Efficiency of carbon removal per added iron in ocean iron fertilization

de Baar et al.: Efficiency of ocean iron fertilization279for quantification of the carbon C stocks and turnoverrates are the same or similar, hence the inherent uncertaintieslikely are similar. On the other hand, the quantificationsof the Fe supply, stocks and turnover ratesare vastly different. For the in situ experiments, thetotal amount of added Fe is accurately known, but thefate of this Fe is very poorly quantified due to some 60to 75% being lost, somehow, somewhere. Budget estimationsof the fate of added Fe are difficult and onlyone budget assessment has been published (Bowie etal. 2001). For the natural fertilization experiments, thequantification of Fe supply from below sediments isdifficult and in KEOPS was shown to be far lower thandesirable.Ranges of estimatesThe oceans are dynamic regions, with much variabilityin time and place, where day-to-day variability ofthe weather (wind, insolation) is a major driver of variabilityof any one upper ocean plankton ecosystem. Inevery single fertilization experiment or natural fertilizationstudy, the complete 3D mapping every day ofall key variables cannot be achieved with merely 1 (orat most 3) ships and their shipboard observers. Foreach experiment the ensuing dataset, therefore, is verylimited in space, in time, and in a very restricted numberof key variables. Thus, we are only scratching thesurface. Nevertheless, for each study, the overarchingsynthesis article (e.g. Tsuda et al. 2003, 2007, Boyd etal. 2004, Coale et al. 2004, Blain et al. 2007) very cleverlycombines these limited observations into an overallcoherent interpretation, based on the underlyingmore specific reports (e.g. Buesseler et al. 2004, Nishiokaet al. 2005, Gerringa et al. 2008) of that 1 study.Previously (de Baar et al. 2005, Boyd et al. 2007) andin this article, an effort has been made to combinethese individual field projects (IronEx II throughCROZEX, KEOPS) into a synthesis, where inevitablyits conclusions carry the sum of all uncertainties of thesingle studies. Moreover, the various estimates as citedhere from work by others, as well as in this article, areall based on many inevitable assumptions, as requireddue to lack of data or knowledge, where a large numberof these assumptions are highly debatable, if notquestionable. In other words, due to the limiteddatasets, there is much room for alternative interpretationsand conclusions.CONCLUSIONSThe most reliable estimate of (C:Fe) large-diatoms-optimalis to date ~23 000 for in situ large Antarctic diatomswithin the Fe-replete SOFeX-South patch. This is ingood agreement with the diffusion limitation (de Baaret al. unpubl. data) of the rate of growth of largeAntarctic diatoms in shipboard and laboratory experimentsof incubations of single species diatoms in filteredultraclean natural Antarctic seawater (Timmermanset al. 2004). In the Fe-limited, suboptimal growthconditions of the open Antarctic Ocean, large diatomssurvive at very low, but likely persistent, rates ofgrowth and may have lower suboptimal intracellularC:Fe ratio values, from ~160 000 at out-patch stationsof SOFeX-South to ~227 000 in the ambient 0.09 nM Fesurface waters of Kerguelen Plateau.In the artificial Fe fertilization experiments, the 4estimates thus far of (C:Fe) gas-flux-efficiency range from 100to 1000 (Table 3). In the period after observation shipshave left, this efficiency may either increase ordecrease, due to either continued net community productionor a shift to final net community respiration,respectively.The estimates of carbon export efficiency at 100 mdepth range from 650 (SERIES) to 6648 (SOFeX-South)with an extrapolated value at 250 m of ~3300 forSOFeX-South (Tables 3 & 5). For EIFEX the export efficiencyat 150 m depth is estimated at ~2780 (Table 5).For these artificial fertilizations the ~75% immediateloss of added Fe may be corrected for by a factorof ~4 when assuming future or natural fertilization asa more stable Fe-organic complex. This wouldincrease the CO 2 drawdown efficiency in the 400 to4000 range and the various export efficiency estimatesto a range from ~2600 (SERIES) to ~26 600(SOFeX-South), or even ~100 000 when including thehigh estimate of SEEDS II based on an assumed verylarge 1000 km 2 patch size.For the artificial in situ Fe fertilization experiments,at least the amount of added Fe is accurately known,such that the uncertainty in the above estimations ofC:Fe efficiency is due solely to uncertainty in theassessment of the C budgets. For natural Fe fertilizationsthe supply of Fe also must be derived from fieldobservations, such that the derived supply of Fe has anuncertainty equal or larger than the uncertainty in theparallel estimation of the C budget. Therefore, the(C:Fe) export-efficiency of natural fertilizations has an inherentlylarger uncertainty than that of the artificial fertilizationexperiments.In the natural Fe fertilization at the Crozet Plateauthe (C:Fe) export-efficiency at 100 m depth was reported tobe ~17 200, within reported upper and lower estimatesof ~60 400 and ~5400, respectively (Pollard et al. 2008).In the natural Fe fertilization at the Kerguelen Plateau,the derived (C:Fe) export-efficiency at either 100 m or 200 mdepth ranged from ~8050 to ~526 000, largely dependingon the assumed or invoked Fe supply term.