Depositional processes, particulate organic carbon contents and origin across the Helgoland Mud Area, SE German Bight.

Fine-grained coastal and marine sediments represent the largest permanent sink for carbon on our planet. They harbor large stocks of particulate organic carbon (POC) derived from marine plankton, land plants and coastal vegetation that build POC via photosynthesis and in this way take up CO₂ from the atmosphere. The Helgoland Mud Area (HMA) represents the main depocenter for fine-grained and organic-rich sediments in the German Bight, North Sea. It covers a seafloor area of approximately 500 km² off the island of Helgoland with a sediment thickness of up to 21 m and water depths between 11 and 27 m. In order to be able to quantify the long-term storage of carbon and its turnover during mineralization in the sediments of the HMA, we reconstruct sedimentation rates for the past decades and determine POC contents and origin. These results on the natural POC dynamics in the HMA are fundamental in order to understand how on-going anthropogenic activities (e.g., bottom trawling, dredging, offshore infrastructure) may affect the POC storage capacity of the HMA. Complementing existing estimates of sedimentation rates, we have determined ²¹⁰Pb_xs and ¹³⁷Cs activities from the upper ~ 30 cm of the sediments in a high spatial and vertical resolution across the HMA. Based on this new dataset, we establish age models and describe the variation of depositional conditions across the HMA. The source of POC is determined by using the pore-water dissolved inorganic carbon concentrations (DIC) and respective stable carbon isotopic compositions with Keeling and Miller-Tans plot analysis as well as thermal gravimetric analysis (TGA). Published sedimentation rates for the HMA range from 0.5 to 11 mm yr^-1 with strong lateral variations. The highest rates have been reported for the NW and central HMA. Our first results on sedimentation rates show ~ 2 mm yr^-1 for these high accumulation areas within the HMA. POC contents in these areas are at ~ 1 wt% throughout the upper ~ 30 cm of the sediments. The source isotopic composition of POC ranges from -2.6 to -28.2 ‰ throughout the study area and needs to be complemented by on-going TGA analyses in order to reliably determine the origin of the POC. Once these analyses are completed, we aim to better understand the sources of organic carbon in this depocenter and to assess how the variation in depositional processes controls the mineralization and long-term burial of POC in the sediments of the HMA.