Water column methane dynamics at an abandoned well (southern North Sea, sector A15)

The coastal ocean is highly dynamic and typically rich in methane, which is produced in sediments by microbial or thermogenic processes. In the North Sea subsurface methane is commercially exploited; once a gas field is exhausted the extraction wells are sealed with concrete. Despite this precaution, these wells may leak, becoming a potential source of methane, which might reach the atmosphere. There are several thousand abandoned wells in the North Sea, but the amount of methane escaping from these and the further fate of this methane including microbial methane oxidation and liberation to the atmosphere are not well constrained. We investigated methane dynamics at an abandoned well in the Dutch sector of the North Sea (A15-3), using a combination of different sampling and investigation tools. We conducted continuous in situ measurements of water column methane concentration, currents and various other physicochemical parameters using a bottom lander (ALBEX). Furthermore, temporally replicated hydro casts were carried out to resolve methane concentrations, vertically. We also recovered discrete water samples to investigate aerobic methanotrophs and associated methane oxidation rates, both vertically and over time. First results show an elevated background methane level of about 30 nM – yet, we recorded several events of high methane concentration reaching up to 300 nM for a duration of 10 - 60 minutes. Intervals of elevated water column methane concentrations and methane oxidation rates were also found during hydro casts. We will discuss these data in relation to environmental forcing factors including tidally induced current and hydrostatic pressure changes, as well as biological factors such as the methanotrophic community dynamics.