Iron fertilization

For information on ocean fertilization schemes not involving iron, see Ocean fertilization.
An oceanic phytoplankton bloom in the South Atlantic Ocean, off the coast of Argentina covering an area about 300 miles (480 km) by 50 miles

Iron fertilization is the intentional introduction of iron to the upper ocean to stimulate a phytoplankton bloom. This is intended to enhance biological productivity, which can benefit the marine food chain and is under investigation in hopes of increasing carbon dioxide removal from the atmosphere. Iron is a trace element necessary for photosynthesis in all plants. It is highly insoluble in sea water and is often the limiting nutrient for phytoplankton growth. Large algal blooms can be created by supplying iron to iron-deficient ocean waters.

A number of ocean labs, scientists and businesses are exploring fertilization as a means to sequester atmospheric carbon dioxide in the deep ocean, and to increase marine biological productivity which is hypothesized by some to decline as a result of climate change. Since 1993, thirteen international research teams have completed ocean trials demonstrating that phytoplankton blooms can be stimulated by iron addition.[1] However, controversy remains over the effectiveness of atmospheric CO
sequestration and ecological effects.[2] The most recent open ocean trials of ocean iron fertilization were in 2009 (January to March) in the South Atlantic by project Lohafex, and in July 2012 in the North Pacific off the coast of British Columbia, Canada, by the Haida Salmon Restoration Corporation (HSRC).[3]

Fertilization also occurs naturally when upwellings bring nutrient-rich water to the surface, as occurs when ocean currents meet an ocean bank or a sea mount. This form of fertilization produces the world's largest marine habitats. Fertilization can also occur when weather carries wind blown dust long distances over the ocean, or iron-rich minerals are carried into the ocean by glaciers,[4] rivers and icebergs.[5]


Consideration of iron's importance to phytoplankton growth and photosynthesis dates back to the 1930s when English biologist Joseph Hart speculated that the ocean's great "desolate zones" (areas apparently rich in nutrients, but lacking in plankton activity or other sea life) might simply be iron deficient.[6] Little further scientific discussion of this issue was recorded until the 1980s, when oceanographer John Martin renewed controversy on the topic with his marine water nutrient analyses. His studies indicated it was indeed a scarcity of iron micronutrients that was limiting phytoplankton growth and overall productivity in these "desolate" regions, which came to be called "High Nutrient, Low Chlorophyll" (HNLC) zones.[6]

In an article in the scientific journal Nature (February 1988; 331 (6157): 570ff.), John Gribbin was the first scientist to publicly suggest that the upcoming greenhouse effect might be reduced by adding large amounts of soluble iron compounds to the oceans of the world as a fertilizer for the aquatic plants.

Martin's famous 1988 quip four months later at Woods Hole Oceanographic Institution, "Give me a half a tanker of iron and I will give you another ice age",[6][7] drove a decade of research whose findings suggested that iron deficiency was not merely impacting ocean ecosystems, it also offered a key to mitigating climate change as well.

Perhaps the most dramatic support for Martin's hypothesis was seen in the aftermath of the 1991 eruption of Mount Pinatubo in the Philippines. Environmental scientist Andrew Watson analyzed global data from that eruption and calculated that it deposited approximately 40,000 tons of iron dust into the oceans worldwide. This single fertilization event generated an easily observed global decline in atmospheric CO
and a parallel pulsed increase in oxygen levels.[8]


Martin hypothesized that increasing phytoplankton photosynthesis could slow or even reverse global warming by sequestering enormous volumes of CO
in the sea. He died shortly thereafter during preparations for Ironex I,[9] a proof of concept research voyage, which was successfully carried out near the Galapagos Islands in 1993 by his colleagues at Moss Landing Marine Laboratories.[6] Since then 9 international ocean studies have examined the fertilization effects of iron:


The maximum possible result from iron fertilization, assuming the most favourable conditions and disregarding practical considerations, is 0.29W/m2 of globally averaged negative forcing,[33] which is almost sufficient to reverse the warming effect of about 1/6 of current levels of anthropogenic CO

