Argument: Algae-sequestered carbon may not remain sequestered for long
"Will Ocean Iron Fertilization Work?". Woods Hole Oceanographic Institution - Only a tiny fraction of the carbon drawn down by blooms sinks from the surface into deeper waters, where it is sequestered from the atmosphere. Estimates of the tonnage of carbon sequestered (measured at 200 meters depth) per ton of iron added hover around 200 to 1, a far cry from early experiments in laboratory beakers that yielded estimates around 100,000 to 1, Boyd said.
"Fertilizing the Ocean with Iron". Woods Hole Oceanic Institute. 7 Sep. 2007 - "In certain regions, including the equatorial and north Pacific and the entire Southern Ocean, a simple iron addition does cause phytoplankton to grow rapidly. But tiny zooplankton, known as “grazers,” eat much of the bloom almost as soon as it starts. This begins a chain of recycling that ensues from the sea surface to the seafloor as grazers, krill, fish, whales, and decomposers feed upon each other. Much of the immense carbon prize won by the iron addition quickly leaks back into the atmosphere as carbon dioxide gas."
"A scientific critique of oceanic iron fertilization as a climate change mitigation strategy". Greenpeace Research Laboratories. September 2007 - Inadequate Carbon Sequestration and Impracticalities To be effective as a carbon sequestration technology, particulate organic carbon (dead plankton and faecal matter) has to be exported efficiently to deeper waters (at least 200 metres). However, published results from the mesoscale iron enrichment studies showed that the amount of carbon exported was either very low or not detectable. This inefficient export of particulate organic carbon to deep waters does not favour iron fertilization as a carbon sequestration technology. For example, during the Southern Ocean Iron Experiment (SOFeX) iron enrichment study, an area of 15 km2 was seeded with iron, resulting in a carbon sequestration of about 900 tonnes of carbon. This is a very small proportion of the carbon released due to human activities (6.5 x 109 tonnes/year), such that it is difficult to see how iron fertilization could scale up to be an effective carbon sequestration method.