The redox speciation of dissolved iron in open ocean seawater was evaluated during two Pacific Ocean research cruises. Using a sensitive flow injection method based on luminol chemiluminescence, vertical profiles of reduced iron concentration, Fe(II), were obtained at 134 stations. In this paper, sampling and analytical methods are discussed and values obtained for Fe(II) are compared to shipboard measurements of total dissolved iron. Concentration profiles are evaluated within the context of various proposed source mechanisms and experimental models of Fe(II) oxidation kinetics.
Samples were collected from rosette-mounted GO-FLO bottles, enabling retrieval of uncontaminated samples from depths of 20 - 1000 m. The length of time required for rosette retrieval coupled with the potential for rapid oxidative loss of Fe(II) complicates the detection of photochemical production processes. Acidification of seawater samples retards oxidation until sample analyses can be completed, but for undetermined reasons it contributes both to the blank response, and to minor instabilities in system response over time that are depth-specific, effects which must be considered and corrected for. The analytical method typically yielded detection limits of 10-15 pM. Potential interferences include reduced vanadium species that contribute to the analytical response, and dissolved organic matter, which has been found to decrease response.
The results from the two cruises suggest a relatively consistent pattern of Fe(II) occurrence and distribution in the Pacific Ocean. Surface water maxima are present in most profiles, with median concentrations of 25-30 pM, accounting for approximately 12% of the dissolved iron. Concentrations decline with depth to undetectable levels in the upper euphotic zone. Fe(II) was frequently detected in deepest samples, constituting a small fraction of the dissolved iron. The concentration profiles are suggestive of photochemical processes; however, no diel bias was observed and Fe(II) persisted in GO-FLOs well beyond the lifetime predicted by oxidation models. Fe(II) in deep samples was found in association with the local oxygen minimum, possibly due to the remineralization of sinking biogenic particles. In the northern Philippine Sea, between the Japanese coast and the Izu-Bonin volcanic arc system, Fe(II) concentrations were found to be atypically high, possibly due to dust and sediment-derived iron.
