The discovery that marine algal blooms deposit organic carbon to the deep ocean answers some — but not all — of the questions about whether fertilizing such blooms is a viable strategy for mitigating climate change. See Article p.313
“Give me half a tanker of iron and I'll give you the next ice age,” is perhaps the best-known quote in ocean science. It comes from the late John Martin1, a leader in the study of iron and its role in sustaining productivity in the ocean. The quip refers to Martin's proposal that the addition of iron to the upper ocean could trigger algal blooms that would ultimately alter climate by sequestering atmospheric carbon dioxide as organic carbon in the deep ocean. Smetacek et al.2 have taken on the challenge of proving Martin's hypothesis experimentally, and on page 313 of this issue they report that carbon formed from iron-fertilized algal blooms does indeed sink to the deep ocean — the first time that this has been convincingly observed.
Productivity in many parts of the global ocean is limited by iron levels, as demonstrated through several studies3 in which the addition of iron to the upper ocean stimulated phytoplankton blooms and greatly increased CO2 uptake into surface waters through photosynthesis. But for ocean iron fertilization (OIF) to have an impact on Earth's climate, organic carbon produced by the phytoplankton must be transported to the deep ocean where it cannot readily re-exchange with the atmosphere — this is the key event in Martin's ice-age-inducing scheme. Proving Martin's iron hypothesis therefore requires the fate of blooms to be followed.
This is what Smetacek et al. have done as part of the European Iron Fertilization Experiment (EIFEX). By tracking phytoplankton biomass using several methods, the authors demonstrated that at least half of the carbon captured by the algal bloom in their OIF experiment sank to depths well below 1,000 metres, some of which is likely to have reached the sea floor. Their findings help to inform us about how the oceans regulate atmospheric CO2, and provide further input to the debate into whether the oceans can, or should, be deliberately modified using OIF to mitigate the effects of climate change — an example of a practice known broadly as geoengineering.
OIF experiments are challenging because the waters used in such studies cannot usually be separated from the rest of the ocean in the way that laboratory experiments can be constrained by beakers. To overcome this problem, Smetacek et al. used an ocean eddy near Antarctica as a 'beaker' (Fig. 1). This solution seems to work well — the authors provide considerable evidence that the upper and lower layers of the eddy moved together coherently, and that the eddy had exchanged less than 10% of its content with the surrounding ocean by the end of the experiment.
The authors introduced dissolved iron(II) sulphate (FeSO4) over a 167-km2 patch in the eddy's core, so that the concentration of iron at the ocean's surface reached a level known to stimulate phytoplankton growth. The consequences were substantial: phytoplankton biomass more than doubled in 24 days, with 97% of the observed increase in chlorophyll associated with large diatoms, a class of phytoplankton that has high iron requirements. Along with this growth, the authors observed a reduction in levels of dissolved inorganic carbon (DIC) and of several nutrients (nitrogen, phosphorus and silicon). Data collected from stations outside the eddy, used as controls to monitor non-fertilized conditions, showed no such effects.
The scientists kept up their study for a full 37 days — longer than any other OIF experiment — and so were able to document the collapse of the diatom bloom through the formation of rapidly sinking aggregates of dead phytoplankton and zooplankton faecal pellets that carried carbon to the deep ocean. The last 13 days of observations were crucial to their success, because they enabled the authors to calculate the depletion of dissolved and particulate carbon at the surface and subsequent increases in particulate organic carbon at depth. Such 'budgets' are notoriously tricky to close in OIF studies, because of the difficulty in quantifying carbon losses that occur through air–sea gas exchange and physical mixing at the fertilized patch's boundaries, and because it is hard to account for variability in carbon levels within and outside the patch. In this case, however, the combination of evidence was clear: the iron-induced diatom bloom led to the export and sequestration of about one mole of carbon per square metre of ocean surface, from the uppermost 100 metres of ocean. In fact, one of the methods used by the authors suggested that, at its peak, carbon flux was the largest ever recorded in the Southern Ocean.
The implications of these findings are several-fold. First, a measure of the efficiency of carbon export in the experiments can be obtained by dividing the amount of DIC removed from the upper 100 metres of ocean by the amount of iron added. This measure — the carbon/iron molar ratio — is crucial for geoengineering proposals, which must specify how much iron will be needed to affect climate. In the laboratory, the ratio can be 100,000 or more4. By contrast, the ratios reported in previous OIF experiments3 have been much lower, in part because iron uptake by plankton in the ocean is inefficient compared with that under laboratory conditions, but also because of differences in the amounts of iron and carbon that are recycled at the surface, or which sink to depth. Smetacek et al. report that the carbon/iron molar ratio in their long experiment was 13,000 — higher than in the previous OIF studies — and argue that this number would have increased further had they followed the bloom for longer.
Furthermore, the authors' results defied expectations5 that the availability of light would limit phytoplankton growth in their experiment. Phytoplankton grow in the 'mixed layer' of the ocean, the region in which the uppermost layers of the ocean are homogenized by wind and other physical effects; the mixed layer in Smetacek and colleagues' experiment was deep, extending down to 100 metres, where little light would penetrate. Comparison of Smetacek and colleagues' study with naturally occurring blooms6,7 in iron-rich waters near islands in the Southern Ocean also suggests that their experiment was similar to natural OIF events, and that higher sequestration was potentially possible.
Although the authors conclude that OIF does indeed sequester carbon in the deep ocean, questions remain about the possible unintended consequences of geoengineering. For example, OIF might cause undesirable effects, such as the production of nitrous oxide (a more potent greenhouse gas than carbon dioxide); oxygen depletion in mid-waters as algae decompose; or stimulation of a toxic algal bloom. And, as with all carbon-removal methods, OIF is no silver bullet for mitigating climate change. The ocean's capacity for carbon sequestration in low-iron regions is just a fraction of anthropogenic CO2 emissions, and such sequestration is not permanent — it lasts only for decades to centuries. However, humans have already embarked on an ocean geoengineering experiment through our energy practices (which are affecting climate and acidifying the seas), by fishing, and through our other uses of ocean resources.
Most scientists would agree that we are nowhere near the point of recommending OIF as a geoengineering tool. But many think8,9 that larger and longer OIF experiments should be performed to help us to decide which, if any, of the many geoengineering options at hand should be deployed. EIFEX certainly does not answer all of the questions about geoengineering, but by showing how the addition of iron to the ocean not only enhances ocean productivity, but also sequesters carbon, it is one of the best OIF studies so far.
Martin, J. H. in US Joint Global Ocean Flux Study Newsletter 1 (2), (US JGOFS Planning Office, Woods Hole Oceanographic Institution, 1990).
Smetacek, V. et al. Nature 487, 313–319 (2012).
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de Baar, H. J. W. et al. J. Geophys. Res. 110, C09S16 (2005).
Blain, S. et al. Nature 446, 1070–1074 (2007).
Pollard, R. T. et al. Nature 457, 577–580 (2009).
Buesseler, K. O. et al. Science 319, 162 (2008).
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Annual Review of Marine Science (2018)
Progress in Oceanography (2015)
Science of The Total Environment (2014)
Journal of Physics A: Mathematical and Theoretical (2013)
Bioresource Technology (2013)