Is iron redox cycling in a high altitude watershed photochemically or thermally driven?

Many studies have shown that the concentration of aqueous Fe2+ increases in surface waters during exposure to sunlight and attribute this phenomenon either to photoreductive dissolution of ferric minerals/colloids or to ligand-to-metal charge transfer within organic complexes of Fe3+. In a multi-summer study of iron redox cycling in a relatively high pH stream (Middle Crow Creek, MCC) that drains a mostly-granitic watershed at an altitude of 2400 m, aqueous Fe3+ (not Fe2+) concentrations were correlated with both sunlight and temperature. A steady state model fails to explain the [Fe2+] and [Fe3+] data from this stream. However, Fe2+ concentrations can be explained using a simple kinetic model in which rate constants for oxidation and reduction were obtained by fitting data from in situ oxidation experiments, including first-order thermal (nonphotochemical) reduction of Fe3+. Rate constants obtained from experiments in the dark result in too much Fe2+ to match the data from illuminated experiments, requiring a net photooxidation process to explain [Fe3+] measured in MCC. The organic content of MCC results in high concentrations of Fe–DOM complexes that not only act as a reservoir contributing to daily changes in [Fetot] as measured by our methods, but whose photochemistry may contribute highly oxidizing reactive oxygen species to the stream. In situ studies suggest that photochemical reduction of organically bound Fe3+ occurs, followed by thermal release of Fe2+ to the water column and subsequent rapid re-oxidation.