Efficiency of carbon removal per added iron in ocean iron fertilization

Abstract

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 CO 2 drawdown during the several weeks of artificial fertilization experiments with concomitant observations is in the range of 100 < (CO 2 :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 the ~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.