Iron fertilization

Iron fertilization is the intentional introduction of iron, an essential nutrient, to the upper ocean to stimulate the marine food chain, and/or to sequester carbon dioxide from the atmosphere . Fertilization supports the growth of marine phytoplankton blooms by physically distributing microscopic iron particles in otherwise nutrient-rich, but iron-deficient blue ocean waters. An increasing number of ocean labs, scientists and businesses are exploring it as a means to revive declining plankton populations, restore healthy levels of marine productivity and/or sequester millions of tons of CO2 to slow down global warming. Since 1993, ten international research teams have completed relatively small-scale ocean trials demonstrating the effect.

Fertilization also occurs when natural or artificial upwellings bring nutrient-rich deep-water up 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. In rare cases, fertilization can occur when weather carries soil long distances over the ocean.


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. 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 micronutrient that was limiting phytoplankton growth and overall productivity in these "desolate" regions, which came to be called "High Nutrient, Low Chlorophyll" (HNLC) zones.

Martin's famous 1991 quip at Woods Hole Oceanographic Institution, "Give me a half a tanker of iron and I will give you another ice age, 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. Martin hypothesized that increasing phytoplankton photosynthesis could slow or even reverse global warming by sequestering enormous volumes of CO2 in the sea. He died shortly thereafter during preparations for Ironex I , 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. Since then 9 other international ocean trials have confirmed the iron fertilization effect:

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 CO2 and a parallel pulsed increase in oxygen levels.


The 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/or 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 wind-driven dust storms from arid lands. This Aeolian dust contains 3~5% iron and its deposition has fallen nearly 25% in recent decades due to modern changes in land use and agricultural practices as well as increased greening of dry regions thanks to increasing levels of atmospheric CO2. (Arid zone grasses and vegetation now lose less water vapor through their stomata to absorb the same amount of carbon dioxide, and thus stay greener longer, reducing dust storm frequency and the amount of iron reaching the deep seas. Increasing sand desertification does little to compensate for this shortfall since sand is primarily silica with relatively low iron content.)

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 CO2). 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, 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 fixation ratio of nearly 300,000 to 1. Assuming that data is on a mass basis, then the normalized atomic ratio would be approximately: "380,000 C: 58,000 N: 3,600 P: 1 Fe".

In "desolate" HNLC zones, therefore, small amounts of iron (measured by mass parts per trillion) delivered either by the wind or a planned restoration program can trigger large responsive 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 micrometre 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.

Carbon sequestration

Plankton that generate calcium or silica 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.

Of the carbon-rich biomass generated by natural plankton blooms and fertilization events, half or more is generally consumed by grazing organisms (zooplankton, krill, small fish, etc.) but 20 to 30% sinks below 200 meters into the colder water strata below the thermocline. Much of this fixed carbon continues falling into the abyss as marine snow, 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 or more. (The surface to benthic depths cycling time for the entire ocean system is approximately 4000 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 sophisticated measurements. Methods currently in use include a combination of ship-borne and remote sampling, submarine filtration traps, tracking buoy spectroscopy, and satellite telemetry.

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 ultimately clouds. This may increase the albedo of the planet and so cause cooling.

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 CO2 uptake and that due to the ocean's albedo increase, however the amount of cooling by this particular effect is very uncertain.

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 sell for approximately €15~20/ton CO2e and European analysts project these prices will nearly double by 2012.

Since NASA scientists have reported a minimum 6~9% decline in global plankton production since 1980 (and other scientists report 10~12% losses), this suggests that a full-scale international plankton restoration program could regenerate approximately 3~5 billion tons of carbon sequestration capacity worth €75 billion or more in carbon offset value. Iron fertilization is a relatively inexpensive carbon sequestration technology compared to scrubbing, direct injection and other industrial approaches, and can theoretically generate these credits for less than €5/ton CO2e.. Given this potential return on investment, some carbon traders and offset customers are watching the progress of this technology with interest.


