Effectiveness of iron fertilisation to cool planet questioned
29 January 2009
A region of the Southern Ocean naturally rich in iron leads to up to three times more carbon being locked away at the bottom of the ocean than a similar area lacking in iron, according to research in the journal Nature.
The team used instruments like this to research natural iron fertilisation in the oceans.
This is the first time scientists have measured the amount of carbon falling to the deep ocean in a natural iron fertilisation system anywhere in the world. But the amount of carbon locked away falls far short of estimates for artificially seeding the oceans with iron calculated by some geoengineers.
Scientists have suggested adding iron to the Earth's oceans as a way to curb the growing emissions of carbon dioxide in the atmosphere. Iron encourages phytoplankton blooms, which use carbon dioxide and sunlight to grow. When these blooms die, they sink to the sea floor, taking their carbon with them, in effect locking it away for thousands of years.
An international team of researchers led by Professor Raymond Pollard of the National Oceanography Centre, Southampton (NOCS) wanted to see if the phytoplankton blooms they saw on satellite images to the north of the Crozet Islands in the Southern Ocean were triggered by iron availability. They also wanted to know if the blooms led to more carbon making its way to the bottom of the ocean.
A bloom the size of Ireland develops to the north of the Crozet Islands.
Pollard and his team visited the region during the spring to analyse the nutrient mix north and south of the islands. They found evidence for much more iron in the waters north of the islands than the south.
'Iron slowly seeps into the ocean from the volcanic Crozet Islands. With no phytoplankton around during the winter because of the lack of sunlight, and a weak circulation north of Crozet, iron builds up over the winter period,' says Pollard. 'This doesn't happen south of the islands, because the ocean circulation is different.'
The volcanic Crozet Islands in the Southern Ocean. Waters to the north have a much high iron content than to the south. Click to enlarge.
In spring, sunlight warms the oceans and a phytoplankton bloom the size of Ireland develops to the north of the Crozet Islands.
During their expedition, the team measured carbon concentrations at 100 metres, 200 metres, 3000 metres and in the sediment on the ocean floor both north and south of the islands. They left sediment traps at 3000 metres at various places around the islands for a year before they were collected by a different group led by George Wolff from the University of Liverpool.
'Once we had analysed the sediment trap material, all three different methods we used to measure carbon - just beneath the surface, at 3000 metres and coring - showed concentrations two-to-three times higher under the iron-fertilised patch,' says Pollard. 'But the numbers we calculated for the carbon-to-iron ratio - a measure of how much carbon is locked away for a given amount of iron input - fall 15-to-50 times short of some geoengineering estimates,' he adds.
'The concentration of carbon in the cores beneath the iron-fertilised area to the north of the Crozet Islands has built up over the last 10,000 years.'
Raymond Pollard, National Oceanography Centre, Southampton
A member of the team, Dr Rachel Mills of the University of Southampton analysed the top ten centimetres of sediment in cores taken from the ocean floor and found a system that's been in place for thousands of years.
'The concentration of carbon in the cores beneath the iron-fertilised area to the north of the Crozet Islands has built up over the last 10,000 years,' says Pollard.
The NOCS-led team is one of only two teams to analyse the link between iron, phytoplankton blooms and so-called carbon sequestration in a natural system. The other team, which was French-led, found a much higher carbon to iron ratio around the Kerguelen Islands - also in the Southern Ocean.
Pollard's results will contribute to a report commissioned by the Royal Society to investigate geoengineering options, to be published in the summer.
An Indo-German experiment to test the effects of seeding an area of ocean between the Cape of Good Hope and Cape Horn started just this week despite earlier condemnation by environmentalists. The team - on a two-month expedition - plans to add six tons of iron sulfate to the ocean. 'This is 40 times less iron than was added naturally near Crozet,' says Pollard.
Scientists say the Indo-German experiment is important, because two months is long enough to tell if artificially adding iron to the oceans does lead to carbon falling into the deep ocean.
'It's only sensible to have a Plan B, in case we do not cut carbon dioxide emissions quickly enough. So, we need to understand how the ocean responds if we add iron,' concludes Pollard.
Southern Ocean deep-water carbon export enhanced by natural iron fertilization. Nature doi:10.1038/nature07716.
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