Image caption The algae Corethron pennatum was one of the species stimulated by the iron addition

Fertilising the oceans with iron to combat climate change can lock carbon away for centuries, research suggests.

Tiny marine plants induced to grow by the iron sink to the ocean floor taking carbon with them, a German-led team reports in Nature journal .

Iron fertilisation is one of the oldest ideas for a climate "technical fix".

But much more research is needed before the approach could be put to use, the scientists say, and cutting emissions should be the priority.

There have been about 12 iron fertilisation experiments at sea down the years, stimulated by the pioneering theory of oceanographer John Martin.

In the 1980s, he proposed that in many parts of the oceans, the growth of phytoplankton - tiny marine plants, or algae - was limited by lack of iron.

Adding iron, he suggested, would enable the plants to make full use of nutrients such as nitrogen and phosphorus; and as they grew, they would absorb carbon dioxide.

This has since become the most researched of all the proposed "geoengineering" approaches - technical fixes for climate change.

Many experiments have shown that adding iron stimulates the phytoplankton to grow and absorb CO2; but whether the carbon is released again as the plants die, or through respiration of tiny animals (zooplankton) that eat them, has never been clear.

The new paper, which relates to the European Iron Fertilization Experiment (EIFEX) performed in 2004 in the Southern Ocean, is the first to give a clear positive answer to that question.

Clearing the waters

The first thing we need to do is reduce emissions, that's absolutely essential Victor Smetacek, AWI

EIFEX deposited about five tonnes of iron sulphate into an eddy in the Southern Ocean currents. Scientists showed that the water in the eddy was pretty much self-contained, its rotation largely isolating it from the rest of the ocean.

Releasing the iron caused a big bloom of algae, which died off again in the days following the release as the iron concentration dwindled.

Over a seven-week period, scientists monitored the water inside and outside the eddy before, during and after deployment of the iron sulphate.

"We had instruments that we could deploy right down to the seafloor, which is at 3,800m depth," said Victor Smetacek, lead researcher on the new paper.

"We also had water bottles that we could close at specific depths, removing the water samples, and we did a huge number of measurements on the phytoplankton and its environment - the nutrients, the iron, and the zooplankton," the Alfred Wegener Institute scientist told the BBC.

Image caption The experiments were carried out using the German Polarstern research ship

These measurements showed that about half of the carbon absorbed from the surface waters was taken down to the sea floor when the phytoplankton died.

"We've quantified this response and were able to guess at the reasons that made the algae sink out of the water column," said Prof Smetacek.

"The organic carbon in the dead algae leaked out and became a sticky mess, you could say, and this picked up other particles and we have these large flocs (flakes of solid matter) sinking out."

Carbon dioxide is constantly being exchanged between the surface of the ocean and the atmosphere.

The presumption is that once the carbon has made it to the ocean floor in solid form, it will remain there for centuries. Meanwhile, the surface water, which is now relatively depleted in carbon, will absorb more from the atmosphere.

'A lof of work ahead'

Dr Michael Steinke from the UK's University of Essex, who was not involved in the study, said it provided "the very first evidence of a man-made conduit between the increasingly CO2-burdened atmosphere and the deep sea".

However, one clear lesson from the number of iron fertilisation experiments down the years is that each patch of ocean is different; to work well, it needs to have the right mix of nutrients and the right kinds of organisms.

The biggest experiment of all, Lohafex, dealt a blow to hopes of utilising ocean fertilisation when it reported three years ago that six tonnes of iron produced little extra plankton growth.

"Will this [new paper] open up the gates to large-scale geoengineering using ocean fertilisation to mitigate climate change?" asked Dr Steinke rhetorically.

"Likely not, since the logistics of finding the right spot for such experiments are difficult and costly. Of the twelve fertilisation experiments of this kind... this group's experiment is the only example to date that demonstrates the all-important carbon burial in the deep sea sediments, away from the atmosphere."

Prof John Shepherd from the UK's National Oceanography Centre, who chaired the Royal Society report Geoengineering the Climate , said impacts on sea life needed to be taken into account before iron fertilisation could be contemplated as a real-life "technical fix".

"Whilst the new research is an interesting and valuable contribution in this evolving field, it does not address the potential ecological side effects of such a technology and it still just a single study in what is a poorly understood field," he said.

Prof Smetacek's own analysis is that even if it were deployed on a vast scale, ocean fertilisation could only take up about a quarter of the extra carbon dioxide being deposited in the atmosphere by humanity's industry, transport and agriculture.

"This is not a solution - the first thing we need to do is reduce emissions, that's absolutely essential," he said.

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