Role of iron

About 70% of the world's surface is covered in oceans, and the upper part of these (where light can penetrate) is inhabited by algae. In some oceans, the growth and reproduction of these algae is limited by the amount of iron in the seawater. Iron is a vital micronutrient for phytoplankton growth and photosynthesis that has historically been delivered to the pelagic sea by dust storms from arid lands. This Aeolian dust contains 3–5% iron and its deposition has fallen nearly 25% in recent decades.[34]

The Redfield ratio describes the relative atomic concentrations of critical nutrients in plankton biomass and is conventionally written "106 C: 16 N: 1 P." This expresses the fact that one atom of phosphorus and 16 of nitrogen are required to "fix" 106 carbon atoms (or 106 molecules of CO
). Recent research has expanded this constant to "106 C: 16 N: 1 P: .001 Fe" signifying that in iron deficient conditions each atom of iron can fix 106,000 atoms of carbon,[35] or on a mass basis, each kilogram of iron can fix 83,000 kg of carbon dioxide. The 2004 EIFEX experiment reported a carbon dioxide to iron export ratio of nearly 3000 to 1. The atomic ratio would be approximately: "3000 C: 58,000 N: 3,600 P: 1 Fe".[36]

Therefore, small amounts of iron (measured by mass parts per trillion) in "desolate" HNLC zones can trigger large phytoplankton blooms. Recent marine trials suggest that one kilogram of fine iron particles may generate well over 100,000 kilograms of plankton biomass. The size of the iron particles is critical, however, and particles of 0.5–1 micrometer or less seem to be ideal both in terms of sink rate and bioavailability. Particles this small are not only easier for cyanobacteria and other phytoplankton to incorporate, the churning of surface waters keeps them in the euphotic or sunlit biologically active depths without sinking for long periods of time.

Atmospheric deposition is an important iron source. Satellite images and data (such as PODLER, MODIS, MSIR)[37][38][39] combined with back-trajectory analyses have been used to identify sources of iron–containing dust. Iron-bearing dusts erode from the soil and are transported by wind. Although most dust sources are situated in the Northern Hemisphere, the largest dust sources are located in northern and southern Africa, North America, central Asia, and Australia.[40]

Heterogeneous chemical reactions in the atmosphere modify the speciation of iron in dust and may affect the bioavailability of deposited iron. The soluble form of iron is much higher in aerosols than in soil (~0.5%).[40][41][42] Several photo-chemical interactions with dissolved organic acids increase iron solubility in aerosols.[43][44] Among these, photochemical reduction of oxalate-bound Fe(III) from iron-containing minerals is important. The process is that the organic ligand forms a surface complex with the Fe (III) metal center of an iron-containing mineral (such as hematite or goethite). On exposure to solar radiation the complex is converted to an excited energy state in which the ligand, acting as bridge and an electron donor, supplies an electron to Fe(III) producing soluble Fe(II).[45][46][47] Consistent with this, several studies documented a distinct diel variation in the concentrations of Fe (II) and Fe(III) in which daytime Fe(II) concentrations exceed those of Fe(III).[48][49][50][51]

Volcanic ash as a source of iron

Large amounts of aeolian (wind deposited) sediment are deposited annually in the world’s oceans. These deposits have long been thought to be the main source of iron to the surface ocean, and therefore the main source of iron for biological productivity. Recent studies suggest that volcanic ash has a significant role in supplying the world’s oceans with iron as well.[52] Volcanic ash is composed of glass shards, pyrogenic minerals, lithic particles, and other forms of ash which release nutrients at different rates depending on structure and the type of reaction caused by contact with water.[53]

Murray et al. recently assessed the relationship between increases of biogenic opal in the sediment record with increased iron accumulation over the last million years.[54] In August 2008, an eruption in the Aleutian Islands, Alaska deposited ash in the nutrient-limited Northeast Pacific. There is strong evidence that this ash and iron deposition resulted in one of the largest phytoplankton blooms observed in the subarctic.[55]

