Science Blogs covers the debate over the LOHAFEX results, which showed poor carbon sequestration as a result of fertilizing an eddy very poor in silica. Diatoms cannot grow without silica, and are the primary engines of the biological pump.
"But biogeochemist Kenneth Coale, director of Moss Landing Marine
Laboratories in California, estimates that the silicon-rich southern
part of the Southern Ocean would deliver up to twice as much potential
carbon sequestration as the northern area Smetacek fertilized, in large
part because of the diatoms and associated ecosystem dynamics. The
predators that eat diatoms, it turns out, have large waste pellets that
sink rapidly. Coale warns that calling iron fertilization a failed
strategy on the basis of an experiment in low-silicon waters is just as
unwise as declaring the technique a home run after a successful
experiment would have been. "I would be reluctant to extrapolate from
any one experiment anything having to do with the efficacy of iron
fertilization as a carbon-sequestration strategy," says Coale.
"Another scientist, Margaret Leinen, is the head of a company, Climos,
that is hoping to commercialize iron fertilization to gain carbon
credits at sea. The former head of geosciences at the National Science
Foundation, she says the 1-gigaton-a-year figure for atmospheric CO2 was based on paleoclimate records. Chemical analyses of ocean cores
show that the Southern Ocean drew down at least that much CO2 millions of years ago during glacial periods. "In the paleorecord, we
find a lockstep correlation between the amount of [phytoplankton
growing] and temperature," says Coale.
"Smetacek had actually tried to find an area of ocean that would
feature diatoms. Levels of silicon are generally higher south of 50°
latitude. But Smetacek says the German government asked him to stay
north of that line due to a treaty called CCAMLR designed to protect marine species in the Southern Ocean. Part of that
restriction was no doubt connected to the fact that the LOHAFEX mission
was controversial from the start, drawing criticism both from
environmentalists and from the German environmental ministry. So
Smetacek says he had to settle on a patch at 48° south latitude.