Wavelength dependence of the photochemical reduction of iron in arctic seawater.

Chemiluminescence measurements of the photochemical reduction of iron in cold, high-latitude waters (79 degrees N) show that a significant fraction (20%) of the dissolved iron is reduced when exposed to sunlight. The reduction is immediately initiated and the transition to a steady-state concentration of approximately 200 pM photochemical Fe(II) is achieved within approximately 40 s. The photochemical Fe(ll) is reoxidized to Fe(III) in less than a minute upon blocking the sunlight, much faster than expected, which is ascribed to reaction with photochemically produced oxidants. Using filters to block different ranges of the incident sunlight it was found that 35% of the photochemical Fe(II) was produced in the UV-B range (300-315 nm), 30% in the range 315-360 nm, and 30% at higher wavelengths. Measurements of light attenuation as a function of depth indicate that photochemical Fe(II) at a depth of 5 m in high-latitude waters should amount to approximately10% of that at the surface. The fast kinetics modulate the paramount importance that photochemical reactions may have on the bioavailability of iron in surface waters.