(HNLC areas), particularly growth of larger phytoplankton (>8 μm), is limited by iron
availability. Iron, while a major constituent element of the Earth as a whole, is very
dilute in seawater. That is because ferric iron (Fe3+, the form absorbable by cells)
forms iron hydroxide (Fe(OH) 3 ) in basic solutions such as seawater (once closely
buffered to pH ∼8.3). Iron hydroxide has a solubility coefficient of the order of 10−12
and forms a flocculant precipitate reducing the levels of free ferric iron to sub-
nanomolar levels (<1 × 10−9 moles Fe3+ L−1). Ferrous iron (Fe2+) is oxidized to ferric
in seawater and joins the precipitates. Iron is required as a cofactor for many enzymes
and as a constituent of some pigments, so its limited availability controls the growth
of larger phytoplankton. A substantial suite of twelve studies involving direct
additions of iron to ∼100 km^2 patches of HNLC ocean (eastern tropical Pacific,
subarctic Pacific, Southern Ocean) have proved that, indeed, iron is the limiting
factor. Reviews of these studies have been provided by de Baar et al. (2005) and Boyd
et al. (2007). Even before those “IRON-EX” studies, addition of iron to HNLC areas
to sequester CO 2 from the atmosphere was suggested. It continues to be a contentious
issue. One of us (Miller, e.g. Strong et al. 2009a) has taken public stands against this
“ocean iron fertilization” (OIF) approach to reduction of atmospheric CO 2 , and the
following reflects that bias. We continue to think opposition to OIF is well founded.
That is not to say that global society should do nothing to reduce CO 2 emissions and
even CO 2 in the atmosphere. This book is not about what can or should be done, but
something must be.
(^) In 1989, with remarkable public notice in 1990 (remarkable considering that rather
esoteric science was at issue), the late John Martin both hypothesized the interaction
of glacial–interglacial cycling with iron-limitation in HNLC areas and suggested that
we might be able to use enhanced iron transport to those areas to remove CO 2 from
the atmosphere. The idea was to add iron-rich dust to broad HNLC stretches of the
Southern Ocean, promoting growth of large phytoplankton. This regional
phytoplankton bloom would more completely strip the waters of major nutrients and
(the theory went) would mostly sink, carrying large quantities of organic matter into
the deep sea. This would amount to a mitigation of the industrial CO 2 problem,
possibly returning the climate to the somewhat cooler conditions prior to, say, 1960.
(^) This created a storm of controversy. The conclusion, which came from a simple
calculation by Michael Pilson and from a box model by Peng & Broecker (1991), was
that even getting all of the nutrients upwelling in the Antarctic converted to organic
matter and then sunk would have a very small impact on atmospheric CO 2 . Moreover,
most of the sequestered carbon would not be at particularly great depths and would
come back to rejoin the atmosphere in only 30–40 years. More elaborate modeling
studies suggested that sustaining even a modest impact on atmospheric CO 2 would