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10 Oceanus Magazine Vol. 46, no. 1, 2008 www.whoi.edu/oceanusin this age of satellites, it’s fairly easy to answer the basic question of whether adding iron to the ocean can stimulate a plankton bloom. When storms over land blow iron-rich dust into the sea, satellite im-ages show marbled swaths of green phyto-plankton spinning across waters previously blue and barren. satellites also show plank-ton blooms near the galápagos and other is-lands where iron-rich deep waters naturally well up to surface. even blooms spurred by experimental additions of iron to the ocean can be detected by satellite, and shipboard scientists conducting the experiments re-ported an almost instantaneous change in the color and even the smell of the water.Twelve experiments so far have not looked so closely at the trickier questions of how much carbon dioxide taken up by a bloom is drawn out of the air and trans-ferred into the deep sea, and how long it re-mains sequestered there. as yet, scientists have turned up only partial answers.Philip Boyd of the new zealand na-tional institute for Water and atmospheric Research summarized the 12 experiments at an ocean iron fertilization conference con-vened at Woods Hole Oceanographic in-stitution (WHOi) in september 2007 and in an article in Science magazine earlier last year. Four took place in the northwest Pa-cific, two were in the equatorial Pacific, and six were in the southern Ocean. all 12 re-ported up to 15-fold increases in the chloro-phyll content of surface waters. (chlorophyll is the sunlight-capturing molecule in pho-tosynthesis and is often measured in lieu of actual plankton counts.) Only a tiny fraction of the carbon drawn down by blooms sinks from the surface into deeper waters, where it is sequestered from the atmosphere. estimates of the tonnage of carbon sequestered (measured at 200 meters depth) per ton of iron added hover around 200 to 1, a far cry from early experiments in laboratory beakers that yielded estimates around 100,000 to 1, Boyd said. But those may be underestimates. al-though scientists have spent up to several weeks monitoring blooms after iron addi-tion, ship schedules and budgets have usu-ally prevented them from monitoring long enough, or deep enough, to obtain good measurements of “export efficiency”—the proportion of carbon that sinks from the surface into deeper waters.The 2002 united states-funded sOFeX Will Ocean Iron Fertilization Work?Getting carbon into the ocean is one thing. Keeping it there is another.Jack Cook, WHOITESTING THE WATERS—Twelve small-scale experiments over the past decade in several ocean locations (red dots) consistently showed that intentional iron additions do result in phytoplankton blooms that help draw down carbon dioxide from the air. But the experiments have not determined how much carbon is transferred and sequestered in the deep sea, rather than quickly recycled back to the atmosphere.From Philip Boyd, New Zealand National Institute for Water and Atmospheric ResearchWOOds HOle OceanOgRaPHic insTiTuTiOn 11Jack Cook, WHOI0 to 100meters100 to 500metersBelow 500metersTHE BOTTOM LINE—Only a small fraction of the carbon drawn into the ocean by plankton blooms makes it into the depths where it no longer can be exchanged with the atmosphere.1 Air and seaexchange CO2.4 Fragments of decaying phyto-plankton and fecal pellets from zooplankton both contain carbon.5 Separately or in aggregations (called “marine snow”), these carbon-containing particles sink.6 Only 5 to 50% of the total carbon from blooms reaches 100 meters. About 2 to 25% sinks between 100 and 500 meters.7 Microbes decompose particles further. Zooplankton eat some of this material.9 CO2 from organic matter respiration recirculates back to surface waters.10 Zooplankton migrate up at night to feed and back to the depths during the day.8 Perhaps only 1 to 15% of the original carbon in surface waters sinks below 500 meters.3 Zooplankton eat phytoplankton and respire CO2.2 Phytoplanktontake up CO2 to grow.CoccolithophoridsRadiolariaSalpsForaminiferaDiatomsOtherPhytoplanktonCopepodEuphausiidPlankton KeySalp pelletCopepod pelletsMicrozooplanktonmini-pelletsMarine snowaggregatesBits of plankton shellsEuphausiid pelletsRemnant ofmucousy websmade by someplankton to feedParticle Key12 Oceanus Magazine Vol. 46, no. 1, 2008 www.whoi.edu/oceanusexperiment did show that more carbon was exported into deeper waters below the fertilized ocean patch, WHOi marine biochemist Ken Buesseler and colleagues reported. and unpublished results from the 2004 european eiFeX experiment showed levels of carbon sequestration that were far higher and far deeper (all the way to the seafloor) than previously observed—but this occurred only in the final days of monitoring, Victor smetacek of the alfred Wegener institute in germany told partici-pants at the WHOi conference. The emerging picture is that iron fertil-ization does in principle work well enough to squirrel away carbon for at least a few decades—possibly useful in the world’s ef-forts to solve its carbon emissions problem. although present yields seem low, improved methods could boost that number in two ways: by refining logistics to make blooms larger, and by increasing “export efficiency,” or the proportion of carbon that sinks from the surface into deeper waters, where it is less easily returned to the atmosphere. Logistics and luckiron addition is simple in principle, but once a ship is loaded up and heading for open waters, even small experiments be-come a tangle of logistics. The sOFeX ex-periment employed three research ships, helicopter scouts, and 76 scientists to moni-tor the results of adding one to two tons of iron to the ocean.The typical method involves drizzling acidified iron sulfate into the ocean as a thin slurry, to reduce the amount that im-mediately sinks out of the sunlit surface waters where photosynthesis happens. add-ing the iron requires a 12-hour zigzagging cruise across a theoretical square of water whose boundaries shift constantly in the ocean currents. in the weeks of monitor-ing that follow, a ship typically spends 12 hours out of every day just mapping out the boundaries of the bloom.Blooms are hard to track because the added iron rapidly dilutes, sinks, and reacts with seawater, becoming virtually undetect-able after a few days, Boyd said. Researchers add minute amounts of an inert tracer, sul-fur hexafluoride (sF6), itself a potent green-house gas.


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