Wataru Tanikawa

Exploring deep microbial life in coal-bearing sediment down to ~2.5 km below the ocean floor

A deep sleep in coal beds Deep below the ocean floor, microorganisms from forest soils continue to thrive. Inagaki et al. analyzed the microbial communities in several drill cores off the coast of Japan, some sampling more than 2 km below the seafloor (see the Perspective by Huber). Although cell counts decreased with depth, deep coal beds harbored active communities of methanogenic bacteria. These communities were more similar to those found in forest soils than in other deep marine sediments. Science, this issue p. 420; see also p. 376 Coal beds more than 2 kilometers below the seafloor host methanogenic bacteria related to those found in forest soils. [Also see Perspective by Huber] Microbial life inhabits deeply buried marine sediments, but the extent of this vast ecosystem remains poorly constrained. Here we provide evidence for the existence of microbial communities in ~40° to 60°C sediment associated with lignite coal beds at ~1.5 to 2.5 km below the seafloor in the Pacific Ocean off Japan. Microbial methanogenesis was indicated by the isotopic compositions of methane and carbon dioxide, biomarkers, cultivation data, and gas compositions. Concentrations of indigenous microbial cells below 1.5 km ranged from <10 to ~104 cells cm−3. Peak concentrations occurred in lignite layers, where communities differed markedly from shallower subseafloor communities and instead resembled organotrophic communities in forest soils. This suggests that terrigenous sediments retain indigenous community members tens of millions of years after burial in the seabed.

Changes to magnetic minerals caused by frictional heating during the 1999 Taiwan Chi-Chi earthquake

We carried out magnetic mineral analyses of samples from the shallowest major fault zone within the Chelungpu fault system, which is the zone that previous researchers believe slipped during the 1999 Taiwan Chi-Chi earthquake. Our aim was to gain an understanding of the changes to magnetic minerals during the earthquake. Magnetic hysteresis and low-temperature thermal demagnetization measurements showed that high magnetic susceptibilities in the black gouge zone within the major fault zone could be attributed not to fining of ferrimagnetic minerals but, rather, to their abundance. Thermomagnetic analyses indicated that the strata in and around the fault zone originally contained thermally unstable iron-bearing paramagnetic minerals, such as pyrite, siderite, and chlorite. We therefore concluded that frictional heating (>400°C) occurred in the black gouge zone in the major fault zone during the slip of the Chi-Chi earthquake and that the resultant high temperature induced thermal decomposition of paramagnetic minerals to form magnetite, resulting in the observed high magnetic susceptibilities.

Influence of fault slip rate on shear-induced permeability

[1] We measured permeability in sandstone and granite sheared at slip rates from 10 ―4 to 1.3 m/s under low-normal stress at confining pressures up to 120 MPa. As the slip rate increased, the permeability of Berea sandstone decreased by an order of magnitude, whereas that of Indian sandstone and Aji granite increased by 3 orders of magnitude at high slip rates. A fine-grained gouge layer of thickness developed during slip, and the wear rate was increased abruptly at high slip rates. Microcracks and mesoscale fractures formed at slip rates above 0.13 m/s. Numerical modeling showed that the slip surface temperature increased by several hundred degrees for slip velocities above 0.13 m/s and exceeded the α-β phase transition temperature of quartz at 1.3 m/s. Both the temperature rise and the temperature gradient at the slip surface were high at fast slip rates. We attributed reduced permeability after slip in porous sandstone to the low-permeability gouge layer. An abrupt permeability increase in low-permeability rocks at high slip rates was caused by heat-induced cracks. An increase in the rate of wear of gouge with increasing slip velocity was caused by frictional heating that reduced the rock strength. The host-rock permeability that separated reductions and increases in permeability was about 10 ―16 m 2 at 10 MPa effective pressure. Our results suggest that abrupt increases in shear stress during slip in a low-permeability fault zone caused by thermal cracking, which may decrease the total slip displacement. The abrupt permeability increase at high slip rates in low-permeability rocks agrees with hydrogeochemical phenomena observed after earthquakes.

