A German research ship laden with 20 tonnes of iron sulphate has whipped up a storm of protest as it sails towards the Antarctic, where it intends to dump its cargo into the ocean late this week.

Scientists on the RV Polarstern, which set sail from Cape Town in South Africa on 7 January, plan an ocean-fertilization experiment that some argue will violate international agreements. But the scientists say that it will yield the very data necessary to assess the impact of the controversial geo-engineering technique, which aims to trap carbon dioxide from the atmosphere by encouraging the growth of algae.

The team, comprising about 50 scientists from Germany, India, Italy, Spain, Chile, France and Britain, is heading for a small patch of the Scotia Sea between Argentina and the Antarctic Peninsula. The eight-week experiment, called LOHAFEX, will be the sixth ocean-fertilization study conducted in the Southern Ocean since 1993.

In response to widespread environmental concerns, the 191 parties to the United Nations' Convention on Biological Diversity last year agreed to a moratorium on all ocean fertilization activities, with only small-scale scientific studies in coastal waters exempted.

Environmental campaigners say that LOHAFEX should not have received permission under these rules. "We're taken aback by this flagrant disregard of international law," says Mariam Mayet, director of the African Centre for Biosafety in Johannesburg.

But the Alfred Wegener Institute for Polar and Marine Research (AWI) in Bremerhaven, Germany, which operates the RV Polarstern, denies that the experiment falls under the UN moratorium.

The study will address, among other things, marine biology, the flow of carbonaceous particles, and biodiversity questions that have barely been analysed in previous experiments, says Karin Lochte, the director of the AWI.