Geochemistry

Iron's Icy Fate

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Science  28 May 2010:
Vol. 328, Issue 5982, pp. 1077
DOI: 10.1126/science.328.5982.1077-c

The scarcity of bioavailable iron [Fe(II)aq] in the oceans limits primary productivity. In most marine environments, Fe(II)aq becomes available to phytoplankton only upon the reduction of Fe(III)-oxide particles added to the ocean from wind-blown dust or river outflows; however, in polar settings, a substantial fraction of iron is trapped in ice or snow before it enters the water column. Through a series of laboratory experiments and outdoor observations in Arctic ice, Kim et al. show that Fe(III) oxides in ice actually generate more Fe(II)aq than particles added directly to open water do. This process occurs through enhanced photoreductive dissolution of concentrated Fe(III) oxides at liquid-like ice grain boundaries, even in the presence of oxygen. When aggregated in a confined space, the semiconducting particles facilitate charge transfer much more rapidly than is possible for single particles in suspension. Therefore, Fe(III) oxides in ice or snow may ultimately contribute more to the bioavailable Fe(II)aq pool after melting than previously expected.

Environ. Sci. Technol. 44, 10.1021/es9037808(2010).

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