Carbon sequestration

Main article: Carbon sequestration
Air-sea exchange of CO

Previous instances of biological carbon sequestration have triggered major climatic changes in which the temperature of the planet was lowered, such as the Azolla event. Plankton that generate calcium or silicon carbonate skeletons, such as diatoms, coccolithophores and foraminifera, account for most direct carbon sequestration. When these organisms die their carbonate skeletons sink relatively quickly and form a major component of the carbon-rich deep sea precipitation known as marine snow. Marine snow also includes fish fecal pellets and other organic detritus, and can be seen steadily falling thousands of meters below active plankton blooms.[56]

Of the carbon-rich biomass generated by plankton blooms, half (or more) is generally consumed by grazing organisms (zooplankton, krill, small fish, etc.) but 20 to 30% sinks below 200 meters (660 ft) into the colder water strata below the thermocline. Much of this fixed carbon continues falling into the abyss, but a substantial percentage is redissolved and remineralized. At this depth, however, this carbon is now suspended in deep currents and effectively isolated from the atmosphere for centuries. (The surface to benthic cycling time for the ocean is approximately 4,000 years.)

Analysis and quantification

Evaluation of the biological effects and verification of the amount of carbon actually sequestered by any particular bloom requires a variety of measurements, including a combination of ship-borne and remote sampling, submarine filtration traps, tracking buoy spectroscopy and satellite telemetry. Unpredictable ocean currents have been known to remove experimental iron patches from the pelagic zone, invalidating the experiment.

The potential of iron fertilization as a climate engineering technique to tackle global warming is illustrated by the following figures. If phytoplankton converted all the nitrate and phosphate present in the surface mixed layer across the entire Antarctic circumpolar current into organic carbon, the resulting carbon dioxide deficit could be compensated by uptake from the atmosphere amounting to about 0.8 to 1.4 gigatonnes of carbon per year.[57] This quantity is comparable in magnitude to annual anthropogenic fossil fuels combustion of approximately 6 gigatonnes. It should be noted that the Antarctic circumpolar current region is only one of several in which iron fertilization could be conductedthe Galapagos islands area being another potentially suitable location.

Dimethyl sulfide and clouds

Schematic diagram of the CLAW hypothesis (Charlson et al., 1987)[58]

Some species of plankton produce dimethyl sulfide (DMS), a portion of which enters the atmosphere where it is oxidized by hydroxyl radicals (OH), atomic chlorine (Cl) and bromine monoxide (BrO) to form sulfate particles, and potentially increase the cloud cover. This may increase the albedo of the planet and so cause cooling - this proposed mechanism is central to the CLAW hypothesis.[58] This is one of the examples used by James Lovelock to illustrate his Gaia hypothesis.[59]

During the Southern Ocean Iron Enrichment Experiments (SOFeX), DMS concentrations increased by a factor of four inside the fertilized patch. Widescale iron fertilization of the Southern Ocean could lead to significant sulfur-triggered cooling in addition to that due to the increased CO
uptake and that due to the ocean's albedo increase, however the amount of cooling by this particular effect is very uncertain.[60]

Financial opportunities

Since the advent of the Kyoto Protocol, several countries and the European Union have established carbon offset markets which trade certified emission reduction credits (CERs) and other types of carbon credit instruments internationally. In 2007 CERs sold for approximately €15–20/ton COe
.[61] Iron fertilization is relatively inexpensive compared to scrubbing, direct injection and other industrial approaches, and can theoretically sequester for less than €5/ton CO
, creating a substantial return.[62] In August, 2010, Russia established a minimum price of €10/ton for offsets to reduce uncertainty for offset providers.[63] Scientists have reported a minimum 6–12% decline in global plankton production since 1980.[34][64] A full-scale international plankton restoration program could regenerate approximately 3–5 billion tons of sequestration capacity worth €50-100 billion in carbon offset value. Given this potential return on investment, carbon traders and offset customers are watching the progress of this technology with interest.[65] However, a recent study indicates the cost versus benefits of iron fertilization puts it behind carbon capture and storage and carbon taxes.[66]

