Release and turn-over of carbon, nitrogen and metals under oxic and suboxic conditions in long-term incubations of Skagerrak sediments

Suspended particulate matter and associated pollutants from the entire North Sea are deposited in the Skagerrak, located between Norway and Denmark. Consequently, the sediments of the Skagerrak play a key role for long-term carbon storage within the North Sea. Due to its location and bathymetry, the bottom sediment redox conditions within the Skagerrak are heterogeneous and cover a wide range from oxic to suboxic conditions. We investigated nitrogen sequestration processes and the mobility of pollutants in these sediments during incubation experiments that simulated oxic and suboxic conditions. Analysis of isotopic fractionation was used as a tool to better understand the nitrogen sequestration pathways (δ¹⁵NO₃^-) and redox conditions (δ^98/95Mo).

Typically, incubation experiments last days to weeks but do not cover long-term effects. In contrast, we incubated different zones of three sediment cores with North Sea water for up to twelve months. The sediments originated from locations with (a) mainly iron reduction, (b) mainly manganese reduction and (c) both iron and manganese reduction. After one, three, six and twelve months, we sampled water and sediments from the incubations for various parameters (e.g., trace elements, carbon and nitrogen content, nutrients, δ¹⁵NO₃^-, δ^98/95Mo). Under aerobic conditions, the sediments with high organic carbon content (2.78 ± 0.05 %) released up to 33 ± 6 µmol g^‑1 NO₃^- during remineralization, while in anaerobic incubations, these sediments released only up to 4.8 ± 0.8 µmol g^-1 NH₄⁺. However, sediments with lower organic carbon contents (1.89 ± 0.05 %) released only 4.8 ± 1.2 µmol g^‑1 NO₃^- and 1.18 ± 0.19 µmol g^-1 NH₄⁺, respectively. In combination with trace element concentrations, δ^98/95Mo ratios allowed to distinct between different organic matter oxidation pathways. The aerobic incubations released mainly copper, lead and nickel while under  anaerobic conditions, also  cobalt but significantly less copper has been released. Hence, the prevailing oxygen conditions also have a strong impact on the remobilization of e.g., legacy pollutants stored in the sediments. The results of our long-term incubations reveal important biogeochemical processes and indicate that some processes are only traceable at larger timescales applied in this study, but not by incubation durations that are usually applied for biogeochemical studies.