nanoscale iron particles
© Riaswell et al./BioMed CentralTwo forms of nanoscale iron particles (rods and granules) can be seen in high-resolution pictures of ice.
The increasing number of icebergs breaking off Antarctica may have an unexpected benefit. According to one team of scientists, the bergs could feed carbon-loving plankton. If they are right, melting icebergs could - theoretically - slow global warming. Just how great an effect this would have remains to be seen.

Rob Raiswell of the University of Leeds, UK and colleagues trained high-resolution microscopes on ice sampled from icebergs in the Southern Ocean and the Antarctic glaciers from which they are born.

They found nano-sized particles of iron, between five and 10 millionths of a millimetre across. The team believe that because of the size and structure of the particles, the iron could be assimilated by phytoplankton.

"Most of the ground-up rock carried by icebergs is thought to be inert," says Raiswell. "However, the high resolution microscopy shows there are small amounts of iron nanoparticles. They simply could not be seen except by these techniques."

Phytoplankton need iron in order to grow, and the Southern Ocean is generally thought to be low on iron. But there is evidence that some Antarctic glaciers are flowing into the ocean faster because of climate change. This means more icebergs. If Raiswell's findings are correct, more icebergs would mean more dissolved iron, therefore more phytoplankton, and more carbon dioxide sucked out of the atmosphere and into the oceans.

Plankton boost

"Dust has been thought to be the main outside source of iron to the Southern Ocean," says Raiswell. He and his colleagues calculated that existing icebergs could double the supply of iron to the region.

The researchers will need to prove that the nano-iron can indeed boost plankton growth. Ken Denman of the Canadian Centre for Climate Modelling and Analysis says there is some debate over what form of iron phytoplankton can use. "For example, only a few percent of air-borne iron deposited in the oceans is believed to be readily utilisable by phytoplankton," he says.

Denman also points out that climatologists think there is typically less iron in the oceans during warm inter-glacial periods. "Why would human-induced warming increase the iron supply whereas recent natural warming occurred at the same time as decreased iron and southern Ocean [phytoplankton], as far as we can tell from the ice cores?"

It is too early to say how much of an impact more icebergs will have. One problem is that not all plankton sinks to the bottom of the ocean and contributes to the deep-ocean carbon sink. Part of it is eaten by marine animals and returned to the water column in their excrement. Geochemists have only a poor idea of the amount of carbon that is cycled in this way.

Journal reference: Geochemical Transactions