Assessing methane emissions from an offshore marine aggregate extraction site near Sylt, Eastern North Sea

Abstract. Coastal regions are estimated to contribute with up to 1 % to the global atmospheric methane (CH₄) budget. However, these emissions remain highly uncertain due to strong spatial and temporal variability. Anthropogenic activities in coastal waters, such as sand mining and beach nourishment, have not yet been investigated or included in the calculation of the resulting greenhouse gas emissions. Since 1984, regular beach nourishment has been carried out every summer on the west coast of the island of Sylt (North Sea, Germany). During this process, dredging vessels extract a mixture of sand and water from the seabed of a spatially confined area (Westerland II), situated approximately 8 km off the coast and deposit the material on the western beach and foreshore of Sylt. High resolution measurements of CH₄ in ambient air at the coastal atmospheric station Westerland (Sylt, Germany) show CH₄ spikes of up to 400 ppb above background concentrations. These spikes occurred mainly during summer season, during low tide and under westerly wind conditions (from the sand dredging area).

To investigate the origin of the observed atmospheric CH₄ spikes, combined in-situ measurements of dissolved and atmospheric CH₄ together with water sampling, were performed on board the research vessel RV Mya II (AWI) along a coastal transect from Sylt to the Westerland II dredging site. In the vicinity of the dredging site, elevated CH₄ concentrations were detected both in ambient air (400–500 ppb above background reaching up to 2450 ppb) and in surface and bottom waters (69–90 nmol L^-1). This resulted in a mean diffusive flux of 45 ± 47 µmol m^-2 d^-1 with a maximum value of 340 µmol m^-2 d^-1 in the dredging area, in contrast to a diffusive flux of 3.0 ± 3.6 µmol m^-2 d^-1 during the transit. Our observations demonstrate significantly higher CH₄ concentrations and fluxes in both the atmosphere and the water column above the sand dredging site, compared to the areas outside the dredging activities. Furthermore, the isotopic composition of dissolved CH₄ within the dredging site characterized by more negative stable carbon and hydrogen isotope values, point to a microbial source of the excess CH₄.