Iron fertilisation

Iron Hypothesis

The Iron Hypothesis was formulated by oceanographer John Martin, based on theories by Joseph Hart and first tested in 1993 and may be a long term solution to the rapid production of anthropogenic carbon dioxide. In 1988, Nature published the results of Martin's experiments as well as his speculations on climate change [1.].

Certain areas of the oceans have high levels of plant nutrients, such as nitrates, phosphates and carbonic acid. However, the phytoplankton does not grow as strongly as it should, given the plentiful supply of sunlight and plant food. It was discovered by John Martin that the lack of micronutrients, trace metals and particularly iron, was a limiting factor for growth of phytoplankton in HNLC (High Nutrient, Low Chlorophyll) ocean surface waters, much in the same way as a lack of vitamins (vital minerals) can cause illness and deficiencies in humans. Scientists Ken Johnson, Dick Barber and Kenneth Coale of the Moss Landing Marine Laboratories in Monterey Bay, California, were able to prove John Martin was correct, in a series of tests conducted near the Galapagos Islands in 1993 and again in 1995. Their results were published in Nature [3.]. By seeding or fertilizing the surface layer of the sea with fine particulates of iron, growth blooms of phytoplankton could be encouraged.

The world's oceans are huge natural carbon dioxide sinks [7.] and represent the largest active carbon sink on Earth. This oceanic sink for carbon dioxide (CO2) is driven by two processes, the solubility pump and the biological pump. The solubility pump is where atmospheric CO2 one of the so-called greenhouse effect gases, is washed out of the air by precipitation. The rainfall dissolves the gas and turns some of it into carbonic acid. In the natural circulation of the solubility pump, CO2 is also released back into the atmosphere - unless it is utilised by the biological pump. The biological pump is where biological organisms, mainly phytoplankton, metabolises and fixes the CO2 into carbohydrate through photosynthesis, which then enters the ocean food chain, the aquatic ecosystem. A 2006 study [16.] suggests that every day, more than a hundred million tons of carbon in the form of CO2 are fixed into organic material by phytoplankton in the euphotic zone and each day a similar amount of this now biological carbon is either grazed by other marine life or sinks to the sea floor as marine snow.

Anthropogenic production of CO2 has imbalanced the solubility pump by placing more CO2 in the atmosphere. About one third of this increase (approximately 2.2 Giga tonnes of Carbon per year) [7.]&[12.] is being dissolved in the oceans, leading to increased acidification and threatening ecosystems. The amount of CO2 that can be held in the oceans is dependent on the temperature and salinity of the water. Cold water holds more CO2 than warm water. Whilst deep cold water is able to hold vast quantities of dissolved CO2, this water does circulate around the world through the thermohaline circulation. As this deep water warms, when it nears the surface, it is less able to contain the dissolved gas and CO2 is released back into the atmosphere. It is said by some prominent oceanographers that the world's oceans have a climatic memory. That changes in CO2 release, are directly related to climatic changes hundreds of years before.

The biological pump also has a circulation cycle. Which can be positively influenced by the careful adjustment of growth limiting factors. Using the Iron Hypothesis and fertilization as a tool, it would be possible to encourage the biological pump to increase the extraction (from the solubility cycle) and fixation of additional anthropogenic carbon dioxide, into living structures. Any dead or deteriorated phytoplankton that was not consumed by other sea life (including bacteria), would eventually sink to the ocean floor as marine snow, where it would be sequestered, taking no further part in the active biological cycle. Some of this carbon is remineralised by bacteria and other sea life, however, for all practical purposes the sequestration is regarded as very long term.

Crude oil is believed to be the result of compression and heating of rocks containing prehistoric zooplankton and phytoplankton. So it is not without irony, that through the Iron Hypothesis and encouraging the growth of phytoplankton, carbon dioxide created by the anthropogenic emissions of fossil fuels, could be incorporated back into zooplankton and phytoplankton, which is then trapped on the ocean floor and could eventually, after considerable time as the result of compression of the floor sediments, become crude oil all over again. One might speculate that it may be possible to farm the phytoplankton blooms, collect the biomass and store it in depleted oil wells, where methanogenesis would not be a problem. Indeed, the production of methane under these circumstances would be a fringe benefit.

