Iron fertilization

Iron fertilization is the intentional introduction of iron to iron-poor areas of the ocean surface to stimulate phytoplankton production. This is intended to enhance biological productivity and/or accelerate carbon dioxide (CO
) sequestration from the atmosphere.

Iron is a trace element necessary for photosynthesis in plants. It is highly insoluble in sea water and in a variety of locations is the limiting nutrient for phytoplankton growth. Large algal blooms can be created by supplying iron to iron-deficient ocean waters. These blooms can nourish other organisms.

Multiple ocean labs, scientists and businesses have explored fertilization. Beginning in 1993, thirteen research teams completed ocean trials demonstrating that phytoplankton blooms can be stimulated by iron augmentation.[1] 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 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 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 be iron-deficient.[6] Little scientific discussion was recorded until the 1980s, when oceanographer John Martin renewed controversy on the topic with his marine water nutrient analyses. His studies supported Hart's hypothesis. These "desolate" regions came to be called "High Nutrient, Low Chlorophyll" (HNLC) zones.[6]

John Gribbin was the first scientist to publicly suggest that climate change could be reduced by adding large amounts of soluble iron to the oceans.[7] Martin's 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][8] drove a decade of research.

The findings suggested that iron deficiency was limiting ocean productivity and offered an approach to mitigating climate change as well. Perhaps the most dramatic support for Martin's hypothesis came with 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 oceans worldwide. This single fertilization event preceded an easily observed global decline in atmospheric CO
and a parallel pulsed increase in oxygen levels.[9]

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".[10] 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).[11]


There are two ways of performing artificial iron fertilization: ship based direct into the ocean and atmospheric deployment[12].

Ship based deployment

Trials of ocean fertilization using iron sulphate added directly to the surface water from ships are described in detail in the experiment section below.

Atmospheric sourcing

Iron-rich dust rising into the atmosphere is a primary source of ocean iron fertilization[13]. For example, wind blown dust from the Sahara desert fertilizes the Atlantic Ocean[14] and the Amazon rainforest[15]. The naturally occurring iron oxide in atmospheric dust reacts with hydrogen chloride from sea spray to produce iron chloride, which degrades methane and other greenhouse gases, brightens clouds and eventually falls with the rain in low concentration across a wide area of the globe[12]. Unlike ship based deployment, no trials have been performed of increasing the natural level of atmospheric iron. Expanding this atmospheric source of iron could complement ship-based deployment.

Iron Salt Aerosol

One proposed method to boost the atmospheric iron level is Iron Salt Aerosol[12]. By adding Iron(III) chloride into the troposphere, iron salt aerosol could increase natural cooling effects including methane removal, cloud brightening and ocean fertilization, helping to prevent or reverse global warming[12].


Martin hypothesized that increasing phytoplankton photosynthesis could slow or even reverse global warming by sequestering CO
in the sea. He died shortly thereafter during preparations for Ironex I,[16] 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] Thereafter 12 international ocean studies examined the phenomenon:

  • Ironex II, 1995[17]
  • SOIREE (Southern Ocean Iron Release Experiment), 1999[18]
  • EisenEx (Iron Experiment), 2000[19]
  • SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study), 2001[20]
  • SOFeX (Southern Ocean Iron Experiments - North & South), 2002[21][22]
  • SERIES (Subarctic Ecosystem Response to Iron Enrichment Study), 2002[23]
  • SEEDS-II, 2004[24]
  • EIFEX (European Iron Fertilization Experiment),[25] A successful experiment conducted in 2004 in a mesoscale ocean eddy in the South Atlantic resulted in a bloom of diatoms, a large portion of which died and sank to the ocean floor when fertilization ended. In contrast to the LOHAFEX experiment, also conducted in a mesoscale eddy, the ocean in the selected area contained enough dissolved silicon for the diatoms to flourish.[26][27][28]
  • CROZEX (CROZet natural iron bloom and Export experiment), 2005[29]
  • A pilot project planned by Planktos, a U.S. company, was cancelled in 2008 for lack of funding.[30] The company blamed environmental organizations for the failure.[31][32]
  • LOHAFEX (Indian and German Iron Fertilization Experiment), 2009[33][34][35] Despite widespread opposition to LOHAFEX, on 26 January 2009 the German Federal Ministry of Education and Research (BMBF) gave clearance for this fertilization experiment to commence. The experiment was carried out in waters low in silicic acid which was likely to affect sequestration efficacy.[36] A 900 square kilometers (350 sq mi) portion of the southwest Atlantic was fertilized with iron sulfate. A large phytoplankton bloom was triggered. This bloom did not contain diatoms because the site was depleted in silicic acid, an essential nutrient for diatom growth.[36] In the absence of diatoms, a relatively small amount of carbon was sequestered, because other phytoplankton are vulnerable to predation by zooplankton and do not sink rapidly upon death.[36] These poor sequestration results led to suggestions that fertilization is not an effective carbon mitigation strategy in general. However, prior ocean fertilization experiments in high silica locations revealed much higher carbon sequestration rates because of diatom growth. LOHAFEX confirmed sequestration potential depends strongly upon appropriate siting.[36]
  • Haida Salmon Restoration Corporation (HSRC), 2012 - funded by the Old Massett Haida band and managed by Russ George - dumped 100 tonnes of iron sulphate into the Pacific into an eddy 200 nautical miles (370 km) west of the islands of Haida Gwaii. This resulted in increased algae growth over 10,000 square miles (26,000 km2). Critics alleged George's actions violated the United Nations Convention on Biological Diversity (CBD) and the London convention on the dumping of wastes at sea which prohibited such geoengineering experiments.[37][38] On 15 July 2014, the resulting scientific data was made available to the public.[39]


