Iron Fertilisation: A way out of the Acid Test?

Having seen so much destructive impacts of the ocean’s acid test so far, is there a way to mitigate the situation? The answer is YES, there is. One of the most well-known geoengineering solution to mitigate ocean acidification is Ocean iron fertilisation (OIF), which is originated from the ‘Iron hypothesis’ proposed by an oceanographer John Martin in 1990.

How does ocean iron fertilisation work?

Martin’s Iron hypothesis is based on his research on the last glacial-interglacial CO₂ changes. He argued that the high concentrations of atmospheric CO₂ during the last and Holocene interglacial, i.e. 280ppm pre-industrial level was due to the high deficiency of the element Iron in the Southern Ocean, hence reducing the potential CO₂ uptake by phytoplankton in the surface ocean via the ‘biological pump’.  In contrast, during the Last Glacial Maximum (LGM), atmospheric dust iron supplies were 50 times higher than that of the last interglacial period. This iron enrichment had enhanced phytoplankton growth, which had utilised large amount of upwelled macronutrients to stimulate the overall productivity in the Southern Ocean, leading to the total decrease in CO₂ during the LGM.

The current geoengineering technology, ocean iron fertilisation is therefore based on the principle of using iron to enhance the productivity of the ocean, so as to increase the anthropogenic CO₂ sequestration in the ocean interior. In order to investigate the effects, efficiency and feasibility of OIF, a number of small-scale field OIF experiments and modeling studies have been conducted in different high-nutrient, low-chlorophyll (NHLC) regions across the world’s oceans, including the large parts of the Southern Ocean, the eastern equatorial Pacific and part of the North Pacific (Cao and Caldeira, 2010).

Figure 1 Effects of Ocean Iron Fertilisation 

Figure 1 illustrates one of those OIF field experiments employed in the northeast Pacific Ocean. Red and yellow colour represents regions with high concentration of chlorophyll a, thus a high biomass of phytoplankton and vice versa for blue colour. Area (a) has not been enriched with iron while area (b) has. It is clearly shown that with iron fertilisation, biotic community in the ocean has shifted from cyanobacteria-dominated to diatom-based i.e. larger phytoplankton productivity (Armbrust, 2009).   

How effective and feasible is OIF?

The effectiveness and feasibility of this OIF technology is largely questioned by the scientific community. For instance, the above OIF experiment at NE Pacific Ocean has only generated a slight increase in organic carbon storage in deep water despite of the expected enhanced phytoplankton bloom, with most of the organic carbon consumed and recycled in the surface ocean (Armbrust, 2009). Modeling studies produced similar conclusions and projections with the use of OIF. According to the model simulations by Cao and Caldeira (2010), a globally sustained OIF could not diminish atmospheric CO₂ concentrations to below 833ppm or reduce the mean surface ocean pH change to less than 0.38 units; compared to 965ppm with 0.44 units reduction in pH under the IPCC A2 emission scenario (Figure 2). Ironically, it has been reported that iron fertilisation cannot even mitigate ocean acidification but acidify the deep ocean further. As OIF stimulates carbon sequestration at the deep ocean, it is very likely that deep ocean pH will be further reduced with this addition of carbon. This can pose undesirable acidic environment for marine organisms in the deep ocean to survive, although its extent and impacts are still not very well known.

Figure 2 Minor effects of OIF in ocean acidification mitigation

The associated environmental risks are even more alarming to scientists, especially the impacts on marine ecosystems. Silver et al. (2010) reported that high levels of amino acid neurotoxin domoic acid (DA) have been produced historically through the iron enrichment experiments. The high levels of DA have been released by the increase in a diatom genus Pseudo-nitzschia. Pseudo-nitzschia spp. are present in coastal harmful algal blooms (HABs) worldwide and are recognised as contaminants to a wide range of fauna – from invertebrates to marine birds and animals (Silver et al., 2010).

In addition, Denman (2010) have highlighted the possible side effects associated with this OIF technology. He suggested that the increase in remineralisation triggered by iron fertilisation results in increased denitrification and the production of nitrogen dioxide (N₂O), the third most anthropogenic greenhouse gases. Furthermore, models have projected a decrease in dissolved oxygen or even anoxic in wide areas of subsurface ocean caused by increased remineralisation during OIF, making marine organisms, especially calcifiers harder to adapt to the changed environment. This increase in remineralisation process also reduces available macronutrients returning to the surface ocean, which causes a reduction in productivity i.e. biological carbon exports and the complexity of marine ecosystems. As illustrated above, OIF tends to favour the growth of diatoms, which eventually dominates that of all other phytoplankton groups, e.g. one of the dimothylsulfonioproprionate (DMSP)-producing phytoplankton species. Several OIF field experiments have shown a short-term increase in surface ocean DMS concentrations, followed by a decrease relative to the concentration outside the experimental area. Dimethylsulfide (DMS) is an important gas to stimulate the formation of cloud condensation nuclei. Therefore, a reduction in DMSP-producing phytoplankton might cause undesirable climatic impacts.

Despite of all the above negative consequences and concerns of OIF, it might still be considered as a viable mitigation measure in the long run. A striking research produced by Blain et al. (2007) have concluded that the efficiency of natural iron fertilisation was at least ten times higher than previous estimates from short-term phytoplankton blooms induced by the short-term iron-addition experiments. However, it has been stressed that the ocean natural system is very sensitive to iron, in which the addition of iron e.g. from dust  has to be occurred slowly and continuously as supposed to adding in large amounts purposely in order to achieve effective carbon sequestration.  The effectiveness of OIF is also highly constrained by the availability of macronutrients in local and surrounding water to support remineralisation.

To conclude, in a very large extent, ocean iron fertilisation is not a viable approach in mitigating ocean acidification; it is both economically and environmentally unsound. Deep ocean carbon sequestration using OIF technology not only causes undesirable environmental consequences, it also acts as a catalyst to encourage more anthropogenic CO₂ emissions in the form of carbon credits (Cao and Calderia, 2010). How would you weigh up the benefits of removing 32ppm of CO₂ and reducing a maximum of 0.06 unit of ocean pH in 100 years with the costs of potential marine ecosystems lost and increased N₂O production? I believe that the answer is very apparent here.