Cruise report RV Pelagia campaign 64PE525 Version 2

Methane (CH4) is an important greenhouse gas, that is 32 times more potent than CO2 on a time horizon of 100 years (Etminan et al., 2016). Though the half-life of atmospheric methane is relatively short (~ 9 years), CH4 concentrations in the atmosphere have risen drastically to current values of ~ 1.9 ppm; i.e. ~ 2.5-fold higher compared to preindustrial values (IPCC, 2021). The exact mechanisms behind this increase are not fully understood, yet it is clear that the increase in the atmospheric methane budget is caused by anthropogenic sources (Nisbet et al., 2019); together, these amount to 336 - 376 Tg yr-1 . Natural CH4 sources are overall on a similar scale contributing about 40% (183–248 Tg yr-1 ) to the total atmospheric budget (Saunois et al., 2020; IPCC, 2021). Though it is likely that the overall contribution of the Ocean to the atmospheric methane content is comparably small, ocean budget estimates vary considerably. Shelf seas for example are with 9 - 22 Tg yr-1 the main contributor of natural oceanic methane emission to the atmosphere though the shelf only accounts for 8 - 10 % of the global ocean surface (Weber et al., 2019; Saunois et al., 2020). Compared to the open ocean, shelf seas are characterized by high nutrient loads, high primary production, high sedimentation rates and subsequent burial of dead organic matter to the seabed. This leads to elevated methane production in sediments by microbial or thermogenic processes (Reeburgh, 2007). Methane emission related to offshore industries on the other hand are virtually unknown. However, methane in shelf sea sediments is of commercial value and thus extracted. After exploitation, wells are sealed to prevent leakage, but recent investigations in the UK and German sector of the North Sea by Vielstädte et al. (2017) and Böttner et al. (2020), however, found acoustic flares in the water column in vicinity of well heads; these acoustic features are caused by plumes of rising bubbles emanating from the sea floor. The authors almost always found such flares if the well heads were above bright spots in seismic data. Bright spots are caused by high amplitude response in sound velocity of the subsurface, e.g. triggered by the presence of shallow gas. These previous studies could not fully clarify why the location of sea floor gas release was often at some distance from the well head (i.e. the leak-tightness of the well head seal itself was not necessarily compromised). However, the statistical co-occurrence of flares and (abandoned) wells suggested a causality. The Dutch sector of the North Sea is rich in shallow and deep subsurface methane. The deep methane is extracted from multiple active wells and many more wells are abandoned and sealed. With respect to the previous findings (Vielstädte et al., 2017; Böttner et al., 2020) this raises the questions: - if and how frequent gas release occurs at or in the vicinity of (abandoned) wells in the Dutch sector of the North Sea; - if the gas release is caused by the previous drilling and/or inappropriate sealing attempts.