How Much Iron?

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How Much Iron? A Rudimentary Examination of CO2 Sequestration via Artificial, Iron Induced, Phytoplankton Blooms By Ivan O'hero

Ever since 1993, when an experiment (the IronEx 1) verified John Martin’s hypothesis that in many regions of the ocean1 iron is the limiting nutrient of phytoplankton, the possibility of removing globally relevant amounts of atmospheric CO2 by forcing phytoplankton photosynthesis with added iron has had the tantalizing potential to mitigate global warming.i The possibility of a potential solution to global warming did not escape the more popular scientific publications;ii however, more recent experiments suggest that while Martin was correct in that added iron spurs phytoplankton productivity, the process actually sequesters very small amounts of CO2, because much of the photosynthesized carbon gets recycled by organisms that eat the phytoplankton, and eventually much is released back into the atmosphere.iii Nevertheless, experiments attempting to measure the efficacy of natural iron fertilization on carbon sequestration yielded results that show natural iron fertilization to be 18 to 1386 times more efficient at carbon sequestration than iron forcingiv (this wide range may be the result of inaccurate estimates of the natural iron supply). Assuming the most efficient environments for carbon sequestration can be artificially created, would dumping iron into the oceans become a viable approach to 1 High-Nutrient Low-Chlorophyl regions, or HNLC.

2 offsetting anthropogenic CO2 emissions? Or, has the possibility offered by iron fertilization merely created false hope, which will forestall meaningful change? These are questions I will attempt to answer.

The first IronEx experiment revolutionized oceanographic research in that it began an era in which scientists conducted experiments using the actual ocean environment as their laboratory.2 While confirming iron to be the limiting nutrient, the “geochemical response was relatively small.” However, Coale et. al. provide some hypotheses as to why they recorded smaller than expected response: rapid loss of iron from the system; subduction of the fertilized patch below 30 meters, limiting available light; zooplankton grazers consuming and then reëmitting the photosynthesized CO2. To test these hypotheses, Coale et. al. carried out IronEx II, again in the equatorial pacific.v A total of 449 kilograms of iron (as acidic ironsulphate) was injected into the ocean over a one week period and over an initial area of 72km2 (eventually spread to 120km2 by day seventeen, but remained cohesive). This time the fertilized patch was not subducted and the results were more encouraging; according to Coale et. al. “it is now time to regard the 'iron hypothesis' as the 'iron theory.'” While zooplankton grazers again hampered the growth of smaller phytoplankton (resulting in only a doubling in biomass of these species), the biomass of larger diatoms increased 85 times.

Ultimately, one wants to know the quantity of CO2 sequestered relative to the added iron, i.e. the export efficiency. Altogether, Coale et. al. estimate that the 2 IronEx 1, conclusion

3 iron fertilization resulted in the drawdown from the mixed layer (25m initially deepening to 50m on day eleven) of between 5 and 12 µmol of carbon/liter. If one assumes that this carbon was eventually sequestered, then one can use the upper and lower values for the area and depth of the fertilized patch, and upper and lower carbon export estimates to calculate a bounded estimate of the total carbon exported: between 9•106 and 7.2•107 moles, which equates roughly to between 396 and 3169 metric tons of CO2.3 While initially these results may seem incredibly inaccurate, given the ranges for our input variables (multiplying the range factors for each variable (i.e. the upper bound over the lower bound) one obtains 120/72*50/25*12/5=8), a factor of roughly eight in difference is not surprising. Coale et. al. infused 449 kilograms if iron into the ocean (in IronEx II), so the efficiency of the iron fertilization is between 882 and 7057 tons of exported CO2 per ton of infused iron.4 According to the United Nations Statistics Division, in 2004, the United States of America emitted roughly six-billion metric tons of CO2.vi To offset this emission by iron fertilization (using the efficiencies calculated above) would require between 8.50•105 and 6.79•106 metric tons of iron. According to the United States Geological Survey, in 2008 the United States produced, an estimated 3.56•107 tons of iron.vii Therefore, for the United States to completely offset its 2004 level emissions would require between 2.4 and 19 percent of its 2008 annual iron production. Clearly, at these efficiencies, iron fertilization does not sequester enough CO2 to be a feasible strategy for offsetting emissions. 3 Lower estimate is 72*.025*1000^4*10^-6*5*(12.01115 + 2*15.9994)/1000^2, upper is 120*.05*1000^4*10^6*12*(12.01115 + 2*15.9994)/1000^2 4 Obtained by computing 396/0.449 and 3169/0.449

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Although iron forcing remains inefficient, natural iron fertilization can attain dramatically higher export efficiency, as evidenced by the Kerguelen plateau (about 3,000km southwest of Australia).viii Although quantifying the iron supply was difficult, Blain et. al. estimate that the export efficiency (i.e. ratio of carbon sequestered to iron added, measured in moles per moles) above the Kerguelen plateau during natural, iron induced phytoplankton blooms lies somewhere between 2.4•104 and 6.6•105. Converting these efficiencies into mass ratios (by multiplying by the ratio of the molar mass of carbon to that of iron) one obtains that the mass of exported CO2 lies between 5.2•103 and 1.4•105 times the mass of added iron. Using these efficiencies, to offset the United States' 2004 emission of six-billion metric tons of CO2 would require between 1.2•106 and 4.2•104 metric tons of iron, or between 0.12 and 3.2 percent of the 2008 production. One should bear in mind that these numbers arose from assuming iron forcing could be made just as efficient as natural iron fertilization, and that phytoplankton would continue to respond to the added iron.

