Iron Fertilization In The Oceans

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There is ample evidence to support the notion that Carbon Dioxide (CO2) is the main driver of climate change, mostly due to the fact that Greenhouse Gases contribute to radiative forcing and the enhanced greenhouse effect; as a result of man-made emissions, mostly CO2, the earth presently has an energy imbalance which contributes additional warming of the surface (Meunier, 2007). This makes reducing and controlling the levels of atmospheric CO2 by enhancing sinks as well as reducing sources a growing concern, particularly as emissions of CO2 have been rapidly increasing since the Industrial Revolution (Figure 1) and CO2 can reside in the atmosphere for thousands of years (Lacis et al, 2010).Figure 1: Graph to show the atmospheric concentrations of Greenhouse Gases from year 0 to year 2005. The red line represents Carbon Dioxide (CO2). Increases since 1750 can be attributed to the Industrial Revolution. (Source: Forster et al, 2007)

The Ocean sink

1.1 The Ocean Sink

The oceans have absorbed approximately 30% of anthropogenic CO2 since 1750 and therefore have a major role as a CO2 sink (Raven and Falkowski, 1999). As a result of the 'biological' and 'solubility' pumps which move carbon to lower levels of the ocean from the surface waters, they allow CO2 to be drawn down from the atmosphere (Houghton, 1997:28). The 'biological pump' relies on phytoplankton; the principle autotrophs, as they absorb dissolved inorganic carbon (DOC) through photosynthesis. Some of this is transferred to fish through the food chain, and some used to build biomass in the form of particulate organic carbon (POC). Once the phytoplankton and fish die, the calcium carbonate shells of phytoplankton are left behind which fall into the deep ocean and transfer carbon; the biological pump.The carbon uptake by phytoplankton is thought to be limited by iron availability.

This is partly based on evidence from paleo-climate records that show biological activity during glacial periods was stimulated by terrestrially derived iron blown to the oceans; suggested by John Martin as the 'iron hypothesis' (Blain et al, 2007; Boyd et al, 2000). Based on this, geo-engineering approaches to enhance the 'biological pump' have been trialled in recent years.

2. Iron Fertilization

It has been proposed that iron fertilization of the oceans could lead to increased carbon sequestration by increasing the growth of phytoplankton and algal photosynthesis, consequently increasing the drawdown of POC (Watson et al, 2000; Barker et al, 2007); laboratory experiments suggested 30,000 to 110,000 tons of carbon could be removed from the air with every ton of iron added to the ocean (WHOI, 2007). It has also been identified that the production of the chemical dimethylsulfide might increase with plankton growth, which encourages cloud formation. Thus iron fertilization could provide a potential climate change mitigation strategy by reducing the gases which contribute to a warming atmosphere and increasing cloud cover, helping to cool the atmosphere.As a result, eleven small-scale field studies have been undertaken since the early 1990's to comprehend how phytoplankton respond to inputs of iron, by adding small amounts of iron sulphate to the surface water (Barker et al, 2007) in high-nitrate, low-chlorophyll (HNLC) regions; the Equatorial Pacific Ocean, the Subarctic Pacific Ocean and the Southern Ocean (Wong et al, 2006).

2.1 Advantages and Successes

The potential for iron fertilization has been illustrated by many experiments. Blain et al (2007) noted, in observations of natural iron fertilization in the Southern Ocean, that a large phytoplankton bloom was sustained over the Kerguelen plateau by an iron supply from iron rich deep-water. Similarly, artificial iron fertilization experiments have found positive results where phytoplankton blooms were enhanced which prove the 'iron hypothesis' theory.Busseler et al (2004) discovered in the SOFeX experiment that the flux of POC was observed to increase within the iron-fertilized waters, and also impacted upon the mixed layer of water below (Figure 2). At the end of the 28 day experiment, fluxes of POC IN were greater than POC OUT; opposite to what was happening at the start where POC OUT was greater than POC IN.

This was most noticeable in the deeper water column (100m), and therefore it could be suggested that this was a success as more CO2 is being drawn down into the ocean from the atmosphere. However, it should be noted that the magnitude of the flux in comparison to the natural blooms in the region was small, and thus it would likely be insignificant in terms of the global carbon budget.Figure 2: POC flux data from SOFeX, measured at 50m and 100m. Fluxes are shown for 'IN' and 'OUT' for the start and end of the experiment (duration of 28 days). (Source: Buesseler et al, 2004).The success of these experiments has led to policy proposals and commercial endeavours to use such techniques to enhance carbon sequestration (Trick et al, 2010), especially in the interest of selling carbon offset credits. For example, California based company CLIMOS has attracted investment for selling carbon offsets from iron fertilization.

However, the Kyoto Protocol's Clean Development Mechanism does not identify ocean fertilization as a mechanism for creating carbon credits for international trade, and therefore carbon credits would probably have to be traded on the voluntary carbon market (Strong et al, 2009) where individuals or companies with no obligations under the Kyoto Protocal can buy credits to, for example, enhance their company image. This could make about $3 per ton of carbon dioxide equivalent (WHOI, 2008) and subsequently could provide good profits for companies like CLIMOS were it to be successful.

2.2 Potential Problems and Consequences

Where iron fertilization may offer a potential climate change mitigation strategy by stimulating phytoplankton growth, and thereby sequestering CO2 in the form of particulate organic carbon, there are still many uncertainties, problems and potential environmental impacts which have been identified with this technique which limits its potential for success.

