Minimal effect of iron fertilization on sea-surface carbon dioxide concentrations

Abstract

IT has long been hypothesized that iron concentrations limit phyto-plankton productivity in some parts of the ocean^1–3. As a result, iron may have played a role in modulating atmospheric CO₂ levels between glacial and interglacial times₄, and it has been proposed₅ that large-scale deposition of iron in the ocean might be an effective way to combat the rise of anthropogenic CO2 in the atmosphere. As part of an experiment in the equatorial Pacific Ocean⁶, we observed the effect on dissolved CO₂ of enriching a small (8x8 km) patch of water with iron. We saw significant depression of surface fugacities of CO₂ within 48 hours of the iron release, which did not change systematically after that time. But the effect was only a small fraction (∼10%) of the CO₂ drawdown that would have occurred had the enrichment resulted in the complete utilization of all the available nitrate and phosphate. Thus artificial fertilization of this ocean region did not cause a very large change in the surface CO₂ concentration, in contrast to the effect observed in incubation experiments³, where addition of similar concentra-tions of iron usually results in complete depletion of nutrients. Although our experiment does not necessarily mimic all circum-stances under which iron deposition might occur naturally, our results do not support the idea that iron fertilization would signifi-cantly affect atmospheric CO₂ concentrations.