The core tubes used for CH4in porewater measurements were predrilled with holes 1 cm
apart from each other and closed by tape in order to keep sediment inside. The sediment was retrieved with a 2-ml syringe in which the tip was cut off and then transferred into 26 ml glass vials pre-filled with 4 ml of 2% NaOH to prevent methane oxidation. The vials were closed immediately with butyl rubber stoppers and aluminium crimp seals. The samples were kept cool until analysis. Methane concentrations were measured by GC-FID