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Into deep water: Tangaroa faces stormy weather as it tackles the climate-change issue. Credit: NIWA

Oceanographers are trying to resolve a decade-long debate over the way iron levels in the oceans influence global climate. Researchers from six countries participating in the Southern Ocean Iron Release Experiment (SOIREE), led by New Zealand'sNational Institute of Water and Atmospheric Research, set sail from Wellington, New Zealand, last week on the research vessel Tangaroa.

The circulation and mixing of the Southern Ocean is such that the availability of iron to phytoplankton may have directly affected its ability to lock up atmospheric carbon dioxide. In the Southern Ocean, as in around one-third of the world'soceans, phytoplankton growth fails to make full use of available nitrate and phosphate nutrients. This suggests that other factors are limiting photosynthesis, and with it the ability of phytoplankton to fix carbon dioxide, an important greenhouse gas.

Iron is essential for photosynthesis, and a shortage has been shown to limit growth in the equatorial Pacific Ocean, where surface waters fertilized with iron ‘bloomed’ (see Nature 383, 495–501; 1996 ). This oceanic region seems to have been rich in iron in the geological past, but this probably had little effect on atmospheric carbon dioxide levels — the physical circulation would have prevented any extra carbon fixed being locked away in the deep ocean.

In contrast, if phytoplankton productivity in the Southern Ocean is limited by a shortage of iron — the main question SOIREE aims to answer — an increase in iron levels may have significantly reduced the concentration of atmospheric carbon dioxide in the past. Much of the consequently fixed carbon in surface waters would be expected to be physically mixed down into deep waters, removing it from contact with the atmosphere for long enough to diminish atmospheric levels.

The major problem facing the expedition is the weather. Storms in the rough Southern Ocean will make it difficult to find the 10-15 day window of calm seas needed to maintain and follow the 50 square kilometres of surface ocean that will be seeded with ferrous sulphate.