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In OIF experiments over decades, it was found that only a small fraction of the carbon fixed by the phytoplankton was transported to the deep ocean.

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Over the past several decades, a series of Ocean Iron Fertilization (OIF) experiments have been conducted to examine the potential of iron addition to stimulate phytoplankton blooms and subsequently enhance carbon sequestration in the ocean. The underlying hypothesis is that adding iron to iron-limited areas of the ocean can trigger phytoplankton growth, thereby increasing the capture of atmospheric CO2 through photosynthesis. The hope was that the organic carbon fixed by phytoplankton would then sink to the deep ocean, effectively acting as a carbon sink. However, detailed empirical data from these experiments revealed a different reality: a significant portion of the carbon fixed by the phytoplankton does not make it to the deep ocean, as the majority of the organic matter decomposes in the upper layers, releasing CO2 back into the ocean and atmosphere.

The following sections provide an overview of key OIF experiments and their findings, illustrating both the potential and the limitations of this geoengineering strategy.

Key OIF Experiments​

Numerous key OIF experiments have provided insights into the functioning and limitations of this geoengineering approach.

Sources​

  1. Martin, J.H., et al. "Testing the Iron Hypothesis in Ecosystems of the Equatorial Pacific Ocean." Nature, vol. 371, 1994, pp. 123-129.
  2. Boyd, P.W., et al. "A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization." Nature, vol. 407, 2000, pp. 695-702.
  3. Assmy, P., & Smetacek, V. "Enhanced carbon export due to silicate limitation in an iron-fertilized Southern Ocean diatom bloom." Deep-Sea Research Part I, vol. 56, no. 7, 2009, pp. 1017-1038.
  4. Tsuda, A., et al. "A mesoscale iron enrichment in the western subarctic Pacific induces a large centric diatom bloom." Science, vol. 300, no. 5621, 2003, pp. 958-961.
  5. Smetacek, V., et al. "Deep carbon export from a Southern Ocean iron-fertilized diatom bloom." Nature, vol. 487, 2012, pp. 313-319.
  6. Martin, P., et al. "The iron fertilization experiment LOHAFEX and its implications for ocean C and N cycles." Biogeosciences, vol. 10, 2013, pp. 4611-4623.
  7. Chisholm, S.W., et al. "Dis-crediting Ocean Fertilization." Science, vol. 294, no. 5541, 2001, pp. 309-310.
  8. Pollard, R.T., et al. "Southern Ocean deep-water carbon export enhanced by natural iron fertilization." Nature, vol. 457, 2009, pp. 577-580.

IronEx I and II (1993, 1995)​

Conducted in the Equatorial Pacific, IronEx I and II were among the first comprehensive studies to examine the effects of iron addition on phytoplankton growth. The results were promising in terms of phytoplankton bloom stimulation, but the findings also showed that the majority of the organic carbon produced was remineralized in the upper ocean layers, meaning that only a small proportion of the carbon was transported to the deep ocean.

Sources:

  • Martin, J.H., et al. "Testing the Iron Hypothesis in Ecosystems of the Equatorial Pacific Ocean." Nature, vol. 371, 1994, pp. 123-129.
  • Coale, K.H., et al. "IronEx-I, an in situ iron-enrichment experiment: Experimental design, implementation and results." Deep-Sea Research Part II: Topical Studies in Oceanography, vol. 45, no. 6, 1998, pp. 919-945. DOI.
  • Coale, K.H., et al. "A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean." Nature, vol. 383, 1996, pp. 495-501.

SOIREE (Southern Ocean Iron Release Experiment, 1999)​

The SOIREE experiment in the Southern Ocean further solidified the notion that iron addition can induce substantial phytoplankton blooms. However, similar to earlier findings, the export of carbon to the deep ocean was limited. Most of the organic matter generated decomposed in the upper ocean layers.

Sources:

  • Boyd, P.W., et al. "A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization." Nature, vol. 407, 2000, pp. 695-702.
  • de Baar, H.J.W., et al. "Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean." Nature, vol. 373, 1995, pp. 412-415.
  • Coale, K.H., et al. "Southern Ocean Iron Enrichment Experiment: Carbon Cycling in High- and Low-Si Waters." Science, vol. 304, no. 5669, 2004, pp. 408-414.

EisenEx (2000)​

Taking place in the Southern Ocean, EisenEx also demonstrated enhanced phytoplankton growth due to iron fertilization. Yet again, the recorded data indicated that only a modest fraction of the fixed carbon made it to the deep ocean, with most of the organic carbon being remineralized in the photic zone.

Sources:

  • Assmy, P., and V. Smetacek. "Enhanced carbon export due to silicate limitation in an iron-fertilized Southern Ocean diatom bloom." Deep-Sea Research Part I, vol. 56, no. 7, 2009, pp. 1017-1038.
  • Coale, K. H., et al. "Southern Ocean Iron Enrichment Experiment: Carbon cycling in high- and low-Si waters." Science, vol. 304, no. 5669, 2004, pp. 408-414. DOI.
  • Smetacek, V., et al. "Diaptomus: Remineralization of organic matter in a mesocosm iron fertilization experiment." Nature, vol. 487, 2012, pp. 313-319. DOI.