Both scenarios examined here assumed that once the CO2 was sequestered (below 25-50m for IronEx II, 200m for the Kerguelen plateau example) it remained there, and that the phytoplankton would continue to respond linearly to added iron. Additionally, the environmental impact of mining and processing the quantity of iron involved in both situations was disregarded, as well as the possible environmental ramifications of inducing such large scale phytoplankton

5 blooms. Further, the CO2 emission estimate used was for 2004. Clearly each assumption creates a more favorable scenario for iron induced CO2 sequestration. Therefore, unless scientists manage to drastically improve current fertilization techniques, obtaining efficiency levels equal to or better than the most efficient natural fertilizations, seeding the oceans with iron remains a most impractical means of sequestering CO2.

i Coale, K. H., K. S. Johnson, S. E. Fitzwater, S. P. G. Blain, T. P. Stanton, T. L. Coley (1998)  IronEx-I, an in situe iron-enrichment experiment: Experimental design, implementation  and results. Deep-Sea Research Part II, 45, 919-945. ii Gas Guzzlers, by Deborah Franklin. Smithsonian, Feb2004, Vol. 34 Issue 11, p25-26, 2p, 2c; (AN 12059640)

iii    Philip W. Boyd, Cliff S. Law, C.S. Wong, Yukihiro Nojiri, Atsushi Tsuda, Maurice Levasseur, Shigenobu Takeda, Richard Rivkin, Paul J. Harrison, Robert Strzepek, Jim Gower,R. Mike McKay, Edward Abraham, Mike Arychuk, Janet Barwell-Clarke, William Crawford, David Crawford, Michelle Hale, Koh Harada, Keith Johnson, Hiroshi Kiyosawa, Isao Kudo, Adrian Marchetti, William Miller, Joe Needoba, Jun Nishioka, Hiroshi Ogawa, John Page, Marie Robert, Hiroaki Saito, Akash Sastri, Nelson Sherry, Tim Soutar, Nes Sutherland, Yosuke Taira, Frank Whitney, Shau-King Emmy Wong & Takeshi Yoshimura, The decline and fate of an iron-induced

subarctic phytoplankton bloom. Nature, 428, 549-553 (2004)

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Raymond T. Pollard, Ian Salter, Richard J. Sanders, Mike I. Lucas, C. Mark Moore, Rachel A. Mills, Peter J. Statham, John T. Allen, Alex R. Baker, Dorothee C. E. Bakker, Matthew A. Charette, Sophie Fielding, Gary R. Fones, Megan French, Anna E. Hickman, Ross J. Holland, J. Alan Hughes, Timothy D. Jickells, Richard S. Lampitt, Paul J. Morris, Florence H. Ne´de´lec, Maria Nielsdo´ttir, He ´le`ne Planquette, Ekaterina E. Popova, Alex J. Poulton, Jane F. Read, Sophie Seeyave, Tania Smith, Mark Stinchcombe, Sarah Taylor, Sandy Thomalla, Hugh J. Venables, Robert Williamson & Mike V. Zubkov, Southern Ocean deep-water carbon export enhanced by

natural iron fertilization, Nature, 457, 577-580 (2009) v Kenneth H. Coale et. al., A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial pacific ocean. Nature, 383, 495-501 (1996) vi http://unstats.un.org/unsd/environment/air_co2_emissions.htm (accessed 5/12/09) vii http://minerals.usgs.gov/minerals/pubs/commodity/iron_&_steel/mcs-2009-feste.pdf (accessed 5/12/09)

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Ste´phane Blain, Bernard Que´guiner, Leanne Armand, Sauveur Belviso, Bruno Bombled, Laurent Bopp, Andrew Bowie, Christian Brunet, Corina Brussaard, Franc¸ois Carlotti, Urania Christaki, Antoine Corbie`re, Isabelle Durand, Frederike Ebersbach, JeanLuc Fuda, Nicole Garcia, Loes Gerringa, Brian Griffiths, Catherine Guigue, Christophe Guillerm, Ste´phanie Jacquet, Catherine Jeandel, Patrick Laan, Dominique Lefe`vre, Claire Lo Monaco, Andrea Malits, Julie Mosseri, Ingrid Obernosterer, Young-Hyang Park, Marc Picheral, Philippe Pondaven, Thomas Remenyi, Vale´rie Sandroni, Ge´raldine Sarthou, Nicolas Savoye, Lionel Scouarnec, Marc Souhaut, Doris Thuiller, Klaas Timmermans, Thomas Trull, Julia Uitz, Pieter van Beek, Marcel Veldhuis, Dorothe´e Vincent, Eric Viollier, Lilita Vong & Thibaut Wagener, Effect of natural iron fertilization on carbon sequestration in the Southern Ocean. Nature, 446,

1070-1075 (2007)