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

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 North Sea by using the Helgoland Mud Area (HMA) as a natural test field. The HMA represents the most significant depocentre of fine-grained and organic-rich sediments in the German Bight (SE North Sea). We examined factors including sedimentation and accumulation rate, sediment-mixing rate, grain size, total organic carbon (TOC) content, and aerobic remineralisation rate. Highest sedimentation rates (SRs) 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, reaching organic carbon burial efficiencies of >65 %. Sedimentation rates 4 times lower and the lowest TOC contents (0.7 wt %–1.0 wt %) 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, thereby enhancing OM preservation. Our data support this finding, demonstrating and confirming that sedimentation rate is the key factor determining organic carbon burial efficiency (OC BE) and long-term sedimentary carbon storage. 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 parts of the HMA, a mixture of marine and terrigenous OM is 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. Sediment-mixing rates were highest in the northwestern HMA, where the highest bottom-trawling activity is also 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 %. The annual burial flux of organic carbon in the HMA amounts 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 total annual organic carbon accumulation amounts to 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.