if you think at global scale.”