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Abstract:
The abundance of microbial life and the sources of energy necessary for deep subsurface
microbial communities remain enigmatic. Here we investigate deep microbial processes and their potential
relationships to tectonic events in sediments from the Nankai Trough offshore Japan, drilled and sampled
during IODP (Integrated Ocean Drilling Program) Expedition 316. Observed methane isotope profiles indi-
cate that microbially mediated methane production occurs at Sites C0006 and C0007 in sediments below
450 meters below seafloor (mbsf) and 425 mbsf, respectively. The active carbon cycling in these deep
subsurface sediments is likely related to the highly dynamic tectonic regime at Nankai Trough. We propose
that transient increases in temperature have restimulated organic matter degradation at these distinct
depths and explore several candidate processes for transient heating. Our favored hypothesis is frictional
heating associated with earthquakes. In concert with transient heating leading to the reactivation of recalci-
trant organic matter, the heterogeneous sedimentary system provides niches for microbial life. The newly
available/accessible organic carbon compounds fuel the microbial community—resulting in an onset of
methanogenesis several hundred meters below the seafloor. This process is captured in the methane C-
isotope signal, showing the efficacy of methane C-isotopes for delineating locations of active microbial
processes in deeply buried sediments. Additionally, simple model approaches applied to observed chemical
pore water profiles can potentially constrain timing relationships, which can then be linked to causative tec-
tonic events. Our results suggest the occurrence of slip-to-the-trench earthquake(s) 200–400 year ago,
which could relate to historical earthquakes (1707 Hoei and/or 1605 Keicho earthquakes).
