Iron Hypothesis - The Hypothesis

The Hypothesis

The world's oceans are huge natural carbon dioxide sinks and represent the largest active carbon sink on Earth. Marine ecosystems are a critical factor in maintaining the oceanic reservoir of carbon, helping sequester large quantities of carbon dioxide away from the atmosphere. Phytoplankton use photosynthesis to fix inorganic carbon in the well-lit surface waters, and organic carbon is then passed through the food web, sustaining higher trophic levels including fish populations. Sinking of particles, vertical mixing of dissolved organic matter and vertical migration by zooplankton, transport a significant fraction of the organic carbon into the deep ocean. Here the carbon is almost entirely respired back to inorganic form by microbes. The biologically mediated transfer of carbon increases the deep ocean carbon pool, and reduces the carbon concentration in the surface ocean and the atmosphere. Known as the biological pump, this process may reduce atmospheric CO₂ concentrations by as much as a factor of two, relative to a hypothetical "dead" ocean.

The photosynthetic uptake and subsequent removal of carbon by the marine biota is controlled not by carbon availability, but by the supply and consumption of other essential nutrient elements, particularly nitrogen and phosphorus. According to the Iron Hypothesis, phytoplankton growth in some regions is limited by the availability of iron, and this prevents the full use of nitrogen and phosphorus, and limits the biological uptake of carbon. Addition of dissolved iron in these regions has been shown to stimulate phytoplankton growth, leading to increased uptake of nitrogen, phosphorus and carbon. The extra carbon biomass may increase the food supply to the marine consumers, including fish and other economically important organisms, or may sink into the deep waters, where it can be held away from the atmosphere for many hundreds to thousands of years.

The Iron Hypothesis has been considered as a means of global engineering, because a small amount of hematites (micrometre-sized iron particles) could potentially have a considerable effect on the atmosphere. "Give me a half a tanker of iron and I'll give you the next ice age," Martin once said jokingly. The reality may be much more modest: tests in 2002 suggested that between 10,000 and 100,000 carbon atoms are sunk for each iron atom added to the water, which means that it might be possible to sequester 1 billion tonnes of CO₂ for as little as 30,000 tonnes of iron. To put this number into perspective, this is about 10% of CO₂ humans have produced in year 2011 alone.

Furthermore, oceanographers realise that the amount of seeding has to be carefully controlled. Too large a bloom of phytoplankton may cause releases of methane, a potent greenhouse gas, and dimethyl sulfide (DMS), which would not be desirable. Furthermore, algae may be not the only beneficiaries of the added iron: in some ocean fertilization experiments the initial increase in algae biomass and uptake of atmospheric CO₂ was quickly reversed by increased bacterial activity, which recycles the carbon back into the surface water and thus reverses any previous uptake of CO₂ brought about by iron fertilisation.

It has been noted that iron seeding takes place naturally. Not only around estuaries where minerals are washed out by rivers, but also as a consequence of atmospheric dust deposition, particularly downwind of desert regions, and during volcanic eruptions.