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

Quick measurements of permeability can be made by using gas as the pore fluid. To apply gas permeability data to the evaluation of water permeability, the difference between gas and water permeabilities needs to be assessed. We measured intrinsic permeability of sedimentary rocks from the western foothills of Taiwan by using nitrogen gas and distilled water as pore fluids in effective-pressure cycling tests at room temperature. The observed difference in gas and water permeabilities was analyzed in view of the Klinkenberg effect. This effect is due to the slip flow of gases at pore walls, which enhances gas flow when pore sizes are very small. Our experimental results showed that (1) gas permeability was larger than water permeability by several times to one order of magnitude, (2) gas permeability increased with increasing pore pressure, and (3) water permeability increased slightly as the pore-pressure gradient across the specimen increased. Results (1) and ( 2) can be explained quantitatively by an empirical power law in relation to the Klinkenberg constant b that is applicable in low permeable range. This correlation enables us to estimate water permeability from gas permeability.The Klinkenberg effect is important when permeability is lower than 10 -18 m 2 and at low pore-pressure differentials, and correction for the effect is essential to estimate water permeability from gas permeability measurement data. A simplified Bingham flow model for water can partially explain the trend of result (3), though non-Darcy flow behavior or inertial forces of water-rock interaction are needed to account for the observed deviation from Darcy's law.

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Slow earthquakes occur at the plate interface in subduction zones. These low-frequency tremors and slow-slip events are often located at about 30 km depth 1-3 , near the boundary between the crust and mantle (Moho) on the overriding plate. Slow earthquakes occur on fault patches with extremely low frictional strength 4-6 . This weakness is generally assumed to result from increased pore-fluid pressures and may be linked to the release of fluids from the descending plate. Here we propose that a contrast in permeability across the Moho results in the accumulation of water and the build-up of pore-fluid pressure at the corner of the mantle wedge that overlies the subducting plate. We use laboratory measurements of permeability to show that gabbroic rock layers in the crust are two orders of magnitude less permeable than serpentinite layers in the underlying hydrated mantle rocks. Inserting our experimental data into a numerical model that simulates pore pressure evolution across the Moho, we show that the porefluid pressure at this boundary can be as high as lithostatic pressure. We suggest that water released from the descending plate is trapped at the corner of the mantle wedge owing to this permeability barrier, and then causes the localized slow earthquakes that are triggered by fault instabilities.The presence of pore fluid has long been recognized as playing an important role in the crustal deformation and earthquake mechanism, because it weakens the frictional strength and can result in fault instability 7,8 . As episodic tremor and slow-slip events found in subduction zones exhibit extremely low effective stresses, the occurrence of these slow earthquakes is attributed to the effect of high pore-fluid pressures on the plate boundaries 4-6 . Low seismicwave velocities and high Poisson's ratios of the source regions also suggest the presence of aqueous fluids that facilitate these slow earthquakes 1-3 . The occurrence of these events is mostly localized at triple junctions between the subducting plate and the island-arc Moho; consequently, a mechanism must be operating to facilitate water accumulation in such localized regions.Water in the subducting plate is released into the overlying mantle wedge by the dehydration of hydrous minerals 9 . These fluids tend to migrate upwards along the plate interface because the hydrated mantle (serpentinite) can develop a strong fabric 10 that results in a significant anisotropy in permeability 11 . At the island-arc Moho, migrating fluids can become trapped at the gabbroic layer, which acts as a cap rock, similar to the structures that trap petroleum resources 12 . Thus, the permeability contrast across the Moho may be the key to water accumulation in the source regions of slow earthquakes (Fig. 1). A similar model of a permeability barrier has also been proposed to account for the anomalous pore pressures in subducted crust [13][14][15] . In this study we Figure 1 | Schematic model of water circulating in a subduction zone.Subducting plates release most of their water in...

Scientific drilling at a submarine mud volcano shows that geofluid migration stimulates methanogenesis in the deep biosphere.