A zonal picture of the water column distribution of dissolved iron(II) during the U.S. GEOTRACES North Atlantic transect cruise (GEOTRACES GA03)

We report measurements of the transient species iron(II) in filtered water column samples collected during the U.S. GEOTRACES North Atlantic transect cruise (GEOTRACES GA03), which was comprised of legs from Lisbon to Cape Verde in October-November 2010, and from Woods Hole to Cape Verde in November-December 2011. Dissolved iron(II) (dFe(II)) was determined at sea in 0.2 µm filtered samples as soon as possible after collection and filtration, using flow injection analysis, with mean detection limits of 0.06 nM and 0.01 nM during the 2010 and 2011 cruise legs, respectively. Water column concentrations along the cruise transects were generally low (<0.2 nM), with the exception of deep water samples collected over the Trans Atlantic Geotraverse hydrothermal field, in which dFe(II) was as high as 70 nM and accounted for more than 80% of the dissolved iron pool in the near-field hydrothermal plume. Smaller local concentration maxima were observed near 1000 m depth in the low-oxygen, iron-rich waters to the east of Cape Verde, where dFe(II) is correlated with apparent oxygen utilization, and in the upper water column at several stations in the Subtropical Gyre, where dFe(II) can account for greater than 50% of the dissolved iron pool in the lower euphotic zone. Elevated dFe(II) concentrations were also observed over much of the water column on the Bermuda platform, although the source of this enrichment remains uncertain, in the absence of data between Woods Hole and Bermuda.