SEEDS I and II (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study, 2001, 2004)​

The SEEDS I and II experiments in the Western Subarctic Pacific showed substantial phytoplankton blooms but, consistent with previous experiments, the efficiency of carbon export to the deep ocean was low. Most fixed carbon remained in the upper ocean layers, subject to decomposition processes.

Sources:

  • Tsuda, A., et al. "A mesoscale iron enrichment in the western subarctic Pacific induces a large centric diatom bloom." Science, vol. 300, no. 5621, 2003, pp. 958-961.
  • Boyd, P.W., et al. "The decline and fate of an iron-induced subarctic phytoplankton bloom." Nature, vol. 428, 2004, pp. 549-553.
  • Takeda, S., and Tsuda, A. "An in situ iron-enrichment experiment in the Western Subarctic Pacific (SEEDS): Introduction and summary." Progress in Oceanography, vol. 64, no. 2-4, 2005, pp. 95-109.

EIFEX (European Iron Fertilization Experiment, 2004)​

EIFEX, conducted in the Southern Ocean, resulted in significant diatom blooms and some carbon export to the deep ocean but still showed that much of the organic matter was remineralized in the upper layers. This experiment continued to highlight the limited effectiveness of OIF in sequestering carbon in the deep ocean.

Sources:

  • Smetacek, V., et al. "Deep carbon export from a Southern Ocean iron-fertilized diatom bloom." Nature, vol. 487, 2012, pp. 313-319.
  • Pollard, R.T., et al. "Southern Ocean deep-water carbon export enhanced by natural iron fertilization." Nature, vol. 457, 2009, pp. 577-580.
  • de Baar, H.J.W., et al. "The European Iron Fertilization Experiment (EIFEX) in the Southern Ocean." Marine Ecology Progress Series, vol. 364, 2008, pp. 269-285.

LOHAFEX (2009)​

The LOHAFEX experiment in the Southern Ocean led to a moderate phytoplankton bloom with again low carbon export efficiency. This experiment emphasized the variability in the response of marine ecosystems to iron fertilization and the challenges in predicting carbon sequestration outcomes.

Sources:

  • Martin, P., et al. "The iron fertilization experiment LOHAFEX and its implications for ocean C and N cycles." Biogeosciences, vol. 10, 2013, pp. 4611-4623.
  • Haumann, F. Alexander, and Nicolas Gruber. “Antarctic Sea Ice and Biological Carbon Pump.” Nature, vol. 563, 2018, pp. 523-526, doi:10.1038/s41586-018-0722-7.
  • Klaas, Christine, and Victor Smetacek. “The Role of Diatoms in Ocean Carbon Sequestration.” Nature Reviews Microbiology, vol. 7, no. 3, 2009, pp. 214-219, doi:10.1038/nrmicro2109.

Conclusion​

The extensive data from numerous OIF experiments delineate the intricate dynamics of iron fertilization as a geoengineering strategy. While iron addition consistently stimulates phytoplankton growth, the anticipated sequestration of fixed carbon in the deep ocean has not been reliably achieved. Predominantly, the organic matter decomposes in the upper ocean layers, resulting in the release of CO2 back into the marine and atmospheric systems—thereby significantly reducing OIF's efficacy as a carbon sequestration method. Ongoing research and innovative methodologies are crucial to gaining a deeper understanding of the biological carbon pump's mechanisms and enhancing strategies to effectively mitigate climate change.

Sources​

  1. Martin, J.H., et al. "Testing the Iron Hypothesis in Ecosystems of the Equatorial Pacific Ocean." Nature, vol. 371, 1994, pp. 123-129.
  2. Boyd, P.W., et al. "A Mesoscale Phytoplankton Bloom in the Polar Southern Ocean Stimulated by Iron Fertilization." Nature, vol. 407, 2000, pp. 695-702.
  3. Assmy, P., and Victor Smetacek. "Enhanced Carbon Export Due to Silicate Limitation in an Iron-Fertilized Southern Ocean Diatom Bloom." Deep-Sea Research Part I, vol. 56, no. 7, 2009, pp. 1017-1038.
  4. Tsuda, A., et al. "A Mesoscale Iron Enrichment in the Western Subarctic Pacific Induces a Large Centric Diatom Bloom." Science, vol. 300, no. 5621, 2003, pp. 958-961.
  5. Smetacek, V., et al. "Deep Carbon Export from a Southern Ocean Iron-Fertilized Diatom Bloom." Nature, vol. 487, 2012, pp. 313-319.
  6. Martin, P., et al. "The Iron Fertilization Experiment LOHAFEX and its Implications for Ocean C and N Cycles." Biogeosciences, vol. 10, 2013, pp. 4611-4623.