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,[40] offsetting 1/6 of current levels of anthropogenic CO
emissions. These benefits have been called into question by research suggesting that fertilization with iron may deplete other essential nutrients in the seawater causing reduced phytoplankton growth elsewhere in other words, that iron concentrations limit growth more locally than they do on a global scale.[41][42]

Role of iron

About 70% of the world's surface is covered in oceans. The part of these where light can penetrate is inhabited by algae (and other marine life). In some oceans, algae growth and reproduction is limited by the amount of iron. 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.[43]

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
). Research 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,[44] 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".[45]

Therefore, small amounts of iron (measured by mass parts per trillion) in HNLC zones can trigger large phytoplankton blooms on the order of 100,000 kilograms of plankton per kilogram of iron. The size of the iron particles is critical. Particles of 0.5–1 micrometer or less seem to be ideal both in terms of sink rate and bioavailability. Particles this small are easier for cyanobacteria and other phytoplankton to incorporate and the churning of surface waters keeps them in the euphotic or sunlit biologically active depths without sinking for long periods.

Atmospheric deposition is an important iron source. Satellite images and data (such as PODLER, MODIS, MSIR)[46][47][48] combined with back-trajectory analyses identified natural sources of iron–containing dust. Iron-bearing dusts erode from 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.[49]

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%).[49][50][51] Several photo-chemical interactions with dissolved organic acids increase iron solubility in aerosols.[52][53] Among these, photochemical reduction of oxalate-bound Fe(III) from iron-containing minerals is important. 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).[54][55][56] Consistent with this, 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).[57][58][59][60]

Volcanic ash as an iron source

Volcanic ash has a significant role in supplying the world’s oceans with iron.[61] Volcanic ash is composed of glass shards, pyrogenic minerals, lithic particles and other forms of ash that release nutrients at different rates depending on structure and the type of reaction caused by contact with water.[62]

Increases of biogenic opal in the sediment record are associated with increased iron accumulation over the last million years.[63] In August 2008, an eruption in the Aleutian Islands deposited ash in the nutrient-limited Northeast Pacific. This ash and iron deposition resulted in one of the largest phytoplankton blooms observed in the subarctic.[64]

Carbon sequestration

Previous instances of biological carbon sequestration triggered major climatic changes, lowering the temperature of the planet, such as the Azolla event. Plankton that generate calcium or silicon carbonate skeletons, such as diatoms, coccolithophores and foraminifera, account for most direct 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 steadily falls thousands of meters below active plankton blooms.[65]

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 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 involves a variety of measurements, combining ship-borne and remote sampling, submarine filtration traps, tracking buoy spectroscopy and satellite telemetry. Unpredictable ocean currents can remove experimental iron patches from the pelagic zone, invalidating the experiment.

The potential of fertilization 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.[66] This quantity is comparable in magnitude to annual anthropogenic fossil fuels combustion of approximately 6 gigatonnes. The Antarctic circumpolar current region is one of several in which iron fertilization could be conductedthe Galapagos islands area another potentially suitable location.

Dimethyl sulfide and clouds

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 cloud cover. This may increase the albedo of the planet and so cause cooling—this proposed mechanism is central to the CLAW hypothesis.[67] This is one of the examples used by James Lovelock to illustrate his Gaia hypothesis.[68]

During 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 CO
uptake and that due to the ocean's albedo increase, however the amount of cooling by this particular effect is very uncertain.[69]

Financial opportunities

Beginning with the Kyoto Protocol, several countries and the European Union established carbon offset markets which trade certified emission reduction credits (CERs) and other types of carbon credit instruments. In 2007 CERs sold for approximately €15–20/ton COe
.[70] 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.[71] In August, 2010, Russia established a minimum price of €10/ton for offsets to reduce uncertainty for offset providers.[72] Scientists have reported a 6–12% decline in global plankton production since 1980.[43][73] A full-scale plankton restoration program could regenerate approximately 3–5 billion tons of sequestration capacity worth €50-100 billion in carbon offset value. However, a 2013 study indicates the cost versus benefits of iron fertilization puts it behind carbon capture and storage and carbon taxes.[74]

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 eventually (days to centuries) returns to the atmosphere, negating the original benefit.[75]

Advocates argue that modern climate scientists and Kyoto Protocol policy makers define sequestration over much shorter time frames. For example, trees and grasslands are viewed as important carbon sinks. Forest biomass sequesters carbon for decades, but carbon that sinks below the marine thermocline (100–200 meters) is 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 quantified. Creating phytoplankton blooms in iron-poor areas 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.

Fertilization advocates respond that 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).

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,[76] but two others found increases in phytoplankton.[77][78] A 2010 study of oceanic transparency since 1899 and in situ chlorophyll measurements concluded that oceanic phytoplankton medians decreased by ~1% per year over that century.[79]

Ecological issues

Algal blooms

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.[80]

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 open oceans (far from shore) where iron deficiency is substantial. Most coastal waters are replete with iron and adding more has no useful effect.[81]

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.[82]

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.[83] A 2010 study showed that iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas[84] which, the authors argue, raises "serious concerns over the net benefit and sustainability of large-scale iron fertilizations". Nitrogen released by cetaceans and iron chelate are a significant benefit to the marine food chain in addition to sequestering carbon for long periods of time.[85]

Ocean acidification

A 2009 study tested the potential of iron fertilization to reduce both atmospheric CO2 and ocean acidity using a global ocean carbon model. The study showed that an optimized regime of micronutrient introduction would reduce the predicted increase of atmospheric CO2 by more than 20 percent. Unfortunately, the impact on ocean acidification would be split, with a decrease in acidification in surface waters but an increase in acidification in the deep ocean.[86]

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Changing ocean processes

Micronutrient iron and ocean productivity

Ocean biomass carbon sequestration

Ocean carbon cycle modeling

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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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