Iron(II) variability in the northeast subarctic Pacific Ocean

Distributions of dissolved iron (< 0.2 μm, dFe) and its reduced form, Fe(II), were measured during 3 cruises along Line P, a transect from the continental slope to the high nitrate, low chlorophyll (HNLC) northeast subarctic Pacific Ocean. Concentrations of Fe(II) ranged from below the detection limit (4.3 pM or less) to 330 pM, and dFe concentrations were as high as 3.6 nM. Maximum concentrations for both Fe(II) and dFe occurred in waters over the continental slope, with Fe(II) consistently increasing towards the bottom, consistent with Fe(II) supply from benthic sources on the continental shelf and slope. Low oxygen concentrations (~ 10 μM) and pH (~ 7.5) in the North Pacific oxygen deficient zone (ODZ) likely serve to stabilize Fe(II) and may contribute to enhanced dFe release from shelf and slope sediments via reductive dissolution. Concentration gradients along isopycnal surfaces indicate that Fe(II) is transported several hundred kilometers from the continental slope at depth, and there is evidence that episodic events may advect shelf-derived Fe(II) similar distances near the surface. Comparison of transport times and Fe(II) half-lives suggests that it is unlikely for pre-formed Fe(II) to be transported these distances, but that sedimentary particles advected off the shelf and slope may constitute a continuous source of Fe(II) both at depth and near the surface. At the offshore stations, the Fe(II) time series reveals deep local maxima that are transient in time and space and are consistent with a sporadic Fe(II) source, such as remineralization of sinking particles.