Depositional controls and budget of organic carbon burial in fine-grained sediments of the North Sea – the Helgoland Mud Area as a test field

Abstract. The burial of organic matter (OM) within fine-grained continental shelf sediments represents one of the major long-term sinks of carbon. We investigated the key factors controlling organic carbon burial in sediments of the Helgoland Mud Area (HMA), which represents the most significant depocentre of fine-grained and organic-rich sediments in the German Bight (SE North Sea). The examined factors include sedimentation and accumulation rates, sediment mixing rates, grain size, total organic carbon (TOC) content and aerobic remineralisation rates. Highest sedimentation rates of up to ~4.5 mm yr^-1 and average TOC contents of 2 wt% were found in the southern part of the HMA which is under the influence of the Elbe river outflow. The overall highest organic carbon burial efficiencies of >65 % were also determined in this area. Four times lower sedimentation rates and lowest TOC contents were found in the shallow, eastern part of the research area, with the lowest organic carbon burial efficiencies being 30 %. High sedimentation rates are known to limit oxygen exposure time and thereby enhance OM preservation. Our data support this finding and demonstrate that sedimentation rate is the key factor determining organic carbon burial efficiency and long-term sedimentary carbon storage. The area of the HMA is characterized by varying mixtures of OM from marine and terrestrial sources. In the southern part of the HMA, close to the outflow of the Elbe river, the OM being degraded is primarily of terrigenous origin, while in the central and northern part of the HMA a mixture of marine and terrigenous OM has been shown to be remineralised. At the sites dominated by the degradation of marine organic matter, as found in the western and northwestern HMA, the organic carbon burial efficiency is lower and fluctuates around 55 %. The burial efficiency of OM is highest in sedimentary habitats characterised by high sedimentation rates and OM of terrigenous sources. Our modelled sediment mixing rates were highest in the northwestern HMA, where also the highest bottom trawling activity is reported. The comparison of sites similar in depositional characteristics but different in bottom trawling intensity suggests that in the area of intense bottom trawling in the northwestern HMA the sequestration of OM is reduced by around 30 %. Furthermore, we have determined the annual burial flux of organic carbon in the HMA that amounted to an average of 22.5 g C m^-2 yr^-1. Considering the strong tidal currents in the shallow HMA, the burial flux is exceptionally high and even compares with those reported for the deeper Skagerrak and Norwegian Trough (~10 to 66 g C m^-2 yr^-1), which are the main depocentres for fine-grained and organic-rich sediments in the North Sea. For the entire HMA the determined burial flux results in a total annual organic carbon accumulation of 0.011 Tg C yr^-1. These findings highlight the importance of depocentres for fine-grained sediments as important carbon sinks: while the area of the HMA represents only 0.09 % of the North Sea it stores 0.76 % of the total annual accumulated organic carbon in this shelf sea area.