Rates and pathways of organic carbon mineralisation in different sedimentary environments of the Helgoland Mud Area, SE German Bight

Fine-grained coastal and continental margin sediments are the largest permanent sink for carbon on our planet. They are typically rich in organic matter and often characterised by high sedimentation rates – favouring the burial of carbon. The ultimate processes that control the preservation of organic matter (OM) and its burial to deeper sediment layers are the different aerobic and anaerobic microbial OM mineralisation pathways that occur in surface sediments. In order to assess the rates and pathways of OM degradation in fine-grained sediments of the North Sea, we have chosen the Helgoland Mud Area (HMA), which represents the most important mud depocenter in the German Bight. The HMA is located at water depths between 11 and 27 m and covers an area of about 500 km2 southeast of the island of Helgoland. We present a high spatial and vertical resolution pore-water dataset for the HMA of surface sediments retrieved using a multi-corer (MUC). This dataset includes oxygen profiles, pore-water profiles of sulfate, sulfide, nitrate, ammonia, dissolved iron, dissolved manganese, dissolved inorganic carbon and its stable carbon isotopic composition. A full diagenetic model for the uppermost 25 cm of the sediments was applied to estimate the rates of the different OM mineralisation pathways and the respective diffusive fluxes towards and across the sediment-water interface. The organic carbon burial flux and organic matter mineralisation rates range from 2.6 to 9.9 mmol m-2 d-1 and 1.9 to 9.1 mmol m-2 d-1, respectively. The highest remineralisation rates are attributed to aerobic respiration and account for up to 86 % of total OM mineralisation in the investigated surface sediments. Sulfate reduction is shown to be the second-most important mineralisation pathway of OM in the study area – except for three sites which are characterised by iron reduction and denitrification as the dominant mineralisation process after aerobic respiration. These results will be discussed in the context of the different depositional conditions, variations in particulate organic carbon (POC) accumulation and POC origin across the HMA.