Page 125 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

CH4 indicates that a local CH4-producing mechanism is active. Previous research indicates that deep methane seeps in the area are of biogenic origin. The activity of methanogenic microorganisms accompanying the observed CH4 concentrations suggests that methanogens are metabolizing some carbon into CH4 at a low rate. Sites with high CH4 concentrations are likely indicative of deep methane seeps. This study provides insights on methane stores at the landscape scale of basal-peat deposits in the North Sea and serves as a step towards reducing uncertainties in the global CH4 budget and better understanding the role of basal-peat deposits in the global CH4 cycle.
The CH4 concentrations of 1-30 μmol l-1 observed in the peat layer of the mid- and southern North Sea in the present study are an order of magnitude higher than background concentrations measured in shallow North Sea sediments (<0.1 μmol l-1; Niemann et al., 2005; Steinle et al., 2016) and much higher than concentrations observed in the water column (maximum of 1.1 μmol l-1) (Borges et al. 2016), with the exception of muddy sediments like those of the Helgoland Bight, where observed CH4 concentrations reached values of up to 6 mmol l-1 (Aromokeye et al. 2020). Borges et al. (2016) reported average CH4 concentrations in the water column of 0.139 μmol l-1 (near-shore) and 0.024 μmol l-1 (off-shore) and a maximum concentration of 1.128 μmol l-1. However, in situ CH4 observations of peat sediments have not yet been measured because peat sediments are now inundated, buried several meters beneath marine sediments.
The CH4 concentrations measured in this study are higher than those measured in the water column in the same area, due to the metabolism of CH4 by methanogenic bacteria present in shallow sediments (Zhuang et al. 2018). Darci’s site is influenced by a known biogenic CH4 gas seep located ±600 m beneath the seafloor (Schroot, Klaver and Schüttenhelm 2005). The CH4 concentrations observed in these peat deposits are lower than but similar in magnitude to those found in the near-surface sediments (less than 0.2 m beneath the sea floor) of a highly active gas seep in the northern North Sea (Niemann et al. 2005). The large methane concentrations observed at the Vittorio site may be the result of a deep gas seep.
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