The search for extraterrestrial life may be facilitated if ecosystems can be found on Earth that exist under conditions analogous to those present on other planets or moons. It has been proposed, on the basis of geochemical and thermodynamic considerations, that geologically derived hydrogen might support subsurface microbial communities on Mars and Europa in which methanogens form the base of the ecosystem1,2,3,4,5. Here we describe a unique subsurface microbial community in which hydrogen-consuming, methane-producing Archaea far outnumber the Bacteria. More than 90% of the 16S ribosomal DNA sequences recovered from hydrothermal waters circulating through deeply buried igneous rocks in Idaho are related to hydrogen-using methanogenic microorganisms. Geochemical characterization indicates that geothermal hydrogen, not organic carbon, is the primary energy source for this methanogen-dominated microbial community. These results demonstrate that hydrogen-based methanogenic communities do occur in Earth's subsurface, providing an analogue for possible subsurface microbial ecosystems on other planets.

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A hydrogen-based subsurface microbial community dominated by methanogens

Gas geochemistry and seismotectonics: a review

Hydrogen generation from mantle source rocks in Oman

Terrestrial gases in groundwater and soil air have been extensively studied in recent years in seismically active areas, especially in USSR, China, Japan, and United States, in search of premonitory changes that might be useful for earthquake prediction. Concentrations of radon, helium, hydrogen, mercury, carbon dioxide, and several other volatiles have been found generally to be anomalously high along active faults, suggesting that the faults may be paths of least resistance for the terrestrial gases generated or stored in the earth to escape to the atmosphere. Temporal gas concentration changes with durations of a few hours to many months have been observed before many large earthquakes at a relatively small number of favorably situated stations at epicentral distances of up to several hundreds of kilometers. These “sensitive” stations are generally located along active faults, especially at intersections or bends of faults, or some other structurally weak zones, possibly because of tectonic strain concentration at such places. However, gas concentrations, especially those measured at or near the ground surface, may also be significantly affected by meteorological, hydrological, and other nontectonic changes in the environment. The nontectonically induced variations must be carefully eliminated or recognized in the search of true earthquake-related anomalies.

Gas geochemistry applied to earthquake prediction: An overview

Studies of earthquake precursory phenomena during the last several decades have found that significant geophysical and geochemical changes can occur prior to intermediate and large earthquakes. Among the more intensely investigated geochemical phenomena have been: (1) changes in the concentrations of dissolved ions and gases in groundwaters and (2) variations in the concentrations of crustal and mantle volatiles in ground gases. The concentration changes have typically showed no conanomalies trend (either increasing or decreasing), and the spatial and temporal distribution of the observed anomalies have been highly variable. As a result, there is little agreement on the physical or chemical processes responsible for the observed anomalies. Mechanisms proposed to account for precursory groundwater anomalies include ultrasonic vibration, pressure sensitive solubility, pore volume collapse, fracture induced increases in reactive surfaces, and aquifer breaching/fluid mixing. Precursory changes in soil gas composition have been suggested to result from pore volume collapse, micro-fracture induced exposure of fresh reactive silicate surfaces, and breaching of buried gas-rich horizons. An analysis of the available field and laboratory data suggests that the aquifer breaching/fluid mixing (AB/FM) model can best account for many of the reported changes in temperature, dissolved ion and dissolved gas concentrations in groundwater. Ultrasonic vibration and pressure sensitive solubility models cannot reasonably account for the geochemical variations observed and, although the pore collapse model could explain some of the observed chemical changes in groundwater and ground gas, uncertainties remain regarding its ability to generate anomalies of the magnitude observed. Other geochemical anomalies, in particular those associated with hydrogen and radon, seem best accounted for by increases in reactive surface areas (IRSA model) that may accompany precursory deformation around the epicenter of an impending earthquake. Analysis of the probable response of these models to the earthquake preparation process, as well as to other environmental factors, suggests that geochemical monitoring programs can provide information that may be valuable in forecasting the probability of an earthquake; however, because of the complexity of the earthquake preparation process, the absolute prediction of seismic events using geochemical methods alone, does not presently appear to be feasible.

