Iron at sea can bury carbon for centuries

Dumping iron at sea can bury carbon for centuries, study shows.

Marine biologist Philipp Assmy analyzing a plankton sample under the microscope.

Marine biologist Philipp Assmy analyzing a plankton sample under the microscope. Photo: Thomas Bresinsky / Caligari Film and Alfred Wegener Institute

Overview picture of the sinking plankton community in subsurface waters (160–400 m) of the iron fertilized eddy. Spiny diatoms dominated the sinking material.

Overview picture of the sinking plankton community in subsurface waters (160–400 m) of the iron fertilized eddy. Spiny diatoms dominated the sinking material. Photo: Philipp Assmy / Norwegian Polar Institute

The diatom Chaetoceros atlanticus was one of the species stimulated by the iron addition.

The diatom Chaetoceros atlanticus was one of the species stimulated by the iron addition. Photo: Marina Montresor / Stazione Zoologica

In a recent Nature article an international team of scientists present multiple lines of evidence from the European Iron Fertilization Experiment (EIFEX) in the Southern Ocean that at least 50% of the carbon captured by an iron-induced phytoplankton bloom was exported to the deep ocean and thus sequestered from contact with the atmosphere for centuries or longer.

The EIFEX experiment was the first out of 12 ocean iron fertilization (OIF) experiments conducted to date that followed the fade of the sinking bloom along the deep water column. This was possible because EIFEX was conducted in a closed ocean eddy that provides vertical coherence between surface and deeper layers. A patch of 150 square kilometres (circle with a diameter of 14 kilometres) within the eddy was fertilized with seven tonnes of iron sulphate on 13/14 February 2004. This corresponds to an iron addition of one hundredth of a gram per square metre. The resultant iron concentration of two nanomol per litre is similar to values measured in the wake of melting icebergs while the iron concentrations in coastal regions tend to be much higher. Despite the addition of a minute iron dosage on a square meter basis EIFEX stimulated a massive bloom dominated by grazer-protected diatoms that sank to the deep ocean and sediments.

However, results from EIFEX can by no means be extrapolated to fertilization of the entire Southern Ocean as a measure to combat global warming. More, larger scale experiments are needed to minimize the effect of dilution with surrounding waters and evaluate the effects on plankton ecology, biogeochemistry and climate. The project was a collaboration between several researchers from leading European research institutions, including Philipp Assmy from the Norwegian Polar Institute.

Reference

Smetacek, V., Klaas, C., Strass, V.H., Assmy, P., Montresor, M., Cisewski, B., Savoye, N., Webb, A., Arrieta, J.M., Bathmann, U., Bellerby, R., Berg, G.M., Croot , P., d’Ovidio, F., Gonzalez, S., Henjes, J., Herndl, G. J., Hoffmann, L.J., Leach, H., Losch, M., Mills, M.M., Neill, C., Peeken, I., Röttgers, R., Sachs, O., Sauter, E., Schmidt, M.M., Schwarz, J., Terbrüggen, A. and Wolf-Gladrow, D. Deep carbon export from a Southern Ocean iron-fertilized diatom bloom, Nature, 19. Juli 2012, Volume 487, pp 313 – 319.