http://www.scientificamerican.com/article/glacial-melt-pours-iron-into-ocean-seeding-algal-blooms/
Glacial Melt Pours Iron into Ocean, Seeding Algal Blooms
The iron fertilizer from glacier melt may help feed plankton blooms that, in turn, suck carbon dioxide out of the sky
A decade ago, a common hypothesis was that rivers and dust supplied the ocean with most of its iron. Since then, scientists have reported in several papers that icebergs and deep-sea hydrothermal vents also may be significant contributors.
Credit: Andrew Bowden via Flickr
Call it natural geoengineering.
Scientists report in a new study this week that glacial melt may be funneling significant amounts of reactive iron into the ocean, where it may counter some of the negative effects of climate change by boosting algal blooms that capture carbon. The paper, published in Nature Communications, adds to a body of research suggesting that melting ice at both poles may have widespread consequences beyond rising sea levels.
"The theory goes that the more iron you add, the more productive these plankton are, and thus the more CO2 is taken out of the atmosphere in photosynthesis," said Jon Hawkings, a doctoral student at the University of Bristol and lead author of the study. "Plankton 'fix' CO2 much like trees."
The work could help improve climate models of the future and fill in data holes about major climate transitions and ice ages in the past, he said. The effects on Antarctica in particular will need additional examination, he said, as iron currently is limited in the Southern Ocean.
Hawkings and a research team from four United Kingdom-based universities tested meltwater collected from the Leverett glacier in Greenland during summer 2012 and detected large amounts of iron nanoparticles known as ferrihydrite. Ferrihydrite is considered to be "bioavailable" iron because it is easily used by plankton in lab experiments, Hawkings said.
Through the detected iron mineral levels in their samples, the team estimated that the flux of bioavailable iron into the ocean from glaciers currently is between 400,000 and 2.5 million metric tons annually from Greenland and up to 100,000 metric tons from Antarctica.
That means that polar regions may rival wind-blown dust as a source of ocean iron. The contribution from Greenland alone could range from 8 to 50 percent of the global ocean flux of bioavailable iron, Hawkings said.
The iron ore counter-effect
A decade ago, a common hypothesis was that rivers and dust supplied the ocean with most of its iron. Since then, scientists have reported in several papers that icebergs and deep-sea hydrothermal vents also may be significant contributors.
A decade ago, a common hypothesis was that rivers and dust supplied the ocean with most of its iron. Since then, scientists have reported in several papers that icebergs and deep-sea hydrothermal vents also may be significant contributors.
A study last year found that a Greenland glacier was releasing iron, but it did not assess as large an area and for as long of a period of time as his study, Hawkings said. The studied area of the Leverett glacier, for instance, is more than 600 kilometers squared, while earlier work assessed a glacier about 5 kilometers squared, he said.
"Our study is the first to date to follow a whole melt season and the first to have looked at a large glacial catchment," he said.
Matt Charette, a senior scientist at the Woods Hole Oceanographic Institution and co-author of an earlier paper on Greenland-supplied iron, said although the new study overlaps somewhat with his prior work, it provides new details.
"A case could be made that a larger system like the one they studied is more appropriate for scaling up to the entire ice sheet," he said.
Kenneth Coale, a scientist at Moss Landing Marine Laboratories, said the paper was "nicely done" and added to understanding of how iron may provide a counter-effect to climate change.
The Greenland iron originates from stored subglacial meltwater that gets "flushed out" by surface waters carried through tunnels and cracks in ice during the melt season, Hawkings said. It's not fully understood how far the iron travels once in the ocean, but it likely stays near both poles. "Evidence exists for transport a few 100 kilometers out to sea, but only limited amounts will reach the open ocean," he said.
It's also not fully understood how the iron will interact with polar ecosystems. Scientists have long known that iron-fueled algae can eat up carbon, leading to speculation that iron fertilization might be a geoengineering option to cool the planet. It also holds the possibility of boosting marine life that feed on plankton. A community in Canada two years ago, for instance, dumped large amounts of iron dust into the ocean to try to boost salmon stocks.
In the case of "natural" iron fertilization via ice sheets, the positive likely outweighs the negative, in the sense that carbon will be removed in an area highly vulnerable to warming, and extra algae may help polar marine life threatened by warming, Hawkings said. He noted that algae can boost krill, which can in turn can feed fish, whales and seals.
However, he pointed to a report from the Woods Hole Oceanographic Institution documenting a range of potential problems with added iron and resulting algae in the ocean in general, such as depleting the ocean surface of other nutrients like nitrogen.
"In theory it's a good thing. However, there may be impacts on species diversity ... and decomposing plankton may use up oxygen in deeper waters, depriving other organisms of it as happens in rivers and lakes when you get an algal bloom," Hawkings said.
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