Acid test for the seas
Box 1 | Iron fertilisation of the oceans
Ocean acidification is expected to make life difficult for phytoplankton such as coccolithophores and diatoms that use carbonate in their skeletal structures. But in a strange twist, some scientists believe that greatly increasing the growth of these organisms could help solve the problem of too much carbon dioxide in the atmosphere.
Phytoplankton are abundant in most parts of the ocean, but there are some 'dead spots' – in the deep equatorial waters of the Pacific, for example, as well as in the Gulf of Alaska and the Southern Ocean – where they are relatively scarce, even when plenty of sunlight and nutrients are available. Scientists debated the reason for this scarcity for many years. Some thought that it was simply predation by zooplankton that kept the phytoplankton in check and prevented them from blooming. Others thought it might be the lack of a micronutrient, iron.
In a series of experiments designed to test the latter hypothesis, scientists tipped a few tonnes of iron into the ocean. The phytoplankton loved it, increasing their natural abundance in the experiment areas by up to 30-fold. This extraordinary result led many to speculate that 'iron fertilisation' could help mitigate global warming. The idea would be to dump thousands of tonnes of iron into the phytoplankton-poor parts of the ocean, thus stimulating phytoplankton blooms. These would use carbon dioxide dissolved in seawater from the atmosphere to build their skeletal structures and then, when the organisms die, they would deposit carbon – in the form of calcium carbonate or silicon carbonate – in the lower reaches of the ocean, effectively removing it from the carbon cycle for hundreds or thousands of years.
Iron fertilisation is not just an idea: it happens naturally on a grand scale. Wind-blown dust has been a significant source of oceanic iron for millions of years. More dramatically, volcanic eruptions can deliver huge quantities of iron into the ocean in short periods, causing sudden pulses of phytoplanktonic growth. The eruption of Mt Pinatubo in the Philippines in 1991 is a modern example: it deposited an estimated 40,000 tonnes of iron dust into oceans worldwide, stimulating growth in phytoplankton that caused a measurable (although temporary) decline in the concentration of atmospheric carbon dioxide.
But the idea of artificial iron fertilisation is also controversial. Some scientists say it would not have a significant long-term impact on carbon dioxide levels. Others fear that such ecological engineering could have unintended consequences, such as the creation of toxic algal blooms, oxygen depletion in the deep ocean and increasing the rate of ocean acidification. Both advocates and opponents seem to agree, though, that artificial fertilisation, even on a grand scale, is no iron bullet for ending global warming.
Related sites
Iron surprise: Algae absorb carbon dioxide (Science News, Vol. 148, 1995)
Fertilising the ocean with iron: An unproven greenhouse remedy (ACE CRC, Australia)
Scientists map evolution of phytoplankton (CNN.com, USA)
Age of iron and ice (New Scientist, 14 October 2000)
Iron soup feeds algal appetite for carbon dioxide (New Scientist, 1 July 1995)
Pumping iron in the Pacific (New Scientist, 3 December 1994)
Ocean iron fertilization – moving forward in a sea of uncertainty (Science, 11 January 2008)
Posted January 2008.