9:20 AM - 9:35 AM
[MIS33-02] Regional variation of CH4 and N2 production processes in the deep aquifers of an accretionary prism
Keywords:accretionary prism, deep aquifer, methanogens, fermentative bacteria, denitrification
Accretionary prisms are thick sediments that are formed at a convergent plate boundary, and it is observed around the world. In Southwest Japan, accretionary prism is distributed in a wide region from the coastal area of the Pacific Ocean side to the mountainous area. Since the sediment contains layers of water-bearing permeable sandstone, groundwater is anaerobically reserved in the deep aquifers. In addition to the groundwater, it has been reported that a large amount of the natural gas, mainly CH4 and nitrogen gas (N2) is present in the deep aquifers. However, the processes of CH4 and N2 production in the deep aquifers associated with accretionary prism is poorly understood. The objectives of this study were to identify the origin of the CH4 that is reserved in the deep aquifers on the basis of the carbon stable isotope analysis. We also determined the processes and potential of microbial CH4 and N2 productions using 16S rRNA gene analysis and culture experiments.
The groundwater and natural gas derived from the deep aquifers of accretionary prism were collected from 14 deep wells situated in Shizuoka Prefecture, Japan. CH4 was the predominant component of the natural gas derived from deep aquifer of the coastal and middle areas of the accretionary prism (>96 vol.%). In contrast, the natural gas collected from deep aquifer of the mountainous area included a considerable amount of N2 (23–50 vol.%) as well as CH4. The stable carbon isotope analysis of CH4 in the natural gas and the DIC in the groundwater, mainly bicarbonate, showed that CH4 included in the natural gas of the coastal area is thermogenic origin. On the other hand, it was suggested that the natural gas of the middle and mountainous areas mainly contains CH4 of biogenic origin. Next generation sequencing analysis of bacterial and archaeal 16S rRNA genes showed the dominance of methanogenic archaea, fermentative bacteria, and denitrifying bacteria in the groundwater. Furthermore, high potential of CH4 production through anaerobic degradation of organic substrates by syntrophic consortium of H2-producing fermentative bacteria and H2-using methanogenic archaea was observed from anaerobic cultivation using the groundwater samples collected from the middle and mountainous areas of the accretionary prism. In addition, high potential of N2 production by denitrifying bacteria was also confirmed from anaerobic cultivation using the groundwater samples collected form the mountainous area.
From these results, it was suggested that CH4 is produced by a thermogenic process especially in the deep aquifer of the coastal area associated with the accretionary prism, and H2-producing fermentative bacteria and H2-using methanogenic archaea contribute to significant CH4 production in the deep aquifer of the middle and mountainous areas. Our results also suggest that N2 production by denitrifying bacteria occurs particularly in the deep aquifer of the mountainous area.