Iron Fertilization

What is Ocean Iron Fertilization?

Ocean Iron Fertilization (OIF) is the intentional introduction of iron into ‘high nutrient, low chlorophyll’ (HNLC) oceans to induce phytoplankton blooms. John Martin developed a two part hypothesis (Strong et al, 2009).   

1.   HNLC regions of the oceans can be explained by limited iron availability and therefore nitrates and phosphates are not depleted because additional phytoplankton growth is limited by iron (Strong et al, 2009).
2. If iron does control productivity in HNLC regions and therefore the burial of organic carbon via the biological pump, it could explain the dust deposition and atmospheric CO₂ during the Last Glacial Maximum (Strong et al, 2009). 

Martin also mentioned how this hypothesis could be important for the potential of drawing down CO₂ from the atmosphere (Martin et al, 1990). 

OIF experiments

Since Martin’s initial hypothesis the last 20 years have seen a series of experiments which attempt to understand more about the process of iron fertilization. The first two experiments called IronExI and IronExII were done in the equatorial Pacific and spread several kilometres. The Southern Ocean was the next to be experimented on because this is where the most HNLC regions are found (Strong et al, 2009).

The experiments were designed to track the fate of carbon fixed in phytoplankton blooms in the surface layer because this is where the CO₂ will be absorbed from the atmosphere and transported to the deep ocean.

The European Iron Fertilization Experiment took place in 2004 and was the longest iron fertilization experiment to date. The aim of the experiment was to investigate the carbon export response and community shifts in the Southern Ocean iron-induced bloom. The experiment showed the highest ratio of carbon export to iron added (Strong et al, 2009). 

The experiments have improved understanding considerably. They have proved Martin’s original hypothesis and there is now more information on the initial phytoplankton response to iron enrichment. However, the results have proven to be inconclusive in regard to carbon sequestration (Strong et al, 2009). There are too many varying factors that affect the blooms in the long term.

Modelling Iron Fertilization

Zahariev et al. (2008) modelled the global elimination of iron limitation in oceans. They found that 0.9Gt C yr^-1 would be taken from the atmosphere. This is equivalent to 11% of global emissions in 2004. Models also show that in order to maintain carbon sequestration continued iron fertilization of the entire Southern Ocean would be needed with enough iron to deplete the ocean of macronutrients.

Martin, J.H., R.M. Gordon, and S.E. Fitzwater (1990) ‘Iron in Antarctic waters’,   Nature, 345, 6271, pp. 156–158

Strong, A.L., J. J. Cullen, and S. W. Chisholm (2009) ‘Ocean Fertilization: Science, Policy, and Commerce’ Oceanography, 22, 3, pp. 236-261

Zahariev, K., J.R. Christian, and K.L. Denman (2008) ‘Preindustrial, historical, and fertilization simulations using a global ocean carbon model with new parameterizations of iron limitation, calcification, and N2 fixation’ Progress in Oceanography, 77(1) pp.56–82