Multilateral reaction

The parties to the London Dumping Convention adopted a non-binding resolution in 2008 on fertilization (labeled LC-LP.1(2008)). The resolution states that ocean fertilization activities, other than legitimate scientific research, "should be considered as contrary to the aims of the Convention and Protocol and do not currently qualify for any exemption from the definition of dumping".[67]

An Assessment Framework for Scientific Research Involving Ocean Fertilization, regulating the dumping of wastes at sea (labeled LC-LP.2(2010)) was adopted by the Contracting Parties to the Convention in October 2010 (LC 32/LP 5).[68]

Sequestration definitions

Carbon is not considered "sequestered" unless it settles to the ocean floor where it may remain for millions of years. Most of the carbon that sinks beneath plankton blooms is dissolved and remineralized well above the seafloor and will eventually (days to centuries) return to the atmosphere, negating the original effect.

Advocates argue that modern climate scientists and Kyoto Protocol policy makers define sequestration in much shorter time frames. For example, they recognize trees and grasslands as important carbon sinks. Forest biomass only sequesters carbon for decades, but carbon that sinks below the marine thermocline (100–200 meters) is effectively removed from the atmosphere for hundreds of years, whether it is remineralized or not. Since deep ocean currents take so long to resurface, their carbon content is effectively sequestered by the criterion in use today.


While ocean iron fertilization could represent a potent means to slow global warming current debate raises a variety of concerns.

Precautionary principle

The precautionary principle (PP) states that if an action or policy has a suspected risk of causing harm, in the absence of scientific consensus, the burden of proof that it is not harmful falls on those who would take the action. The side effects of large-scale iron fertilization are not yet known. Creating phytoplankton blooms in naturally iron-poor areas of the ocean is like watering the desert: in effect it changes one type of ecosystem into another.

The argument can be applied in reverse, by considering emissions to be the action and remediation an attempt to partially offset the damage.

20th-century phytoplankton decline

While advocates argue that iron addition would help to reverse a supposed decline in phytoplankton, this decline may not be real. One study reported a decline in ocean productivity comparing the 1979–1986 and 1997–2000 periods,[69] but two others found increases in phytoplankton.[70][71] A study in Nature [2010] of oceanic transparency since 1899 and in situ chlorophyll measurements concluded that oceanic phytoplankton medians have indeed decreased by ~1% per year over the past century.[72]

Comparison to prior phytoplankton cycles

Fertilization advocates respond that similar algal blooms have occurred naturally for millions of years with no observed ill effects. The Azolla event occurred around 49 million years ago and accomplished what fertilization is intended to achieve (but on a larger scale).

Sequestration efficiency

Fertilization may sequester too little carbon per bloom, supporting the food chain rather than raining on the ocean floor, and thus require too many seeding voyages to be practical.[15][73] In the Lohafex experiment, a 2009 Indo-German team of scientists examined the potential of the south-western Atlantic to sequester significant amounts of carbon dioxide, but found few positive results.[74]

The counter-argument to this is that the low sequestration estimates that emerged from some ocean trials are largely due to these factors:

  1. Data: none of the ocean trials had enough boat time to monitor their blooms for more than five weeks, confining their measurements to that period. Blooms generally last 60–90 days with the heaviest "precipitation" occurring during the last two months.
  2. Scale: most trials used less than 1,000 kilograms (2,200 lb) of iron and thus created small blooms that were quickly devoured by opportunistic zooplankton, krill, and fish that swarmed into the seeded region.
  3. Location: some trials have been conducted in locations of the ocean where iron is not the only limiting nutrient. In the 2009 Lohafex experiment, it is believed the lack of carbon sequestration was caused by a shortage of silicic acid in the test area due to previous blooms in the area. Silicic acid is a crucial ingredient needed by Diatoms to grow their shells.[74]

Some ocean trials reported positive results. IronEx II reported conversion of 1,000 kilograms (2,200 lb) to carbonaceous biomass equivalent to one hundred full-grown redwoods within two weeks. Eifex recorded fixation ratios of nearly 300,000 to 1.

