2:00 PM - 2:15 PM
[BPT05-14] Origin of hydrocarbons from serpentinite-hosted Hakuba Happo hot spring: Molecular and intramolecular 13C analyses
Keywords:Serpentinite, Abiogenic hydrocarbon, Intramolecular isotopic analysis, Stable carbon isotope
The concentrations of C1 to C4 hydrocarbons were determined by Gas Chromatograph (GC) with two detectors of Pulsed Discharge Detector (PDD) and Thermal Conductivity Detector (TCD). Quantitative analysis of dissolved organic acids was performed by High Performance Liquid Chromatograph (HPLC). The δ13C values of C1 to C5 hydrocarbons were determined by using the on-line pre-concentration system coupled with the GC-C (combustion)-IRMS (isotope ratio mass spectrometer) method. For the first time, we apply a novel analytical method, namely intramolecular 13C analysis of propane (C3H8), to a natural sample derived from the serpentinite-hosted system. The intramolecular 13C composition in propane was measured using the GC-pyrolysis (Py)-GC-C-IRMS system (Gilbert et al., 2016).
The isotopic trend of depletion in 13C with increasing molecular weight (δ13C1 > δ13C2 > δ13C3 > ...) was observed in Happo sample, showing similar isotopic trend compared with that of Lost City field, a seafloor serpentinite–hosted hydrothermal system (Proskurowski et al., 2008). We developed an abiotic polymerization model to account for isotopic compositions of Happo hydrocarbons. The carbon isotopic compositions of Happo hydrocarbons can be explained by the successive addition of a single carbon compound (C1-compound) with constant isotopic fractionation. The result of the intramolecular analysis revealed that the difference of δ13C values between terminal and central carbon atom positions of propane (site preference; SP in ‰) for Happo sample showed a significantly high SP value compared with that of thermogenic origin. Such a highly SP value corresponded to that expected from a polymerization model developed in this study. Thus, both molecular and intramolecular 13C compositions of hydrocarbons for serpentinite–hosted sample suggested that the production mechanism of these hydrocarbons was abiotic polymerization of C1-compound with a constant isotopic fractionation. The δ13C value of methane suggested that methane was a reactant C1-compound for polymerization reaction. On the other hand, the formate and acetate were detected in Hakuba Happo hot spring. Formate was the second highest concentrated dissolved C1-compound after methane in Happo water, implying that formate was an alternative carbon source for abiotic polymerization in the serpentinite–hosted systems.