Efficiency of carbon removal per added iron in ocean iron fertilization
de Baar, H.J.W.; Gerringa, L.J.A.; Laan, P.; Timmermans, K.R. (2008). Efficiency of carbon removal per added iron in ocean iron fertilization. Mar. Ecol. Prog. Ser. 364: 269-282. http://dx.doi.org/10.3354/meps07548
The major response to ocean iron fertilization is by large diatoms, which at Fe-replete ambient seawater show an optimum C:Fe elemental ratio of ~23 000 and a higher ratio of ?-160 000 or more under Fe-limited conditions. The efficiency of CO2 drawdown during the several weeks of artificial fertilization experiments with concomitant observations is in the range of 100 < (CO2:Fe) < 1000 and is unknown in direction (positive or negative) and magnitude in the period after observations. The efficiency of biogenic carbon export into deeper water layers ranges from ~650 < (C:Fe)export < ~25 000 for reported export depths in the 100 to 250 m range. Variations in ocean initial conditions and variable weather during an experiment cause this range of ~2 orders of magnitude. Approximately 75% of Fe added in fertilization experiments is lost very rapidly. Hence the above efficiencies can be multiplied 4-fold, to ~2600 < (C:Fe)export < ~100 000, for the sake of comparison with natural fertilization with Fe-organic complexes, which stabilize Fe in solution. Quantification of the Fe source of natural fertilization is difficult, leading to an export efficiency in ~2400 < (C:Fe)export < ~800 000 range. Due to severe under-sampling, the existing datasets of artificial experiments and natural fertilizations may allow a wider range of alternative assessments than reported here.
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