Efficiency of carbon removal per added iron in ocean iron fertilization

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 similar to 23 000 and a higher ratio of similar to 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 similar to 650 < (C:Fe)(export) < similar to 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 similar to 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 similar to 2600 < (C:Fe)(export) < similar to 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 the similar to 2400 < (C:Fe)(export) < similar to 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.