Iron could cut down global warming

Iron fertilisation in the Southern Ocean has been found to increase the growth of algae on the water’s surface, and consequently to increase the up-take of the greenhouse gas, carbon dioxide.

The research, published in Geochemistry Geophysics Geosystems G3, the electronic journal of the American Geophysical Union and the Geochemical Society, was conducted during February 1999, when a 50km square patch of the Tasman Sea in the Southern Ocean was seeded with iron sulphate. After two weeks the patch of algae had expanded to over 200km square, with a 400% increase in chlorophyll concentrations at the centre of the patch, and a corresponding 50% increase of particulate organic carbon.

“Iron fertilisation 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 Ken O Buesseler, one of the researchers.

However, the authors of the paper, and a related report published in the 12 October issue of Nature, are quick to emphasise that, as far as the control of atmospheric carbon dioxide is concerned, the study does not necessarily mean that the addition of iron will lead to a decrease in ocean surface carbon, and that the fate of the carbon in the ocean remains a major unanswered question.

The natural tracer thorium-234 was used in order to allow the researchers to follow the export of carbon to the deep sea. By the end of the two weeks, the ecosystem response had lead to a 10% drawdown of surface water carbon dioxide. Delays in the response of the algae, and the ability of the researchers to speculate about potential particle export resulted from the cold water temperatures, and the longevity of the iron-induced phytoplankton bloom, say Buesseler and co-author, Matthew A Charette. According to ocean colour satellite images, the algae patch continued to expand and persisted for at least 30-45 days.

“There are two major concerns,” said Buesseler. “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 [the Southern Ocean Iron Release Experiment] 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,” added Buesseler. “More discussion is needed between a number of groups, among them commercial interests, governments, ocean scientists, fisheries biologists and climate modellers, before we alter the ocean in unforeseen ways.”

Further work around Antarctica is scheduled for January 2002 as part of the Southern Ocean Iron Experiment (SOFEX), a new programme funded by the National Science Foundation that will follow the ocean response to iron fertilisation for a month.

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