Deep subsurface carbon cycling in the Nankai Trough (Japan)—Evidence of tectonically induced stimulation of a deep microbial biosphere

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 indicate 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 recalcitrant 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 Cisotope 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 tectonic 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).

Evolution of (Bio‐)Geochemical Processes and Diagenetic Alteration of Sediments Along the Tectonic Migration of Ocean Floor in the Shikoku Basin off Japan

“…Several studies on deep subseafloor sediments show that organic matter that is highly refractory at seafloor temperature (∼2-3°C) becomes accessible for microbial processes with increasing burial and related heating of the sediments (Burdige, 2011;Parkes et al, 2007;Wellsbury et al, 1997). Riedinger et al (2015) suggested that frictional heating associated with earthquakes might reactivate recalcitrant organic matter in deep sediments from IODP Sites C0006 and C0008 in the Nankai Trough, resulting in the onset of methanogenesis several hundred meters below the seafloor. Although, we cannot provide evidence on the reactivity of the organic material, we hypothesize that the higher temperatures prevailing since ∼0.5 Ma increased the bioavailability of organic matter at Site C0023, which caused enhanced rates of biogenic methanogenesis and higher methane fluxes in both, up-and downward directions.…”

“…In addition to variations in organic matter quantity and reactivity related to oceanographic and climatic changes, changes in biogeochemical processes can also be induced by tectonic dynamics including fluid flow along faults, hydrothermal circulation and frictional heating (e.g., Fischer et al., 2013; Wehrmann & Riedinger, 2016). For example, recalcitrant organic matter in subseafloor sediments from the Nankai accretionary prism might be reactivated caused by frictional heating associated with earthquakes or advective fluid flow (Ijiri et al., 2018; Riedinger et al., 2015). Marine sediments represent one of the most valuable archives to reconstruct changes in paleoceanographic, depositional and climatic conditions over Earth's history.…”

Biogeography, Ecology, and Evolution of Deep Life

“…The establishment and propagation of the sediment-hosted deep biosphere is still under investigation, but likely includes a combination of selection from the surface environment (71) and persistence of cells with depth (72). What stimulates the deep biosphere, in addition to chemical interfaces, may include the continued influence of depositional conditions (68), tectonic activity (73), geological shifts under pressure (74), and the internal heating of Earth (75). It is still unknown exactly how the community in deep sediment responds to the stress of sedimentation.…”

Patterns of in situ Mineral Colonization by Microorganisms in a ~60°C Deep Continental Subsurface Aquifer

“…Little is known, however, about the changes in deep subsurface systems of yearly or longer timescales. It may be possible that deep biosphere organisms persist with little to no growth until a rare nutrient or energy delivery, perhaps mediated by tectonic disturbance (Riedinger et al, 2015). Controlled substrate addition experiments performed in situ offer a novel experimental approach to understand the influence of transient energy influx on deep biosphere community structure and help constrain the energy requirements and maximum metabolic rates of these organisms.…”