The iron hand in global temperature

THERE must be something in the Galapagos Islands, off the coast of Ecuador, that attracts scientists in pursuit of life's mysteries. Charles Darwin spent a good chunk of his life wandering in these islands before formulating his theory of natural selection. And now, the waters around the islands have yielded crucial evidence in favour of a controversial idea that the growth of plankton -- microscopic organisms that constitute the basic food for all marine life -- is partly limited by the level of iron in the sea water.

Besides being a source of food for marine creatures, plankton have a direct and an important bearing on the global ecosystem, for they soak up carbon dioxide from surface waters, which in turn take it from the atmosphere. Therefore, if iron controls the growth of plankton and thereby the atmospheric level of carbon dioxide, a greenhouse gas, it follows that iron supply to the oceans could affect global temperatures.

In a series of experiments to test if plankton growth depended on the levels of iron in sea water, US researchers filled large containers with sea water and to some of these containers added minute quantities of iron in the form of a solution, leaving the others as controls. Over the course of a few days, the scientists found the iron-spiked water supported much more plankton growth than did the controls.

For decades, it was thought that iron, an element essential to life, was plentiful in sea water. But later studies falsified this view -- earlier observers had been measuring not the actual iron concentration, but contamination from the ships (made mostly of iron) whence observations had been made.

When it was established that iron concentration in sea water was low -- a few billionths of a gram per litre of sea water -- scientists began speculating that perhaps the lack of it put a limit on plankton growth, in some regions at least. There are large tracts of the global ocean in which the limiting nutrient is not iron but nitrates and phosphates. In more than 70 per cent of the oceans, the concentration of these chemicals is reduced to near zero by the growth of plant-plankton.

But there are certain oceanic regions (equatorial regions and much of north Pacific, for example) where nitrate and phosphate concentrations are not zero in surface water, suggesting that something else stops plankton growing before these nutrients run out. Scientists guessed it was iron and hence these experiments near the Galapagos Islands. These regions, say scientists, are particularly important in setting the natural atmospheric carbon dioxide level.

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