Current estimates of the amount of iron required to restore all the lost plankton and sequester 3 gigatons/year of CO
range widely, from approximately 2 hundred thousand tons/year to over 4 million tons/year. The latter scenario involves 16 supertanker loads of iron and a projected cost of approximately €20 billion ($27 billion).

Ecological issues

Algal blooms

Main article: Harmful algal bloom
A "red tide" off the coast of La Jolla, San Diego, California.

Critics are concerned that fertilization will create harmful algal blooms (HAB). The species that respond most strongly to fertilization vary by location and other factors and could possibly include species that cause red tides and other toxic phenomena. These factors affect only near-shore waters, although they show that increased phytoplankton populations are not universally benign.

Most species of phytoplankton are harmless or beneficial, given that they constitute the base of the marine food chain. Fertilization increases phytoplankton only in the deep oceans (far from shore) where iron deficiency is the problem. Most coastal waters are replete with iron and adding more has no useful effect.

A 2010 study of iron fertilization in an oceanic high-nitrate, low-chlorophyll environment, however, found that fertilized Pseudo-nitzschia diatom spp., which are generally nontoxic in the open ocean, began producing toxic levels of domoic acid. Even short-lived blooms containing such toxins could have detrimental effects on marine food webs.[75]

Deep water oxygen levels

When organic bloom detritus sinks into the abyss, a significant fraction will be devoured by bacteria, other microorganisms and deep sea animals which also consume oxygen. A large enough bloom could render certain regions of the sea deep beneath it anoxic and threaten other benthic species.However this would entail the removal of oxygen from thousands of cubic km of benthic water beneath a bloom and so this seems unlikely.

The largest plankton replenishment projects under consideration are less than 10% the size of most natural wind-fed blooms. In the wake of major dust storms, natural blooms have been studied since the beginning of the 20th century and no such deep water dieoffs have been reported.

Ecosystem effects

Depending upon the composition and timing of delivery, iron infusions could preferentially favor certain species and alter surface ecosystems to unknown effect. Population explosions of jellyfish, that disturb the food chain impacting whale populations or fisheries is unlikely as iron fertilization experiments that are conducted in high-nutrient, low-chlorophyll waters favor the growth of larger diatoms over small flagellates. This has been shown to lead to increased abundance of fish and whales over jellyfish.[76] A 2010 study shows that iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas [77] which, the authors argue, raises "serious concerns over the net benefit and sustainability of large-scale iron fertilizations". Whale feces have been referred to as "marine ecosystem engineers". Nitrogen released by cetacean species and iron chelate are a significant benefit to the marine food chain in addition to sequestering carbon for long periods of time.[78]

However, CO
-induced surface water heating and rising carbonic acidity are already shifting population distributions for phytoplankton, zooplankton and many other creatures. Optimal fertilization could potentially help restore lost/threatened ecosystem services.

Conclusion and further research

Critics and advocates generally agree that most questions on the impact, safety and efficacy of ocean iron fertilization can only be answered by much larger studies.

A statement published in Science in 2008 maintained that it would be

premature to sell carbon offsets from the first generation of commercial-scale OIF experiments unless there is better demonstration that OIF effectively removes CO2, retains that carbon in the ocean for a quantifiable amount of time, and has acceptable and predictable environmental impacts.[79]

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Changing ocean processes

Micronutrient iron and ocean productivity

Ocean biomass carbon sequestration

Ocean carbon cycle modeling

Further reading

Secretariat of the Convention on Biological Diversity (2009). Scientific Synthesis of the Impacts of Ocean Fertilization on Marine Biodiversity. Montreal, Technical Series No. 45, 53 pages




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