2.2.1 Uncertainties

Firstly, the transport of phytoplankton biomass to lower depths below the main thermocline, and therefore its efficiency to reduce atmospheric CO2, has not been explored in much detail. One experiment by Boyd et al (2000) did note, through the aid of satellite imagery, that a significant proportion of iron remained in the surface waters which suggests that sequestration of carbon by phytoplankton is not very efficient; much of the biomass is moved back into the upper water column.

This therefore brings concern as to whether, with the addition of iron, export of carbon to deeper waters will occur to allow more carbon to be drawn down at the surface and suggests that estimates of the amounts of carbon which could be sequestered could be over stated (Barker et al, 2007).Boyd et al (2000) also alleged it is difficult to confirm the iron hypothesis, as iron availability, light availability and silicate availability could, in combination or separately, all exert controls on phytoplankton growth in the Southern Ocean and thus iron supply may not be the only mechanism to enhance primary productivity. Suggestions that Krill can act as a long-term reservoir of iron have also been made, as they eat diatoms, and Baleen whales eat Krill; a study indicated high iron content in seven species of Krill and muscle tissue from two Baleen whales.

The study suggests that Krill can store iron in their bodies, and that Antarctic Krill could hold 24% of the total iron in the surface waters. This suggests that an alternative process of enhancing iron in Southern Ocean waters could be to allow whales, which have previously suffered exploitation, to recover to enhance iron levels naturally in surface waters (Nicol et al, 2010).

2.2.2 Economic Costs

Models have also advocated that even if this method does sequester some CO2, it would be nowhere near enough to balance the CO2 from anthropogenic emissions; if the Southern Ocean were constantly fertilized with enough iron to eliminate limitations to phytoplankton growth, it would still only sequester less than 1 of the 8 gigatonnes of carbon a year which is currently produced through emissions (Strong et al, 2009) and this emissions figure is expected to continue to rise. Consequently, there would be little economic benefit in this method as the impact on global carbon budgets would be small compared to the costs, and it may turn out to be more economically feasible to cut down on emissions to the atmosphere as opposed to storing them in the oceans, using such methods as building more renewable energy sources.Alongside this, ocean fertilization can only be deployed in 30% of the world's oceans; those HNLC oceans which are iron deficient (Barker et al, 2007).

Therefore, the effect would be very small, particularly in the future with rising emissions and the need for even more carbon to be sequestered. This would mean other techniques would need to be deployed in the other 70% of the oceans and over the land surface to make a more significant impact on the global carbon cycle and consequently to contribute to mitigating climate change. To add to this, the time period and geographical extent of effective carbon sequestration on the large-scale is poorly understand from the small-scale experiments (Watson et al, 2000), and so it could become a costly exercise if iron is needed to be consistently added to the waters.However, the economic cost is not the only concern.

There are also potential environmental impacts of ocean fertilization, which could impact on marine life and the marine food web.

2.2.3 Environmental Costs

Increasing organic carbon export as a result of fertilization could lead to an oxygen deficiency (anoxia) in the deeper ocean, which could give rise to outgassing of Nitrous Oxide and Methane through enhanced dentrification. These emissions could contribute more to the greenhouse effect as the increase in greenhouse gases could be more than the decrease from the drawdown of CO2 (Raven and Falkowski, 1999). This method consequently could deliver more greenhouse gases to the atmosphere than it sequesters and would thus be adding to the problem and not resolving it.

It could also cause changes elsewhere in the oceans, as the blooms would take up other nutrients as well as iron, such as nitrate and phosphate, which could potentially deplete neighbouring waters of essential nutrients for phytoplankton growth (WHOI, 2007). Alongside the lack of oxygen, this could be a threat to most marine organisms as it would subsequently affect the whole food chain (Strong et al, 2009) in the nearby and adjacent waters.Trick et al (2010) discovered that iron fertilization could furthermore threat marine life, as the addition of iron increases plankton of the genusPseudonitzschia which can produce toxic acid known as domoic acid, which rises in concentrations in conjunction with the algal 'blooms' (Vastag, 2010). This toxic acid accumulates in fish, which are immune to it, yet it is known to cause poisoning of mammals and birds in the waters such as sea lions and pelicans, of which fish are their main source of food (Black, 2010), so it could have a significant effect on the marine food chain.Nevertheless, none of these effects have been directly observed by the small-scale experiments (Barker et al, 2007) and therefore to understand the long-term side effects and uncertainties about the risks, more large-scale experiments would need to be conducted (Strong et al, 2009).

3. Conclusions

Iron fertilization has proven to be controversial; while in principle it could sequester some CO2 reducing the amount in the atmosphere and therefore reducing the level of greenhouse gases, the economic and environmental costs are uncertain and potentially severe and should therefore be explored before any large scale fertilization is undertaken. Whilst many studies have proven the iron hypothesis and consequently provided a good case for iron fertilization, it should still be considered with caution, especially given the potential environmental risks, and the possibility that phytoplankton growth could be controlled by other factors and iron concentrations could be regulated through other systems.The experiments which have been conducted so far have focused on how phytoplankton is stimulated by iron availability, and have not observed closely enough the much more essential results of exactly how much CO2 is absorbed and transferred into the deep ocean, or fundamentally, how long it can remain sequestered there (WHOI, 2008). Longer observations, larger scales, comparisons of amounts of iron addition, and deeper flux measurements are needed to reduce uncertainty and obtain greater understanding of how iron fertilization might affect the global carbon cycle before iron fertilization should be considered as a climate change mitigation strategy.

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