Scientists who fertilized a small patch of the Southern Ocean near Antarctica in 1999 to determine if the iron would stimulate growth of algae that consume carbon dioxide, a major greenhouse gas, say their results show that iron supply does control algal growth during the summer but that the long-term fate of the carbon remains unknown.
The results of the 1999 experiment were reported in today's issue of the journal Nature. Woods Hole Oceanographic Institution scientists Ken O. Buesseler and Matthew Charette, co-authors of the Nature study, say the results are important to our understanding of the ocean's role in carbon cycling. They also caution that the results of this study don't necessarily mean that adding iron in short bursts will lead to a decline of carbon from the surface ocean, and note that a major unanswered question is the fate of the carbon in the ocean.
"Iron fertilization is a hot topic not only within the ocean research community but also among ocean entrepreneurs and venture capitalists who see the potential for enhancing fisheries through large-scale ocean manipulations," said Buesseler, who has been studying this issue for years. He and Charette have a related paper to be published October 13 in the electronic journal "Geochemistry Geophysics Geosystems G3 " which is published by the American Geophysical Union and the Geochemical Society."
Their upcoming G3 paper, "Does Iron Fertilization Lead to Rapid Carbon Export in the Southern Ocean?" provides insight by using the natural tracer thorium-234 to follow the export of carbon to the deep sea. Thorium is a naturally occurring element that by its chemical nature is "sticky," and due to its natural radioactive properties, is relatively easy to measure. Buesseler and colleagues have been studying thorium as a proxy for carbon export from the surface waters to the deep sea in a wide variety of natural ocean systems, including the North Atlantic, equatorial Pacific, near Hawaii, in the Arabian Sea, south of New Zealand and in the Ross Sea. Further work around Antarctica is scheduled for January 2002 as part of the Southern Ocean Fe Experiment (SOFEX), a new program funded by the National Science Foundation that will follow the ocean response to iron (Fe) fertilization for close to one month.
The Southern Ocean Iron Release Experiment (SOIREE) reported in today's Nature took place in the 1999 summer season and was the first detailed study of particle export during an iron fertilization experiment. Similar experiments were conducted in 1993 in the equatorial Pacific and in 1995. The earlier studies found a strong biological response to the additional iron supply and a chemical drawdown of the carbon dioxide in the water column, meaning the algae or phytoplankton absorbed the carbon dioxide. The 1999 SOIREE study showed a much slower biological response to the additional iron, which Buesseler, Charette and their colleagues say may be due to colder water temperatures that promote slower uptake of the iron by the phytoplankton. Scientists were also surprised by the persistence of the iron induced algal bloom, which could be seen by satellite six weeks after the iron was added to the surface ocean.
Buesseler's work was funded by the National Science Foundation. Charette's work was supported by a Vetlesen Foundation Postdoctoral Fellowship.
"There are two major concerns," Buesseler said of iron fertilization experiments. "There are potential major ecological changes, and we don't know if it will work. Growing algae is not enough to be an effective solution to removing greenhouse carbon dioxide. You need to remove algal carbon from the surface to the deep ocean. In SOIREE we saw no increase in carbon export on sinking particles, which implies no removal or sequestration of carbon from the atmosphere. Carbon could stay in the surface waters for weeks or months but end up being returned to the atmosphere."
"This study could raise interest in large-scale manipulations by commercial interests, but we need to be careful," Buesseler added. "More discussion is needed between a number of groups, among them commercial interests, governments, ocean scientists, fisheries biologists and climate modelers, before we alter the ocean in unforeseen ways."
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