Mineral assemblage anomalies in the slip zone of the 1999 Taiwan Chi‐Chi earthquake: Ultrafine particles preserved only in the latest slip zone

We determined mineral assemblages of samples from the Taiwan Chelungpu fault and from milling and heating experiments by using X-ray diffraction and scanning and transmission electron microscopy. The fault system contains three dominant fault zones, the shallowest of which slipped during the 1999 Chi-Chi earthquake. The quartz and clay mineral contents of the primary slip zone were low, and it contained partly amorphous ultrafine particles (several tens of nanometers). Up to 30 weight percent of materials in that zone could not be fit to standard diffraction patterns, whereas nearly 100 weight percent of those in surrounding samples could be. The unfitted component could be attributed to the observed ultrafine particles produced by comminution during the earthquake, because weak diffraction intensities are caused from mineral lattice distortion, granulation, and amorphous coatings. Such particles are a potential proxy for identifying the slip zone of the most recent earthquake along a fault.

Thermal conductivities, thermal diffusivities, and volumetric heat capacities of core samples obtained from the Japan Trench Fast Drilling Project (JFAST)

We report thermal conductivities, thermal diffusivities, and volumetric heat capacities determined by a transient plane heat source method for four whole-round core samples obtained by the Japan Trench Fast Drilling Project/Integrated Ocean Drilling Program Expedition 343. These thermal properties are necessary for the interpretation of a temperature anomaly detected in the vicinity of the plate boundary fault that ruptured during the 2011 Tohoku-Oki earthquake and other thermal processes observed within the Japan Trench Fast Drilling Project temperature observatory. Results of measured thermal conductivities are consistent with those independently measured using a transient line source method and a divided bar technique. Our measurements indicate no significant anisotropy in either thermal conductivity or thermal diffusivity.

Dynamic weakening of smectite‐bearing faults at intermediate velocities: Implications for subduction zone earthquakes

The hydrous clay mineral smectite, which is pervasive in sediments on subducting oceanic plates, is thought to weaken and stabilize subduction thrust faults. However, these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench. Here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 µm/s to 1.3 m/s for water-saturated smectite-quartz mixtures with various smectite contents, so as to shed light on the frictional response of smectite-bearing faults at intermediate to high slip rates. At slip rates of 30 to 150 µm/s, the friction coefficients decreased gradually from 0.5–0.6 to 0.1 with an increase in smectite content from 20 to 50 wt %. In contrast, at slip rates higher than 1.3 mm/s, friction exhibited marked slip weakening, resulting in low friction coefficients of 0.1–0.05, even for low smectite contents (roughly <30 wt %). Drastic slip weakening occurred at smectite contents of 10–30 wt % at slip rates of ~10 mm/s, which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types. The intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear-enhanced compaction and microscopic thermal pressurization of pore fluids. This process could weaken the fault even below seismic slip rates, leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench.

Biological CO2 conversion to acetate in subsurface coal-sand formation using a high-pressure reactor system