The Iron Hypothesis is elegant from a global engineering standpoint, because a small amount of hematites (micrometre-sized iron particles) could have a huge effect on the atmosphere. Tests in 2002 suggested that between 10,000 and 100,000 carbon atoms are sunk for each iron atom added to the water. With these figures in mind, one might believe it possible to sequester 1 billion tonnes of CO2 for as little as 30,000 tonnes of iron. "Give me a half a tanker of iron and I'll give you the next ice age," Martin once said jokingly. However, oceanographers realise that the amount of seeding has to be carefully controlled. Too large a bloom of phytoplankton and you could release methane and dimethyl sulfide (DMS), which would not be desirable. German reports in 2005 indicate that any biomass carbon in the oceans, whether trapped on the ocean floor or recycled in the euphotic zone, represents long term storage of carbon. The application of iron fertilisation in select parts of the oceans, at carefully controlled levels, could have the combined effect of restoring ocean productivity while at the same time correcting the negligent anthropogenic production of carbon dioxide.

It has been noted that iron seeding takes place naturally. Not only around estuaries where minerals are washed out by rivers, but also during volcanic eruptions. When Mount Pinatubo erupted in the Philippines in 1991, it ejected ten cubic kilometres of material, ten times more than Mount St. Helens. Volcanic ash containing trace metals, was spread by the winds over the world's oceans. It was reported that there was a noticeable increase in the levels of oxygen (O2) in the years that followed. The minerals were washed into the oceans, where the iron fertilised the phytoplankton, which enabled them to fix and metabolise the CO2 and release O2.

As a footnote, John Martin died of prostate cancer in 1993. But his legacy could be a way of ensuring the control of anthropogenic emissions, and ultimately a way of re-invigorating the oceans by global engineering, using the solubility pump, the biological pump and photosynthesis of the natural energy of the Sun, our greatest long term renewable energy source.

References

1. Iron-deficiency limits phytoplankton growth in the Northeast Pacific Subarctic. Martin J. H. and Fitzwater S. E., (1988) Nature 331, 341-343.

2. Iron as a limiting factor in oceanic productivity Martin J. H. (1992) Environmental Science Research, 43, 123-37.

3. Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean. Martin J. H., Coale K. H., Johnson K. S., Fitzwater S. E., Gordon R. M., Tanner S. J., Hunter C. N., Elrod V. A., Nowicki J. L., Coley T. L., Barber R. T., Lindley S., Watson A. J., Van Scoy K., Law C. S., Liddicoat M. I., Ling R., Stanton T., Stockel J., Collins C., Anderson A., Bidigare R., Ondrusek M., Latasa M., Millero F. J., Lee K., Yao W., Zhang J. Z., Friederich G., Sakamoto C., Chavez F., Buck K., Kolber Z., Greene R., Falkowski P., Chisholm S. W., Hoge F., Swift R., Yungel J., Turner S., Nightingale P., Hatton A., Liss P. & Tindale N. W. (1994) Nature 371, 123-129.

4. A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Coale K.H., Johnson K.S., Fitzwater S.E., Gordon R.M., Tanner S., Chavez F.P., Ferioli L., Sakamoto C., Rogers P., Millero F., Steinberg P., Nightingale P., Cooper D., Cochlan W.P., Landry M.R., Constantinou J., Rollwagen G., Trasvina A. & Kudela R. (1996) Nature, 383, 495-501.

5. Phytoplankton bloom on iron rations. Frost B.W.,(1996) Nature 383, 475-476.

6. Iron-limited diatom growth and Si:N uptake ratios in a coastal upwelling regime. Hutchins D.A. and Bruland K.W. (1998) Nature 393, 561-564.

7. Oceanic sinks for atmospheric CO2. Raven, J. A. and Falkowski P. G., (1999) Plant, Cell & Environment 22, 741–755.

8. Differential Response of Equatorial Pacific Phytoplankton to Iron Fertilization, Cavender-Bares, Kent K., Mann E. L., Chisholm S. W., Ondrusek M. E., and Bidigare R. R. (1999) Limnology and Oceanography, 44(2), 237–246.

9. Diatoms and the Ocean Carbon Cycle, Victor Smetecek, March 1999, Protist, Vol. 150 No. 1

10. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Boyd P.W., Watson A.J., Law C.S., Abraham E.R., Trull T., Murdoch R., Bakker D.C.E, Bowie A.R., Buesseler K.O., Chang H., Charette M., Croot P., Downing K., Frew R., Gall M., Hadfield M., Hall J., Harvey M., Jameson G., LaRoche J., Liddicoat M., Ling R, Maldonado M.T., McKay R.M., Nodder S., Pickmere S., Pridmore R, Rintoul S., Safi K., Sutton P., Strzepek R., Tanneberger K., Turner S., Waite A. and Zeldis J. (2000) Nature 407, 695-702.

11. Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2. Watson A.J., Bakker D.C., Ridgwell A.J., Boyd P.W. and Law C.S., (2000) Nature 407, 730-733.

12. Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Takahashi, T., S. C. Sutherland, C. Sweeney, A. Poisson, N. Metzl, B. Tilbrook, N. Bates, R. Wanninkhof, R. A. Feely, C. Sabine, J. Olafsson and Y. C. Nojiri (2002) Deep-Sea Research. Part. II 49, 1601-1622.

13. A mesoscale iron enrichment in the western subarctic Pacific induces a large centric diatom bloom. Tsuda A., Takeda S., Saito H., Nishioka J., Nojiri Y., Kudo I., Kiyosawa H., Shiomoto A., Imai K., Ono T., Shimamoto A., Tsumune D., Yoshimura T., Aono T., Hinuma A., Kinugasa M., Suzuki K., Sohrin Y., Noiri Y., Tani H., Deguchi Y., Tsurushima N., Ogawa H., Fukami K., Kuma K. and Saino T. (2003) Science 300, 958-961.

14. The decline and fate of an iron-induced subarctic phytoplankton bloom. Boyd P.W., Law C.S., Wong C.S., Nojiri Y., Tsuda A., Levasseur M., Takeda S., Rivkin R., Harrison P.J., Strzepek R., Gower J., McKay M., Abraham E., Arychuk M., Barwell-Clarke J., Crawford W., Crawford D., Hale M., Harada K., Johnson K., Kiyosawa H., Kudo I., Marchetti A., Miller W., Needoba J., Nishioka J., Ogawa H., Page J., Robert M., Saito H., Sastri A., Sherry N., Soutar T., Sutherland N., Taira Y., Whitney F., Wong S.K. and Yoshimura T. (2004) Nature 428, 549-553.

15. Iron Seeding Creates Fleeting Carbon Sink in Southern Ocean. Schiermeier Q., 2004 Nature, 428, 788.

16. Southern Ocean iron enrichment experiment: carbon cycling in high- and low-Si waters. Coale K.H., Johnson K.S., Chavez F.P., Buesseler K.O., Barber R.T., Brzezinski M.A., Cochlan W.P., Millero F.J., Falkowski P.G., Bauer J.E., Wanninkhof R.H., Kudela R.M., Altabet M.A., Hales B.E., Takahashi T., Landry M.R., Bidigare R.R., Wang X., Chase Z., Strutton P.G., Friederich G.E., Gorbunov M.Y., Lance V.P., Hilting A.K., Hiscock M.R., Demarest M., Hiscock W.T., Sullivan K.F., Tanner S.J., Gordon R.M., Hunter C.N., Elrod V.A., Fitzwater S.E., Jones J.L., Tozzi S., Koblizek M., Roberts A.E., Herndon J., Brewster J., Ladizinsky N., Smith G., Cooper D., Timothy D., Brown S.L., Selph K.E., Sheridan C.C., Twining B.S. and Johnson Z.I. (2004) Science 304, 396-397.

17. Role of Marine Biology in Glacial-Interglacial CO2 Cycles. Kohfeld K.E., Le Quéré C., Harrison S.P. and Anderson R.F. (2005) Science 308, 74 - 78.

18. Iron, phytoplankton growth, and the carbon cycle. Street J.H. and Paytan A. (2005) Metal ions in biological systems. 2005 43, 153-93.

19. Controls on tropical Pacific Ocean productivity revealed through nutrient stress diagnostics. Behrenfeld M.J., Worthington K., Sherrell R.M., Chavez F.P., Strutton P., McPhaden M. and Shea D.M. (2006) Nature 442, 1025-1028.

20. Climate-driven trends in contemporary ocean productivity. Behrenfeld M.J., O'Malley R.T., Siegel D.A., McClain C.R., Sarmiento J.L., Feldman G.C., Milligan A.J., Falkowski P.G., Letelier R.M. and Boss E.S. (2006) Nature 444, 695-696.

21. Globalizing results from ocean in situ iron fertilization studies Aumont O. and Bopp L. (2006) Global Biogeochemical Cycles, VOL. 20, GB2017, doi:10.1029/2005GB002591.

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