The influence of UV irradiation on the photoreduction of iron in the Southern Ocean

An iron enrichment experiment, EisenEx, was performed in the Atlantic sector of the Southern Ocean during the Antarctic spring of 2000. Deck incubations of open ocean water were performed to investigate the influence of ultraviolet B (UVB: 280-315 nm) and ultraviolet A (UVA: 315-400 nm) on the speciation of iron in seawater, using an addition of the radioisotopes 59Fe(III) (1.25 nM) or 55Fe(III) (0.5 nM). Seawater was sampled inside and outside the iron-enriched region. The radioisotopic Fe(II) concentration was monitored during daylight under three different light conditions: the full solar spectrum (total), total minus UVB, and total minus UVB+UVA. A distinct diel cycle was observed with a clear distinction between the three different light regimes. A clear linear relationship was found for the concentration of radioisotopic Fe(II) versus irradiance. UVB produced most of the Fe(II) followed by UVA and visible light (VIS: 400-700 nm), respectively. UVB produced 4.89 and 0.69 pM m2 W−1 radioisotopic Fe(II) followed by UVA with 0.33 and 0.10 pM m2 W−1 radioisotopic Fe(II) and VIS with 0.04 and 0.03 pM m2 W−1 radioisotopic Fe(II).