22 Jun 2023
 | 22 Jun 2023

Microbial methane formation in deep aquifers associated withthe sediment burial history at a coastal site

Taiki Katayama, Reo Ikawa, Masaru Koshigai, and Susumu Sakata

Abstract. Elucidating the mechanisms underlying microbial methane formation in subsurface environments is essential to understand the global carbon cycle and to explore natural gas deposits. This study examined how microbial methane formation (i.e. methanogenesis) occurs in natural gas-bearing sedimentary aquifers throughout the sediment burial history. Water samples collected from six aquifers of different depths exhibited ascending vertical gradients in salinity from brine to freshwater and in temperature from mesophilic to psychrophilic conditions. Analyses of gas and water isotopic ratios and microbial communities indicated the predominance of methanogenesis via CO2 reduction. However, the hydrogen isotopic ratio of water changed along the depth and salinity gradient, whereas the ratio of methane changed little, suggesting that in situ methanogenesis in shallow sediments does not significantly contribute to the methane in the aquifers. The population of methane-producing microorganisms (methanogens) was highest in the deepest saline aquifers, where the water temperature, salinity, and the total organic carbon content of the adjacent mud sediments were highest. Cultivation of the hydrogenotrophic methanogens that dominated in the aquifers showed that the methanogenesis rate was maximized at the temperature corresponding to that of the deepest aquifer. These results suggest that high-temperature conditions in deeply buried sediments are associated with enhanced in situ methanogenesis, and that methane formed in the deepest aquifer migrates upwards into the shallower aquifers by diffusion.

Taiki Katayama et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1285', Anonymous Referee #1, 16 Jul 2023
    • AC1: 'Reply on RC1', Taiki Katayama, 25 Jul 2023
  • RC2: 'Comment on egusphere-2023-1285', Anonymous Referee #2, 31 Jul 2023
    • AC2: 'Reply on RC2', Taiki Katayama, 31 Aug 2023

Taiki Katayama et al.


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Short summary
Methane produced by microorganisms in subsurface environments may account for a large fraction of global natural gas reserves. To understand how microbial methane is produced during sediment burial history, we examined methane-bearing aquifers in which temperature and salinity increase with depth. Geochemical and microbiological analyses showed that microbial methane is produced at depth, where microbial activity is stimulated by the increased temperature, and subsequently migrates upwards.