Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence—although each with important uncertainties—lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.
At a glance
- Hain, M. P. & Haug, G. H. The polar ocean and glacial cycles in atmospheric CO2 concentration. Nature 466, 47–55 (2010)
- Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science 323, 1443–1448 (2009) et al.
- Glacial-interglacial CO2 changes: the iron hypothesis. Paleoceanography 5, 1–13 (1990)
- Southern Ocean iron enrichment experiment: carbon cycling in high- and low-Si waters. Science 304, 408–414 (2004) et al.
- Mesoscale iron-enrichment experiments 1993–2005: synthesis and future directions. Science 315, 612–617 (2007) et al.
- The Southern Ocean biological response to aeolian iron deposition. Science 317, 1067–1070 (2007) et al.
- Volcanic ash fuels anomalous plankton bloom in subarctic northeast Pacific. Geophys. Res. Lett. 37, L19604 (2010) et al.
- Material supply to the abyssal seafloor in the Northeast Atlantic. Prog. Oceanogr. 50, 27–63 (2001) et al.
- Extensive phytoplankton blooms in the Atlantic sector of the glacial Southern Ocean. Paleoceanography 21, PA1013 (2006) , , , &
- Role of marine biology in glacial-interglacial CO2 cycles. Science 308, 74–78 (2005) , , &
- 2009) . Geoengineering the Climate: Science, Governance and Uncertainty. RS policy document 10/09 (The Royal Society,
- Global observations of large oceanic eddies. Geophys. Res. Lett. 34, L15606 (2007) , , &
- Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin. Deep Sea Res. Part I Oceanogr. Res. Pap. 56, 15–31 (2009) , , , &
- Mixing in cyclonic eddies in the Antarctic Circumpolar Current. J. Mar. Res. 67, 1–23 (2009) , , &
- Mixed layer analysis of a mesoscale eddy in the Antarctic Polar Front Zone. J. Geophys. Res. 113, C05017 (2008) , , &
- Mesopelagic carbon remineralization during the European Iron Fertilization Experiment. Glob. Biogeochem. Cycles 22, GB1023 (2008) , , , &
- The oceanic phosphorus cycle. Chem. Rev. 107, 563–576 (2007) &
- Robotic observations of enhanced carbon biomass and export at 55° S during SOFeX. Science 304, 417–420 (2004) , , &
- A model of the formation of marine algal flocs by physical coagulation processes. Deep-Sea Res. 37, 1197–1211 (1990)
- The relationship between physical aggregation of phytoplankton and particle flux: a numerical model. Deep-Sea Res. A 39, 1085–1102 (1992) &
- Synthesis of iron fertilization experiments: from the iron age in the age of enlightenment. J. Geophys. Res. 110, C09S16 (2005) et al.
- Effect of natural iron fertilization on carbon sequestration in the Southern Ocean. Nature 446, 1070–1074 (2007) et al.
- Southern Ocean deep-water carbon export enhanced by natural iron fertilization. Nature 457, 577–580 (2009) et al.
- 171–232 (Taylor & Francis, 2002) in Oceanography and Marine Biology. An Annual Review (eds , & )
- Supplementary Information 1 (13.8M)
This file contains Supplementary Text and Data 1-5 (see contents). Each section also includes Supplementary Figures, Supplementary Tables and additional references.
- Supplementary Information 2 (6.1M)
This file contains Supplementary Methods, additional references, Supplementary Figures 1-7 and Supplementary Tables 1-3. This file was replaced on 20 July 2012, as the figures that appeared in the original file were incorrect.