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Geochemical Precursors to Seismic Activity

The hydrogen concentration in soil gas has been measured in the area around the Yamasaki Fault, one of the active faults in southwestern Japan. Degassing of a significant amount of hydrogen (up to more than 3 percent by volume) has been observed for sites along the fault zone. The hydrogen concentration in soil gas at sites away from the fault zone was about 0.5 part per million, almost the same as that found in the atmosphere. The spatial distribution of sites with high hydrogen concentrations is quite systematic. A hypothesis on the production of hydrogen by fault movements is postulated.

http://science.sciencemag.org/content/sci/210/4466/188.full.pdf

Hydrogen release: new indicator of fault activity.

Summary. In the continental lithosphere, the stress field near the junction of two arc-trench systems often shows a different state from the regional trend. By assuming that the drag forces and nose-shaped contortion of the subducting slab control the state of stress in such a region, the vertical displacements and stress distribution in a continental lithosphere with a concave seaward boundary are calculated using a three-dimensional finite element method. It is also assumed that the drag is applied to a depth of about 60 km which is the depth limit of thrust-type earthquakes along the interplate boundary. It is demonstrated that the anomalous stress field resulting from the contorted 'nose' of the subducting slab extends inland some distance from the corner of an arc-arc junction. The area of anomalous stress field is characterized by surface upheavals and a stress regime conductive to strike-slip faulting, though the extent of this area depends on the model parameters. The degree of contortion, i.e. the width of the contorted 'nose' of the subducting slab, remains arbitrary even when appropriate restrictions such as the conservation of surface area and fixed meeting angle and dip angles of the two neighbouring subduction zones are made in symmetrical models. Applying the model to the Hokkaido corner, i.e. the junction of the Kurile and Japan trenches, the anomalous stress field appears in the south-eastem part of Hokkaido and coincides more or less with geophysical observations.

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The effect of the contorted ‘nose’ of a subducting slab on the stress field in the continental lithosphere at an arc-arc junction

Elemental and isotopic compositions of rare gases in gas samples from “Helium spots” and CO2-spring in the fault zone formed by the Matsushiro earthquake swarms, reported by WAKITA et al. (1978), have been analyzed mass spectrometrically. Isotopic compositions of rare gases were atmospheric except for 3He/4He and 40Ar/36Ar ratios. 3He/4He ratios were (9.12 ± 0.58) × 10-6 and (9.10±0.69) × 10-6, and 40Ar/34Ar ratios were 308 ± 9 and 304 ± 8 for “Helium spots” and CO2-spring, respectively. Rare gas elemental abundance patterns for both samples were similar to a pattern produced by the dissolution equilibrium of atmospheric rare gases in water. The origin of rare gases is discussed. The 3He/4He ratio found in the fault zone suggests that the systematic observation of 3He/4He ratio may be useful for the detection of precursory phenomena of an earthquake.

Isotopic compositions of rare gases in the Matsushiro earthquake fault region.

Recent seismic researches have revealed the variation of shallow seismic activity across the trench-arc system of Japan, which is characterized by, A) a high activity throughout the crust and upper mantle between trench axis and "aseismic front", B) a low activity between "aseismic-front" and volcanic front, and C) a moderate activity within the upper crust behind volcanic front. This variation of shallow seismicity across the island arc is explained by the effects of pore pressure in the crust, based on laboratory experiments of stick-slip and stable-sliding movements combined with temperature distribution estimated from terrestrial heat flow and thermal model of subducting slab. High pore pressure is expected in the low activity area (region B). Pore pressure is controlled mainly by 1) fluid supply from below (released from subducting oceanic crust), and 2) permeability distribution in the crust and upper mantle. Dehydration process in subducting slab has a great influence on shallow seismic activity.

Naoyuki Fujii

Papers

Hydrogen release: new indicator of fault activity.

The effect of the contorted ‘nose’ of a subducting slab on the stress field in the continental lithosphere at an arc-arc junction

Isotopic compositions of rare gases in the Matsushiro earthquake fault region.

Seismic activity and pore pressure across island arcs of japan

Softwares for Digital Image Processing: (3) Cluster Labeling and Perimeter Extraction