Introduction

The accumulation of atmospheric CO₂ (carbon dioxide) well above levels estimated for both the recent and geological past is of concern to environmental scientists, and may be one of the most compelling indicators of global climate change due to human activity. The atmospheric CO₂ concentration currently stands at 360 ppm (parts per million). This figure is substantially higher than the atmospheric CO₂ concentration of 270 ppm 100 years ago, before widespread combustion of large quantities of fossil fuel was the global norm.

Even the relatively low atmospheric concentration of greenhouse gases a century ago is considerably larger than the CO₂ level estimate of less than 200 ppm given for the period 15,000 years ago, at the height of the last glacial period.

Dusty Winds and Carbon Sinks

One compelling hypothesis attempting to explain the low glacial-age atmospheric CO₂ concentrations suggests that strong winds (which scientists assert would have been typical of glacial times) carried massive amounts of iron-containing dust and sand particles from continental land masses to oceans regions that were iron-limited. Iron fertilization in this manner would have stimulated increased primary production (photosynthesis) by marine phytoplankton, resulting in enhanced uptake of atmospheric CO₂ uptake by these rapidly growing and reproducing populations. Also enhanced would have been the seqestration of carbon within the deep ocean upon sinking and subsequent remineralization of sinking planktonic biomass. The net result of this flux would have been a decrease in atmosphheric CO₂ levels as photosynthetic carbon-fixation continued to convert inorganic carbon to living biomass, portions of which were continually exported to the deep sea via sinking and other processes.

Support for the Ice Age Iron-fertilization hypothesis has come from research efforts that have demonstrated adding iron to small patches in severely iron-deficient oceans, such as the Southern Ocean encircling Antarctica, can promote an increase in phytoplankton photosynthesis and growth.

Such results have led to suggestions—some more dubious than others—that perhaps the largest bioremediation project ever engineered should be the fertilization of the anemic Southern Ocean with massive quantities of iron, the anticipated result being that similarly massive amounts of human produced atmospheric CO₂ would be moved through the phytoplankton to ultimately be sequestered in the abyssal depths.

Issues to be Ironed Out

Fortunately, scientists tend to be rational folks, and such a monumental, logistically and technically difficult, and uncertain undertaking would not be pursued without first doing some more focused research on considerably smaller spatial and temporal scales. The most comprehensive of these proof-of-concept studies was the multi-institutional Southern Ocean Iron Experiment, or SOFeX, carried out in 2002 and whose results have recently been published in the journal Science (April 16 2004).

The various experiments of the SOFeX project indicated that, indeed, the addition of sizeable quantities of iron (just over one metric ton added to each of two 15 km x 15 km sites) did produce small but measurable increases in carbon uptake from the atmosphere and subsequent transport to the deep sea. The key conclusion, however, was that "the relatively modest increase in carbon export does not appear to be large enough to make iron fertilization a viable method for sequestering anthropogenic CO₂." Study authors cite concerns that other micronutrients (particularly silica) might in some instances become limiting once iron was added. They also note that the negligible reduction in the roughly 6.5 billion tons/year of human-related atmospheric carbon input expected from a scaled up (i.e., ocean wide) Southern Ocean iron fertilization would not justify the undertaking of such a major endeavor.

In the end, perhaps it is best that this remains the largest bioremediation project never attempted.