In situ monitoring reveals episodic water column methane anomalies at abandoned wells in the Dutch North Sea

Continental margins harbor substantial reservoirs of methane, generated by microbial activity or thermogenic processes. In the North Sea, commercial extraction of subsurface methane is common, and wellheads of depleted gas field are typically sealed with concrete. Despite these measures, abandoned wells may still leak methane to the water column and potentially to the atmosphere contributing to atmospheric methane levels. With several thousand of such wells scattered across the North Sea, the scale of these emissions and the processes leading the fate of the released methane—whether through microbial oxidation or direct escape into the atmosphere—are still not well understood. We investigated methane dynamics at 3 different  locations in the Dutch sector of the North Sea (A15-03 and B17-05 abandoned wells, B17-04 likely natural seepage), combining various methods, including  autonomous tools. For a time period of 3 days, we continuously measured in situ bottom water methane concentrations and near-bed hydrodynamics using a laser spectrometer and ADCP mounted on a mini-lander. We recorded several episodic events characterized by increasing methane concentrations peaking at 550nM at A15-03 and 800nM at B17-05. In contrast, maximum concentrations remained comparably low at B17-04 with values of up to 80nM. To further resolve vertical methane distribution, we conducted repeated hydro casts that also showed events of rising water column methane concentrations. Discrete water samples were additionally taken to quantify microbially mediated methane oxidation rates, revealing the presence of methanotrophs that could act as a filter for methane escaping to the atmosphere. In this presentation, we will discuss our data in relation to environmental drivers, including tides, currents and biological factors such as methanotrophic community dynamics.