Distribution of iron(II) along the US GEOTRACES Eastern Pacific Zonal Transect

Abstract:

Iron(II) was determined at sea on the US GEOTRACES Eastern Pacific Zonal Transect (EPZT), by chemiluminescence, along with total dissolved iron determined independently. Very high concentrations of Fe(II) (up to 25nM) were observed at all shelf stations, and constituted up to 100% of the total dissolved iron. West of the shelf, a subsurface maxima in Fe(II) coincident with the secondary nitrite maximum persisted as far as 95⁰ W. Surprisingly, the largest subsurface plume of dissolved Fe was below the oxygen minimum zone (OMZ), in the deep waters. Fe(II) does not appear to be responsible for the magnitude of that deep plume, although it probably contributes to high Fe observed within the OMZ. Further west, a large plume of dissolved Fe was observed that was derived from hydrothermal activity along the east Pacific Rise. There was no Fe(II) present, even at the station closest to the origin of the plume, indicating that Fe(II) declines rapidly under these oxygenated conditions despite reports that Fe(II) persists in particles of hydrothermal origin. The only exception was very near the seafloor along the ridge axis, where Fe(II) benthic fluxes were sampled directly. But this sampling was not conducted at actual vent sites. The absence of any Fe(II) signal within the main plume, where total dissolved and particulate Fe was high, provides confidence that the method does not have a significant Fe(III) interference. These results suggest that while Fe(II) plays an important role in benthic sources of both of the major deep sea Fe plumes observed on the cruise, their persistence is likely due to other factors, such as organic complexation.