[U-08] Biogeochemistry of CO world

[U08-P02] Carbon isotopic analysis of formate in Hakuba-Happo hot spring

*Haruna Wakabayashi¹, Yuichiro Ueno^1,2,3, Alexis Gilbert¹, Taguchi Koudai ¹ (1.Tokyo Institute of Technology, 2.Earth Life Science Institute, 3.Japan Agency for Marin-earth Science and Technology)

Keywords:Hydrothermal system, Serpentinite, isotope

Hydrocarbons as well as organic acids have been found in serpentinite-hosted hydrothermal systems, which may have formed abiologically under the high H₂ environment (Nothaft et al., 2021). However, the origins of the organic acids are largely unknown. In this study, hot spring water was collected from an on-land serpentite-hosted hot spring in Hakuba-Happo area, Japan. Fluid chemistry and isotopic compositions of organic molecules were studied in laboratory. The hot spring water was strongly alkaline (pH > 10) and its temperature was about 50°C. The spring water contained high concentrations of hydrogen (H₂ > 150 µmol/L) and methane (CH₄ > 80 µmol/L). Also, ion chromatography analysis showed that the hot spring water contains acetate (3.1 μmol/L) and formate (23 μmol/L). In order to measure the carbon isotope ratios of these trace amounts of organic acids, we have developed an enrichment and extraction method using a rotary evaporator and Solid Phase Micro-Extraction (SPME). The newly-established method allows us to determine bulk and intramolecular carbon isotope compositions of these organic acids using Gas Chromatography - Pyrolysis - Gas Chromatography - Combustion - Isotope Ratio Mass Spectrometry (GC-C-Py-GC-IRMS). As a result, in the Happo #1 well, δ¹³C values of formate was -10‰, whereas methyl and carboxyl positions of acetate were -27‰ and -13‰, respectively. On the other hand, in the Happo #3 well, δ¹³C values of formate was -11‰, whereas methyl and carboxyl positions of acetate were -28‰ and -6‰, respectively. All the observed δ¹³C values of organic acids are significantly higher than that of CH₄ (-34‰: Suda et al., 2014) in the Hakuba Happo spring, and rather similar to ambient DIC (-7‰: Suda et al., 2022). Therefore, we would argue that the formate was originated from the reduction of dissolved carbonate. Based on these isotopic systematics, we will propose a potential abiological route to form organic acids in Hakuba-Happo hot spring.