The Science behind the Theory

The principle of Ocean Iron Fertilization is that additional carbon dioxide will be drawndown from the atmosphere into the oceans, and ultimately buried in sediments in the deep ocean. This page briefly covers the processes involved

The Oceanic Carbon Pump

About half of all photosynthesise on Earth occurs in the seas. Carbon dioxide from the atmosphere dissolves into surface waters, where phytoplankton use it to create biomass.

Most of this phytoplankton enters the ocean food web, as other marine organisms feed on it. Some, however, aggregates with waste-products and other solids. This forms a heavier mass, which sinks to the ocean floor as 'marine snow'.
Reaching the ocean floor, a small proportion of this aggregated organic carbon gets buried in sediments and ultimately lithifies (turns into rock). However, most remains in the water, and returns to the surface during upwelling.

Carbon drawdown

It is commonly said, as I have above, that "increased phytoplankton will draw-down increased amounts of atmospheric carbon dioxide into the ocean" - but there is obviously no suction or pump.

This drawdown is due to a 'gas equilibrium' at the water-air (ocean-atmosphere) interface. Although the oceanic CO₂ level is not equal to the atmospheric CO₂, there is equilibrium between them. If one increases or decreases, the other will also change to reach a new equilibrium.

Already, because of increasing levels of atmospheric CO₂, changes in the oceanic-CO₂ level have been detected. As oceanic CO₂ increases, so does sea water acidity. The effects of this have already been seen in deformities in corals and coccoliths (see "marine safety below the surface").
Correspondingly, as phytoplankton 'uses up' oceanic CO₂, the water-air balance is again upset, as the ocean loses some of its gas content. To regain equilibrium, more is drawn-down from the atmosphere.

The gas interchange across the ocean-atmosphere interface is simply an exchange of gases in both directions. If the status quo is not equilibrium, then the total movement in one direction will be different than in the other, until equilibrium is reached.
Stormy seas and breaking waves are a particularly active area for gas interchange, as sea water and air are vigorously mixed.

Because the ocean takes in more CO₂ than it gives out, it is termed a 'carbon sink'. The terresttrial carbon sinks are the forests and vegetation, which 'lock-up' carbon in their organic matter for the long term.

High nutrient low chlorophyll (HNLC) areas

Many seas are unproductive, even those with high levels of the common limiting nutrients nitrogen, phosphorus and silicate. Already in the 1930's, it was suggested that this low productivity was due to an iron-deficit.[1]

This was particularly noticeable around the Galapagos Islands, where areas down-wind (or down-current) of land were green and supported life, whereas those up-wind were clear blue, and 'dead'. The aim of iron-fertilisation is to mimic the productive, nutrient-rich coastal zones in the open ocean.

Research to date indicates that in 40% of the world's ocean, iron concentrations are so low that the production of phytoplankton is iron limited. This offers vast areas for potential iron-fertilisation.

Palaeo-environmental Records

Both geological records and recent experimentation support the concept of iron-deficiency limiting phytoplankton production.

Analysis of ocean sediment cores from the past 180,000 years suggest that ice ages were preceded by unusually-high levels of ocean iron, usually attributed to wind-blown dust from dry continents. The inference is that this terrigenous iron rectified the natural oceanic iron-deficit, and resulted in phytoplankton blooms. This phytoplankton used carbon dioxide from the surface waters, which resulted in a greater draw-down from the atmosphere. 'It is believed' that this drawdown of the greenhouse gas was so great that it led to global cooling, and ice ages.