Geological CO2 sequestration in unmineable subsurface oil/gas fields and coal formations has been proposed as a means of reducing anthropogenic greenhouse gasses in the atmosphere. However, the feasibility of injecting CO2 into subsurface depends upon a variety of geological and economic conditions, and the ecological consequences are largely unpredictable. In this study, we developed a new flow-through-type reactor system to examine potential geophysical, geochemical and microbiological impacts associated with CO2 injection by simulating in-situ pressure (0–100 MPa) and temperature (0–70°C) conditions. Using the reactor system, anaerobic artificial fluid and CO2 (flow rate: 0.002 and 0.00001 ml/min, respectively) were continuously supplemented into a column comprised of bituminous coal and sand under a pore pressure of 40 MPa (confined pressure: 41 MPa) at 40°C for 56 days. 16S rRNA gene analysis of the bacterial components showed distinct spatial separation of the predominant taxa in the coal and sand over the course of the experiment. Cultivation experiments using sub-sampled fluids revealed that some microbes survived, or were metabolically active, under CO2-rich conditions. However, no methanogens were activated during the experiment, even though hydrogenotrophic and methylotrophic methanogens were obtained from conventional batch-type cultivation at 20°C. During the reactor experiment, the acetate and methanol concentration in the fluids increased while the δ13Cacetate, H2 and CO2 concentrations decreased, indicating the occurrence of homo-acetogenesis. 16S rRNA genes of homo-acetogenic spore-forming bacteria related to the genus Sporomusa were consistently detected from the sandstone after the reactor experiment. Our results suggest that the injection of CO2 into a natural coal-sand formation preferentially stimulates homo-acetogenesis rather than methanogenesis, and that this process is accompanied by biogenic CO2 conversion to acetate.

Thermal conductivities under high pressure in core samples from IODP NanTroSEIZE drilling site C0001

We examined the effects of high pressure on thermal conductivity in core samples from the slope–apron facies and the upper part of the accretionary prism at site C0001 of the NanTroSEIZE drilling program and in other samples of five terrestrial rock types. Thermal conductivity clearly increased with increasing pressure for both wet (water saturated) and dry samples. We determined the rate of thermal conductivity change of the NanTroSEIZE sediments to be 0.014 Wm−1K−1/MPa when pressure was increased, and 0.01 Wm−1K−1/MPa when pressure was decreased. Using the rate determined for decreasing pressure, we estimated that thermal conductivities measured at atmospheric pressure rather than at in situ pressure may be underestimated by 7% for a core sample from around 1 km depth and by 20% for a core sample from around 3 km depth. In general, the rate of thermal conductivity change with pressure showed a positive correlation with porosity. However, the relationship of the rate of thermal conductivity change to porosity is also dependent on the fabric, mineral composition, and pore structure of the sediments and rocks. Furthermore, for two sandstones we tested, the effect of pressure on thermal conductivity for dry samples was greater than that for wet samples.

Helium anomalies suggest a fluid pathway from mantle to trench during the 2011 Tohoku-Oki earthquake

Geophysical evidence suggests that fluids along fault planes have an important role in generating earthquakes; however, the nature of these fluids has not been well defined. The 2011 magnitude 9.0 Tohoku-Oki earthquake ruptured the interface between the subducting Pacific plate and the overlying Okhotsk plate. Here we report a sharp increase in mantle-derived helium in bottom seawater near the rupture zone 1 month after the earthquake. The timing and location indicate that fluids were released from the mantle on the seafloor along the plate interface. The movement of the fluids was rapid, with a velocity of ~4 km per day and an uncertainty factor of four. This rate is much faster than what would be expected from pressure-gradient propagation, suggesting that over-pressurized fluid is discharged along the plate interface.

Near-trench slip potential of megaquakes evaluated from fault properties and conditions

Near-trench slip during large megathrust earthquakes (megaquakes) is an important factor in the generation of destructive tsunamis. We proposed a new approach to assessing the near-trench slip potential quantitatively by integrating laboratory-derived properties of fault materials and simulations of fault weakening and rupture propagation. Although the permeability of the sandy Nankai Trough materials are higher than that of the clayey materials from the Japan Trench, dynamic weakening by thermally pressurized fluid is greater at the Nankai Trough owing to higher friction, although initially overpressured fluid at the Nankai Trough restrains the fault weakening. Dynamic rupture simulations reproduced the large slip near the trench observed in the 2011 Tohoku-oki earthquake and predicted the possibility of a large slip of over 30 m for the impending megaquake at the Nankai Trough. Our integrative approach is applicable globally to subduction zones as a novel tool for the prediction of extreme tsunami-producing near-trench slip.