Rates and pathways of organic matter remineralisation in different sedimentary environments of the Helgoland Mud Area, North Sea

Coastal and shelf-sea sediments are hotspots for organic carbon (OC) remineralisation and burial. Using high-resolution solid-phase and pore-water data and diagenetic modelling, we investigated early diagenetic processes within sediments of the Helgoland Mud Area, SE German Bight. This included assessing the effects of bottom trawling and dumping of harbour sediments on rates and pathways of OC remineralisation. Our results show that aerobic respiration is the most important pathway of OC remineralisation, followed by sulphate reduction. Surprisingly, we observed no effect of different depositional and geochemical environments on the relative contribution of the different pathways of OC remineralisation in large parts of the study area, despite the wide range of mud contents, sedimentation rates, benthic OC fluxes and average reactivities of the OC reaching the sediment-water interface. This observation indicates that either (1) the variations within these parameters do not significantly influence the relative contribution of aerobic and anaerobic processes, or (2) the differences in these parameters are too insignificant to affect the contribution of the pathways. Exceptions are intensive bottom trawling, resulting in a lower contribution of aerobic respiration, enhanced sulphate reduction and higher overall OC remineralisation, and dumping of harbour sediments showing 7 to 12 times lower OC contents compared to harbour sediments and hence low OC remineralisation rates. This indicates a significant impact of benthic and pelagic remineralisation and ultimately CO 2 formation during dredging and dumping of harbour sediments.