Ideas and perspectives: The benthic iron flux from sandy advective bioturbated sediments

Abstract. Multiple investigators have suggested that the benthic flux of dissolved iron (Fed) from continental shelf sediments represents an important source of this micronutrient to ocean waters. The magnitude, biogeochemical controls, and seasonal dynamics of Fed fluxes to date, however, have mostly been studied for muddy cohesive sediments dominated by molecular diffusion. Data from these studies have been included in global biogeochemical models to determine the contribution of this Fe source to the ocean. Fed fluxes from sandy advective sediments have received little consideration, although these sediments cover 50–60 % of the continental shelves. Sandy permeable deposits function as dynamic catalytic filters characterized by the rapid exchange of solutes and infiltration of particles —including labile Corg and reactive metal oxides— and high biogeochemical reaction rates. In this article, we discuss how the fundamentally different modes of solute and particle transport in sands affect the sedimentary Fe cycle and Fed flux. We present a case study in which we simulate bioirrigation in sands in summer and winter. In our experiments, Fed fluxes from non-irrigated sediments under diffusive conditions did not exceed 6 and 13 μmol Fe m-2 d-1 in winter and summer, respectively. Fluxes from irrigated cores reached values of 150 μmol Fe m-2 d-1 (winter) and 115 μmol Fe m-2 d-1 (summer). The results indicate that the pumping activity of the benthic macrofauna plays a key role in controlling the extent of the benthic Fed flux from permeable sediments, and that both biogenic and physical advection enhance fluxes. We argue that bioturbated sandy advective sediments constitute an important benthic Fe source to coastal waters and advocate for a more differentiated treatment of sediment type (muddy diffusive vs. sandy advective) and macrofaunal activity -reflecting different functional groups of the macrobenthos- in global biogeochemical Fe models. A better understanding of the benthic Fe cycle in sandy advective sediments is particularly important to help predict how anthropogenic effects such as changes in the deposition patterns of Corg and metals, the expansion of oxygen minimum zones, and changes in benthic biodiversity will affect the tightly coupled benthic-pelagic ecosystem along continental shelves.