CH4 emissions from a marine aggregate extraction site offshore Sylt (eastern North Sea, Germany)

CH4 emission from coastal regions contribute with up to 1 % to the global CH4 budget but show large uncertainties in terms of spatial resolution and temporal variability. In-situ measurements of CH4 and CO2 in ambient air are carried out by the German Federal Environment Agency (UBA) at the coastal station Westerland (Sylt, Germany). High-resolution CH4 data from 2022, with a temporal resolution of several seconds, show CH4 spikes of up to 400 ppb above background. These spikes occurred under west wind conditions (marine air from the North Sea) during low tide and mainly in summer. Beach nourishment at the west coast of Sylt have been regularly carried out since 1984. From April to October, dredging vessels extract a mixture of sand and water from the seafloor of a spatially confined area (Westerland II), located 8 km off the coast and transport it to the beach and foreshore on the west coast of Sylt. To investigate the relation between sand dredging and the observed atmospheric CH4 spikes, continuous measurements of dissolved and atmospheric CH4 concentrations, were conducted on board of RV “Mya II” (AWI) from Sylt along the coast to the dredging site Westerland II. In addition, water samples were taken at different depths to determine dissolved CH4 concentrations and isotopic signatures. Near the dredging area, we observed elevated CH4 concentrations in ambient air (400-500 ppb above background) and in surface and bottom waters (80 – 100 nmol/L). The saturation of dissolved CH4 in surface waters ranged from around 100% in the control area to up to 3600% in the dredging area. Depending on the wind strength this resulted in a diffusive flux of 48 ± 47 µmol m2 d-1 in the dredging area, in contrast to a diffusive flux of 25 ± 30 µmol m2 d-1 in the control area. Taking the severe storm just before our cruise which should have led to a strong degassing of the whole area into account, the dredging seems to be a strong point source for dissolved CH4. Our measurements clearly confirmed elevated concentrations and fluxes of CH4, both in the atmosphere and the water column above the sand dredging site, compared to the surrounding areas. Moreover, dissolved CH4 within the dredging site was characterized by more negative stable carbon and hydrogen values compared to outside Westerland II, indicating a microbial origin of the excess CH4 there.