Distribution and redox speciation of dissolved iron on the European continental margin

Abstract

To investigate the biogeochemistry of iron in the waters of the European continental margin, we determined
the dissolved iron distribution and redox speciation in filtered (,0.2 mm) open-ocean and shelf waters. Depth
profiles were sampled over the shelf slope southeast of the Chapelle Bank area (47.61uN, 4.24uW to 46.00uN,
8.01uW) and a horizontal surface-water transect over the shelf and through the English Channel (la Manche) and
the southern North Sea (46uN, 8uW to 52uN, 4uE). An abrupt trace-metal front was found near the shelf slope,
indicated by a horizontal gradient of dissolved iron (DFe) and aluminium (DAl), which correlated with changing
salinities (r2 5 0.572 and 0.528, respectively, n 5 92). Labile Fe(II) concentrations varied from ,12 pmol L21 in
North Atlantic surface waters to .200 pmol L21 in the near bottom waters of the shelf break. Labile Fe(II)
accounted for ,5% of the dissolved iron species in surface shelf waters (mean 5.0 6 2.7%), whereas higher Fe(II)
fractions (i.e., .8%) were observed near the sea bottom on the shelf break and during a midday solar maximum in
surface waters in the vicinity of the Scheldt river plume. Benthic processes (resuspension and diagenesis)
constituted important sources of Fe(II) and DFe in this region, and photoreduction of Fe(III) species in shelf
waters caused enhanced labile Fe(II) concentrations. These processes increased the lability of iron and its
potential availability to marine organisms in the shelf ecosystem.

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