While many advocates of ocean iron fertilization see it as modern society's last best hope to slow global warming long enough to change our consumption patterns and energy systems, a number of critics have also arisen including some academics, deep greens and proponents of competing technologies who cite a variety of concerns.

Precautionary principle

Critics argue the precautionary principle as follows:-

We do not know the possible side-effects of large-scale iron fertilization. Not enough research has been done. We should not risk iron fertilization on the scale needed to affect global CO2 levels or animal populations. Creating blooms in naturally iron-poor areas of the ocean is like watering the desert: you are completely changing one type of ecosystem into another. Advocates argue that iron addition would help to reverse a supposed decline in phytoplankton, but this decline may not be real. While one study (Gregg and Conkright, 2002) reported a decline in ocean productivity between the period 1979–1986 and 1997–2000, another study (Antoine et al., 2005) found a 22% increase between 1979–1986 and 1998–2002. Gregg et al. 2005 also reported a recent increase in phytoplankton.

Advocates argue as follows.

Similar blooms have occurred naturally for millions of years with no observed ill effects and that not even trying to remedy these industrial impacts is far more irresponsible considering the known pace of increasing harm.


According to certain ocean iron fertilization trial reports, this approach may actually sequester very little carbon per bloom, with most of the plankton being eaten rather than deposited on the ocean floor, and thus require too many seeding voyages to be practical.

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

  1. Timing: none of the ocean trials had enough boat time to monitor their blooms for more than 27 days, and all their measurements are confined to those early weeks. Blooms generally last 60~90 days with the heaviest precipitation occurring during the last two months.
  2. Scale: most trials used less than 1000 kg of iron and thus created small blooms that were quickly devoured by opportunistic zooplankton, krill and fish that swarmed into the seeded region.
  3. Academic conservatism: having an obviously limited data set and unique sequestration criteria (see Sequestration Definitions below), many peer-reviewed ocean researchers are understandably reluctant to project or speculate upon the results their experiments might have actually achieved during the full course of a bloom.

Some ocean trials did indeed report remarkable results. According to IronEx II reports, their thousand kilogram iron contribution to the equatorial Pacific generated a carbonaceous biomass equivalent to one hundred full-grown redwoods within the first two weeks. Researchers on Wegener Institute's 2004 Eifex experiment recorded carbon dioxide to iron 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 CO2 range widely, from approximately two hundred thousand tons/year to over 4 million tons/year. Even in the latter worst case scenario, this only represents about 16 supertanker loads of iron and a projected cost of less than €20 billion ($27 Billion). Considering EU penalties for Kyoto non-compliance will reach €100/ton CO2e ($135/ton CO2e) in 2010 and the annual value of the global carbon credit market is projected to exceed €1 trillion by 2012, even the most conservative estimate still portrays a very feasible and inexpensive strategy to offset half of all industrial emissions.

Sequestration definitions

Critics: In ocean science, carbon is not considered removed from the system unless it settles to the ocean floor where it is truly sequestered for eons. Most of the organic and inorganic carbon that sinks beneath plankton blooms is dissolved and remineralized at great depths and will eventually be re-released to the atmosphere, negating the original effect.

Advocates: Ocean science does traditionally define "sequestration" in terms of sea floor sediment that is isolated from the atmosphere for millions of years. Modern climate scientists and Kyoto Protocol policy makers, however, define sequestration in much shorter time frames and recognize trees and even 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 or thousands of years, whether it is remineralized or not. Since deep ocean currents take so long to resurface, their carbon content is effectively "sequestered" by any terrestrial criterion in use today.

Ecological issues

Harmful Algal Blooms (HAB)

Critics: Some plankton species cause red tides and other toxic phenomena. How do we know what kind of plankton will bloom in these events? What will prevent toxic species from poisoning lagoons, tide pools and other sensitive ecosystems along our coasts? Highly increased and intensified Red Tide Blooms have been wreaking havoc on the west coast of Florida for the last ten years. The argument that red tide blooms cause no harm is ridiculous. Once the chain of a HAB gets started, no one knows how to end it. The Red Tide Bloom in Maine over the last three years is testament to this. Even though the water is too cold to be very favorable to red tide (K. Brevis), these Red Tide Blooms have flourished. When even harmless species of plankton die they decompose and this brings about a very bad situation like the giant and growing dead zone in the Gulf of Mexico.

