Proposals for ocean iron fertilization (OIF) geoengineering contemplate the seeding of swaths of high nutrient-low chlorophyll ocean regions to stimulate the production of phytoplankton blooms that can sequester carbon dioxide when sinking from the surface to the deep ocean. To date, twelve OIF experiments have been carried out, with most demonstrating that the addition of iron to high nutrient-low chlorophyll regimes, primarily in the Southern Ocean, can generate substantial phytoplankton blooms that sequester carbon. However, the critical second element of the OIF hypothesis had not yet been established, that is, whether the bloom biomass would ultimately sink in the same manner as natural blooms to depths where the carbon could be stored for centuries or longer. Many have expressed profound doubts about this second element, arguing that a substantial portion of the phytoplankton produced through OIF would be consumed by zooplankton species prior to sinking to the ocean’s bottom and/or that a substantial portion of the bloom would be rapidly mineralized and respired.
In a recent article in the journal Nature, researchers report the results of a five-week experiment conducted in the closed core of a mesoscale eddy of the Antarctic Circumpolar Current during an OIF experiment carried out in 2005 (the European Iron Fertilization Experiment, EIFEX). Among the take-aways from the study:
- Large diatoms accounts for 97% of the observed chlorine increase;
- Massive phytoplankton blooms can develop in a mixed layer as deep as 100 meters;
- The results in EIFEX support the second element of the OIF hypothesis, with at least half of the bloom biomass sinking below a depth of 1000 meters, with a substantial amount likely reaching the sea floor, where it may sequester blooms for centuries in ocean bottom water, and even longer in sediments.
This is a very technical article that would not be appropriate for non-science students; however, the results would be worthwhile to include in geoengineering lectures.