SPATIAL AND TEMPORAL DYNAMICS OF EARLY DIAGENETIC PROCESSES IN GLACIALLY INFLUENCED ARCTIC FJORDS

Glacially influenced Arctic fjords represent intriguing environments to study the coupled early diagenetic cycling of carbon, sulfur and trace metals in dynamic coastal settings. Biogeochemical processes in these sedimentary systems are strongly controlled by the interplay between delivery rates of glacially derived iron and manganese oxides, which generally decrease along the fjord head-to-mouth axis, and the input of labile marine organic carbon to the sediment, which increases fjord outward. As a result, the balance between the dominant anaerobic carbon mineralization pathways (dissimilatory metal oxide reduction and sulfate reduction), the importance of oxidative pathways of the sulfur cycle, and ultimately the benthic behavior and fate of trace metals in the surface sediment change along this transect. Dissimilatory metal oxide reduction and the reaction of Fe oxides with H₂S lead to elevated pore-water concentrations of Fe²⁺ (up to 850 μM) and Mn²⁺ (up to 650 μM) in the inner fjords; benthic Fe²⁺ fluxes are highest in a “sweet spot” mid-fjord, where the deposition of both glacial material and labile organic carbon is high. Benthic Fe²⁺ fluxes in these fjord areas reach 130 mmol m^-2 d^-1, which is comparable to fluxes along continental shelf settings with lower bottom water O₂ concentrations. The composition of the bedrock, the duration of sea ice coverage and the distribution of benthic macrofauna exert further controls on biogeochemical processes in fjord surface sediments. Strong seasonal dynamics in key environmental processes, including the development of phytoplankton blooms in late spring and the delivery of large amounts of sediment-bearing meltwater in summer, are additionally reflected in a heterogenous distribution of microbially mediated diagenetic processes in the sediments. This is particularly important close to the glaciers, where labile organic carbon-rich layers are buried and preserved over multi-year timescales. Our studies highlight that short-term seasonal events and strong spatial environmental gradients across coastal systems control the rates and distribution of early diagenetic processes and warrant a careful assessment of, for example, benthic metal flux measurements, underlining the fact that these are highly non-steady state depositional environments.