Advocates: Most species of phytoplankton are entirely harmless, and indeed beneficial. Red tides and other harmful algal blooms are largely coastal phenomena and primarily affect creatures that eat contaminated coastal shellfish. Iron stimulated plankton blooms only work in the deep oceans where iron deficiency is the problem. Most coastal waters are replete with iron and adding more has no effect. Since all phytoplankton blooms last only 90~120 days at most, in the open ocean fertilized patches of any species will dissipate long before reaching any land.

Deep water oxygen depletion

Critics: 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 bloom could, therefore, render certain regions of the sea deep beneath it anoxic and threaten other benthic species.

Advocates: The largest plankton replenishment projects now being proposed are less than 10% the size of most natural wind-fed blooms. In the wake of major dust storms, many extremely vast natural blooms have been studied since the beginning of the 20th century and no such deep water dieoffs have ever been reported.

Ecosystem alterations

Critics: Depending upon the composition and timing of delivery, these iron infusions could preferentially favor certain species and alter surface ecosystems to unknown effect. Population explosions of jellyfish, disturbance of the food chain with a huge impact on whale populations or fisheries are cited as potential dangers.

Advocates: CO2-induced surface water heating and rising carbonic acidity are already shifting population distributions for phytoplankton, zooplankton and many other creatures on a massive scale.

If certain infusions or space/time coordinates do show asymmetrical selective impacts in certain regions, the effect is inherently constrained by the limited size and 90-day lifespan of each bloom. Only larger scale research will show if this is really a problem, what factors tilt the playing field, and/or whether this issue can be effectively addressed.

Conclusion and further research

Advocates say that using this technique to restore ocean plankton to recent known levels of health would help solve half the climate change problem, revive major fisheries and cetacean populations, and alleviate several other urgent ocean crises. Critics say global warming must be solved at the source, large scale iron fertilization experiments have never been attempted, the effects could be inadequate, and/or too little is known to press ahead.

Critics and advocates generally agree that most outstanding questions on the impact, safety and efficacy of ocean iron fertilization can only be answered by much larger studies. One pilot project planned by a U.S. company called Planktos was cancelled in 2008 after it was opposed by environmental groups.

See also


Changing ocean processes

Micronutrient iron and ocean productivity

Ocean biomass carbon sequestration

  • Oceanic Sinks for Atmospheric CO2, J.A. Raven and P.G. Falkowski, June 1999, Plant, Cell and Environment 22, No. 6.
  • Zooplankton Fecal Pellets, Marine Snow and Sinking Phytoplankton Blooms, Jefferson T. Turner, February 2002, Aquatic Microbial Ecology, Vol. 27 No. 1.
  • Phytoplankton and Their Role in Primary, New and Export Production, Paul Falkowski et al., 2003, Ocean Biogeochemistry, Chapter 4, Ed. Michael J.R. Fasham, Springer 2003.
  • Markels, M and R T Barber (2001) Sequestration of CO2 by Ocean Fertilization. Proc 1st Nat. Conf. on Carbon Sequestration, Washington, DC.

Ocean carbon cycle modeling

  • Carbon Dioxide Fluxes in the Global Ocean, Andrew Watson and James Orr, 2003, Ocean Biogeochemistry, Chapter 5, Ed. Michael J.R. Fasham, Springer 2003.
  • Three-Dimensional Simulations of the Impact of Southern Ocean Nutrient Depletion on Atmospheric CO2 and Ocean Chemistry, J.L. Sarmiento and J.C. Orr, December 1991, Limnology and Oceanography 36, No. 8.

Further reading



  • Fertilizing the Ocean with Iron - First article in a six part series from Woods Hole Oceanographic Institution's Oceanus magazine
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