Iron biogeochemistry in sediments on the western continental shelf of the Antarctic Peninsula

Abstract

We examined iron biogeochemistry in continental margin sediments along the western side of the Antarctic Peninsula (WAP) to explore the connections between iron redox cycling and other sediment biogeochemical processes, and to specifically examine the role of sediments as a source of iron to support Southern Ocean primary productivity. Total iron (FeT) in WAP sediments ranged from ∼4 to 6 wt% Fe and showed no consistent depth variations. Total iron oxides (FeOx) represented ∼20–30% of FeT in WAP surface sediments and was comprised of two types of materials: Feox1, which is thought to contain amorphous, highly reactive, and poorly crystalline iron oxides such as ferrihydrite and lepidocrocite, and Feox2, which is thought to contain magnetite and less reactive, more crystalline iron oxides such as goethite and hematite. Absolute and relative concentrations of FeOx declined with sediment depth, presumably due to reductive dissolution, and there was also a major depth change in the speciation of FeOx. Near the sediment surface ∼30% of FeOx was Feox1, while below ∼30 cm Feox1 was almost completely consumed and therefore here virtually all FeOx was Feox2. Consistent with these observations, a simple reaction-transport model applied to the depth distributions of Feox1 and Feox2 iron in WAP shelf sediments suggested a ∼20-fold difference in the relative reactivity of these different oxides towards reductive dissolution (Feox1 > Feox2). At several stations profiles of pore water dissolved iron and solid phase Feox1 can be explained by steady state iron redox cycling. In contrast, profiles at other stations showed evidence of non steady-state iron redox cycling. Seasonal changes in the sediment organic carbon rain rate at these sites may explain this latter behavior. Differences in the behavior of iron at different sites may be related to the interplay between changes in sediment redox conditions and their impact on bioturbation and iron redox cycling. Sediment iron sources to the water column include dissolved iron (∼Fe2+) benthic fluxes as well as sediment resuspension processes that mix iron oxides (i.e., Feox1) found in surface sediments into the bottom waters. The impact these benthic iron sources will have on surface water iron concentrations (and eventually primary production) will depend on transport processes that move this iron from deeper waters to surface waters. They will also depend on chemical transformations in the water column that affect iron bioavailability. While sediment processes (in general) appear to be an important source of iron to Southern Ocean surface waters, there is large uncertainty in their magnitude. However, the results presented here demonstrate that the suspended sediment flux of amorphous iron oxides needs to be considered when discussing sediment iron sources to Southern Ocean surface waters.