Results for deuterium and methane carbon isotope ratios. Left panel: δDCH4 values determined for different gas samples from IODP Expedition 338 . Horizontal bars indicate the standard deviation of duplicate measurements. Right panel: δ13CCH4 data obtained during drilling borehole C0002F (modified from ). Starting at 1700 mbsf, the δ13CCH4 data indicate a shift to methane of thermogenic origin (boundaries after ).