Variations in sediment production of dissolved iron across a continental margin not dominated by major upwelling or riverine inputs

Abstract

Despite the undeniable effect of iron on shaping patterns of ocean productivity, the relative importance of the different sources of this limiting nutrient to the ocean is still under debate. Although global estimates indicate that the benthic input of iron to the oceans is significant, most studies have investigated continental margins exposed to either upwelling or large riverine inputs, environments that are not representative of the majority of the oceans. Additionally, the number of studies that report dissolved iron concentrations in continental slope sediments is limited, despite the fact that these regions between the shelf edge and the continental rise make up >5% of the sedimentary surface area of the global ocean. The sedimentary flux of iron has traditionally been considered negligible due to the rapid oxidation of Fe²⁺ in oxic waters and poor solubility of the Fe(III) product. The recent realization that ferric iron may be stabilized in solution by organic ligands during oxidation near the sediment-water interface suggests that a significant fraction of the dissolved iron pool may be present under the form organic-Fe(III) complexes that could eventually reach the overlying waters. In this study, the speciation and biogeochemical importance of iron was determined in intact sediment cores along a transect across the entire continental margin near Cape Lookout, North Carolina, a region not dominated by upwelling or riverine inputs that is representative of most passive continental margins. Rates of diffusive oxygen uptake (DOU) and maximum diffusive fluxes of both dissolved Fe²⁺ and organic-Fe(III) complexes decreased from the coastal zone to the continental shelf, remained low on the shelf and the upper continental slope, but rebounded to reach a maximum in mid-slope sediments where concentrations of Fe(III) oxides were the highest along the transect. In turn, DOU decreased and dissolved iron was below detection in lower-slope sediments, indicating that mid-slope sediments represent depocenters where Fe(III) oxides and organic matter may accumulate. Pore water sulfate and sulfide concentrations as well as separate sediment incubations confirmed that sulfate reduction does not greatly influence the cycling of iron in these sediments. The production of dissolved organic-Fe(III) in these continental margin sediments is likely regulated by a combination of aerobic oxidation in the presence of natural organic ligands near the sediment-water interface, dissimilatory iron reduction, or chemical oxidation of Fe(II) complexed to natural organic ligands. Fluxes of Fe²⁺ and organic-Fe(III) complexes across the sediment-water interface were not observed. However, diffusive fluxes of Fe²⁺ and organic-Fe(III) complexes into the oxic zone of these sediments (<1 cm from the sediment-water interface) and production of dissolved Fe(III) in sediment slurry incubations suggest that complexation of Fe(III) in these sediments may contribute to the stabilization and potential transport of dissolved iron into oxygenated deep ocean waters. Extrapolation to the global ocean suggests that mid-slope depocenters contribute considerably to the iron inventory of the ocean, thus warranting the need for measurement of benthic iron fluxes and dissolved iron speciation in these environments.

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