Iron and manganese speciation and cycling in glacially influenced high-latitude fjord sediments (West Spitsbergen, Svalbard): Evidence for a benthic recycling-transport mechanism

Glacial environments may provide an important but poorly constrained source of potentially bioavailable iron and manganese phases to the coastal ocean in high-latitude regions. Little is known about the fate and biogeochemical cycling of glacially derived iron and manganese in the coastal marine realm. Sediment and porewater samples were collected along transects from the fjord mouths to the tidewater glaciers at the fjord heads in Smeerenburgfjorden, Kongsfjorden, and Van Keulenfjorden along Western Svalbard. Solid-phase iron and manganese speciation, determined by sequential chemical extraction, could be linked to the compositions of the local bedrock and hydrological/weathering conditions below the local glaciers. The concentration and sulfur isotope composition of chromium reducible sulfur (CRS) in Kongs- and Van Keulenfjorden sediments largely reflect the delivery rate and isotope composition of detrital pyrite originating from adjacent glaciers. The varying input of reducible iron and manganese oxide phases and the input of organic matter of varying reactivity control the pathways of organic carbon mineralization in the sediments of the three fjords. High reducible iron and manganese oxide concentrations and elevated metal accumulation rates coupled to low input of “fresh” organic matter lead to a strong expression of dissimilatory metal oxide reduction evidenced in very high porewater iron (up to 800 μM) and manganese (up to 210 μM) concentrations in Kongsfjorden and Van Keulenfjorden. Sediment reworking by the benthic macrofauna and physical sediment resuspension via iceberg calving may be additional factors that promote extensive benthic iron and manganese cycling in these fjords. On-going benthic recycling of glacially derived dissolved iron into overlying seawater, where partial re-oxidation and deposition occurs, facilitates the transport of iron across the fjords and potentially into adjacent continental shelf waters. Such iron-dominated fjord sediments are likely to provide significant fluxes of potentially bioavailable iron to coastal waters and beyond. By contrast, low delivery of reducible iron (oxyhydr)oxide phases and elevated organic carbon mineralization rates driven by elevated input of “fresh” marine organic matter allow organoclastic sulfate reduction to dominate carbon remineralization at the outer Smeerenburgfjorden sites, which may limit iron fluxes to the water column.