These iron-enrichment experiments engendered considerable interest both inside and outside the scientific community. They raised the possibility of bioengineering of the oceanic ecosystem as a palliative measure against rising levels of fossil fuel CO2 by the controlled addition of iron to HNLC areas of the ocean. This became known as the 'Geritol fix', named after a popular iron-containing tonic for 'tired blood' that was popular many decades ago.
Initially, this idea looked promising. Calculations based on the biological response in these enrichment experiments indicated that a single Fe atom could theoretically initiate the uptake of many thousands of CO2 molecules. This means that to sequester billions of tonnes of fossil fuel carbon (the current global input from fossil fuels) might only require a few million tonnes per year of iron. This is a very small fraction of the total amount of iron smelted each year. Iron is, after all, an extremely abundant element in the Earth's crust. It has even been reported that John Martin quipped ''Give me half a tanker of iron and I'll give you an ice age.''
However, things are not that simple. It is not sufficient for iron enrichment to stimulate new CO2 uptake through additional phytoplankton growth. It is also necessary for the biological carbon sequestered in this way to survive respiration long enough to sink out of the mixed layer, thus removing the sequestered carbon from the ocean-atmosphere system. Once 'pumped' into deep water in this manner, the sequestered carbon will not return to equilibrate with the atmosphere for 1000-2000 years, the turnover time of the deep water circulation system.
Thus, experiments were conducted to measure the flux of biological carbon sinking into deep water as a result of iron enrichment, and here things began to look less promising. In the initial IronEx experiment, some sinking of carbon into deep water was observed, but this may have merely been a result of subduction of the water itself. In the remaining experiments, especially during SOIREE, no increase in the flux of biological carbon to deep water was observed. This was in spite of the fact that the majority of organisms that bloomed were relatively large diatoms.
In reality, the sinking of biological carbon into deep water is a much more complex process because an entire food web is involved. Much of the carbon flux is mediated by grazing zooplankton which produce large, rapidly settling fecal pellets. During artificial experiments like SOIREE, the principal grazers of the large diatoms are probably not very abundant before iron infusion stimulates the rapid growth in numbers of their prey, and the predominant grazers may have been too small to take advantage of a bloom of very large phytoplankton. However, if iron infusions were carried out on a semi-continuous basis, who knows what permanent changes to the food web might be induced? Although this offers tantalizing benefits for mitigating climate change, it does seem to be a very dangerous experiment.
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