The IsoJar™ container is widely used in headspace gas analysis for gases adsorbed on cuttings or bore cores from oil and gas fields. However, large variations in the carbon isotopic ratios of CH4 and CO2 are often reported, especially for data obtained from depths of <1000 m. The IsoJar™ method leaves air in the headspace that allows microbial oxidation of CH4 to CO2, meaning that isotopic fractionation occurs during storage. This study employed the IsoJar™ method to investigate the causes of differences in δ13C data reported by previous studies in the Horonobe area of Japan. It was found that after 80 d storage, δ13CCO2 values decreased by ~2‰, while δ13CCH4 values increased by >30‰, whereas samples analyzed within a week of collection showed no such fluctuations. The conventional amount of microbial suppressant (~0.5 ml of 10% benzalkonium chloride (BKC) solution) is insufficient to suppress microbial activity if groundwater is used as filling water. The significant variations in carbon isotopic compositions previously reported were caused by microbial methane oxidation after sampling and contamination by groundwater from different depths. To avoid these problems, we recommend the following: (1) if long-term sample storage is necessary, >10 ml of 10% BKC solution should be added or >0.3% BKC concentration is required to suppress microbial activity; (2) analyses should be performed within one week of sampling; and (3) for CO2 analyses, it is important that samples are not contaminated by groundwater from different depths.
To reveal the thermal history of the Allende (CV3) meteorite, we pyrolyzed insoluble organic matter (IOM) from the meteorite and examined the chemical and isotopic composition of the pyrolysates against the pyrolysis temperature. Major pyrolysates from the Allende IOM were sulfur (S)-bearing compounds (H 2 S, SO 2 , CS 2 , and OCS), oxygen (O)-bearing compounds (H 2 O, CO, and CO 2 ), and hydrogen gas. S-bearing compounds mainly appeared in a pyrolysis temperature range of 250-300°C, O-bearing compounds mainly appeared at all pyrolysis temperatures, and hydrogen gas mainly appeared in a range of 550-800°C. The IOM scarcely released aliphatic and aromatic hydrocarbons and nitrogen (N)bearing compounds, although they were major pyrolysates of the Murchison IOM at 450-550°C. Regarding the calculated isotopic data of the Allende pyrolysates, the δ 13 C value was almost constant for all the pyrolysis temperatures. The δ 15 N value was constant up to 550°C and then drastically decreased at 550-800°C. The δD values of the pyrolysates at all pyrolysis temperatures were lower than the δD values of the starting IOM. If we assumed that the Allende IOM originated from a primitive IOM (such as the Murchison IOM), our results suggested that the Allende IOM suffered a two-stage thermal process: the first stage was a thermal event below 550-800°C that caused the loss of primary materials (aliphatic and aromatic hydrocarbons, N-bearing compounds) enriched in 13 C, D and 15 N, and the second stage was a thermal event below 300°C that led to the addition of secondary S-and O-bearing compounds depleted in D. Based on our data and previously reported data (the peak metamorphic temperature of the Allende IOM is 550-590°C), it was proposed that the Allende meteorite experienced thermal metamorphism at 550-590°C followed by an alteration below 300°C.
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