Highly isotopically depleted hydrocarbon gases in methane-derived carbonate of the Cretaceous age

We report δC compositions of the light hydrocarbon gases (methane: C1, ethane: C2, propane: C3, n-butane: n-C4, and iso-butane: iso-C4) included in the Cretaceous methanederived carbonate rocks. The sample aliquots used for this study was taken from a carbonate lens including fossil seep community found in the upper Cretaceous aged in the Omagari Formation, northern part of Hokkaido, Japan (44 39′26′′N, 142 2′25′′E). Based on their low δCcarbonate: -41.2~-44.4 (‰ PDB), a major portion of the carbonate seems to be generated by the oxidation of methane, probably during seafloor seeping activities of methane-rich fluids. Hydrocarbon gas was extracted by the acidification of carbonate sample in a glass vial bottle that had been already crused and flushed by highly purified helium, and δC of hydrocarbon gas liberated in gas phase of glass vial was analyzed directly by using isotope-ratio-monitoring-gas chromatography / mass spectrometer (irm-GC/MS). The concentrations of methane were 0.0207~0.434 ( ng/g) (nanogram per crushed carbonate) with the C1/[C2+C3] ratios of 1.2~4.2. The C1/[C2+C3] ratios are 2~4 orders of magnitude lower than the typical bacterial hydrocarbons. The δC values of methane were typical values for bacterial methane (-63.9~-70.2‰). On the other hand, the other hydrocarbon gases showed highly C depleted carbon isotopic compositions (C2: -55.0~-91.7‰, C3: -58.2~-99.4‰, n-C4: -91.8~-163.7‰, iso-C4: -25.7~-65.4‰). The relationships of the δC values for these hydrocarbons in each sample showed a pattern of δC1>δC2>δC3>δCn-4 which is in contrast to the widely accepted isotope distribution pattern of natural gases. There may be two possibilities to explain such δC compositions: 1) syntheses of the hydrocarbons by way of polymerization of bacterial methane (δC: -63.9~-70.2‰), 2) fortuitous mixture of thermally generated hydrocarbons (δC1 <δC2 <δC3 <δCn-4) from a highly C-depleted precursor organic molecules (δC: ~-160‰) probably produced by methanotrophic and those from ordinary marine organic compounds (δC: ca. –20‰). Migration and fixation of organic carbon in vegetation – forest soil system