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Adding Iron To Oceans Not Effective - Study

19 March, 2004

Adding Iron To ‘Anaemic’ Oceans Not Effective In Battle Against Global Warming, Study Finds

Iron fertilisation of microscopic plants in the surface ocean may not be the answer to removing excess “greenhouse gas” carbon dioxide from the atmosphere, according to a paper published by Nature and released on-line yesterday.

The results of experiments in the Gulf of Alaska, led by University of Otago-based oceanographer Dr Philip Boyd, show that iron is not the only factor in the carbon cycle equation. Silicate is also key to the blooming of microscopic plants (phytoplankton) in the sea and the corresponding fixing of carbon – a discovery which may rule out the possibility of fertilising the “ocean deserts” of the world in a bid to offset increased levels of carbon dioxide in the atmosphere.

Almost half the Earth’s photosynthesis is carried out by phytoplankton in the sea. These tiny cells harvest sunlight to fix carbon which is then either re-mineralised to carbon dioxide in the surface waters of the ocean and released back into the atmosphere, or “pumped” down to the deep ocean layer as the plankton sink into the abyss. The strength of this biological pump plays a key role in regulating our climate. In parts of the world where the surface ocean is lacking in iron, phytoplankton are anaemic and therefore are unable to make use of the available nutrients. This means the ocean’s potential to regulate our climate is not being fully realised.

An earlier experiment carried out in the Southern Ocean in 1999 by Boyd and a team of New Zealand and international scientists -- where around 8,000 kg of an iron compound in solution was distributed over a patch of ocean eight km in diameter -- had such dramatic results (a five-fold increase in phytoplankton stocks during the developing bloom), it was thought that simply adding iron might be the answer to increasing the amount of atmospheric carbon dioxide locked up in the ocean.

“What we found, however,” says Boyd, “is that adding iron to the ocean produces a very different picture in the longer-term.” After 18 days of a similar experiment in the Gulf of Alaska, the iron-induced bloom declined and satellite pictures show merely a ghost of the plankton-rich patch that blossomed initially. “We think the decline was initiated by the drop-off in iron levels, but the secondary factor is the removal of all of the silicate by phytoplankton. Until now, we had not realised the importance of silicate in causing the bloom’s decline,” he says.

“And while it might be feasible for us to add iron to the ocean to stimulate blooms, for every ton of it we throw overboard, we’d need to add at least 5000 tons of silicate to enable the blooms to persist for long enough to impact on atmospheric carbon
dioxide levels. It’s just not practical.”

The other important implication of this work is the significance of silicate supply to climate change in the geological past. “We have been pointing to increased natural supplies of iron (from atmospheric dust) as being responsible for decreases in atmospheric carbon dioxide in the distant past, but this new evidence suggests that natural supplies of silicate must also have been greater at the same time.”

During the bloom initiated in the Gulf of Alaska waters, sediment traps (underwater particle collectors) were also deployed below the bloom at various depths to determine how much of the carbon fixed by the bloom sinks to into the subsurface ocean layer. This small but significant transfer of carbon is the key transfer mechanism involved in the climate regulating process.

Most of the carbon ‘pumped’ to these deep waters via sinking particles is re-mineralised back to carbon dioxide by bacteria, just as it is in the surface layer, but the difference is, it remains here for extremely long time periods – on average, this atmospheric carbon dioxide is locked up for 1000 years.

“With these underwater rain gauges in place below the iron-enriched area and outside it, we could determine whether adding iron helps more carbon to settle to depth and therefore increases the efficiency of operation of the pump,” says Boyd. “Our experiment showed that only an additional 10 per cent of the carbon fixed by phytoplankton following iron enrichment of surface waters actually settled to depth.

“The fact that we were not able to significantly improve the efficiency of this transfer of carbon below the surface waters strengthens the argument that adding iron to the ocean is not going to be an effective mitigating strategy for atmospheric CO2 levels thought to be increasing global warming.”


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