Koichiro Nagamine

Stratigraphy and sedimentary petrology of an Archean volcanic–sedimentary succession at Mt. Goldsworthy in the Pilbara Block, Western Australia: implications of evaporite (nahcolite) and barite deposition

Abstract A approximately 3.3 Ga sedimentary succession at Mt. Goldsworthy in the northeastern Pilbara Block, Australia contains beds composed of silicified pseudomorphs of nahcolite (NaHCO3) or barite (BaSO4). This sedimentary succession correlates with the Corboy Formation and consists of lower, middle and upper sedimentary units, which conformably overlie mafic to ultramafic volcanic and volcaniclastic rocks. The lower and middle units are predominantly siliciclastic, whereas the upper unit is characterized by ferruginous cherts and banded iron-formations with minor proportions of intercalated mature sandstone. The succession defines an overall upward fining and deepening trend. The detrital materials were derived from older greenstone successions, volcaniclastics and reworked precipitative beds. Silicified pseudomorphs of nahcolite crystals up to 40 cm in length occur in a 20 m thick bed in the upper portion of the lower unit. The formation of this unit was initiated by deposition of immature terrigenous clastic sediments and followed by the precipitation of nahcolite from Na+-HCO3− brines in a closed or semi-closed evaporitic basin. Bladed barite deposits occur mainly in the middle unit, which was deposited in a sub-aerial to shallow marine environment. Barite may have precipitated as a result of mixing of SO42−-rich seawater and Ba2+-rich hydrothermal fluids. The Mt. Goldsworthy sedimentary succession records an Archean shallow to sub-aerial sedimentary environment that probably developed in a continental margin setting.

Gas geochemical changes at mineral springs associated with the 1995 southern Hyogo earthquake (M = 7.2), Japan

Abstract Gas concentrations monitored at a mineral spring in central Japan over the past 15 years showed similar changes before and after two large earthquakes (September 1984, M = 6.9, 50 km distant: and January 1995, M = 7.2, 220 km distant); namely, a concurrent and abrupt drop in all three gas ratios, He Ar , N 2 Ar , and CH 4 Ar , several months before the events. Improved measurements of gas compositions since 1991 and the addition of gas discharge rate monitoring since 1993 disclosed the following features for anomalies in response to small earthquakes as well as the large 1995 event: (1) the gas ratios increased for both large and small earthquakes, but they did not return to the preseismic values for the large event, as they did within a few hours for the small events; (2) at the 1995 event, the gas discharge rate decreased remarkably 3 h before the event and then increased to higher than preseismic level for several months. In contrast, coseismic rates at small events generally increased for less than 15 min; (3) during a seismically quiet period, from January 1993 to December 1994, the variation in the He Ar ratio was well correlated with that of gas discharge rate. Such correlation disappeared in August and September, 1994, when the three gas ratios dropped before the large event; (4) gas compositions at an auxiliary monitoring site revealed a coseismic change similar to that described above for the 1995 event. These lines of evidence show that subsurface fluids may habitually respond to changes in physical conditions relating to earthquake occurrences, and thus suggest the usefulness of monitoring subsurface fluids for earthquake prediction.

Preseismic hydrogen gas anomalies caused by stress‐corrosion process preceding earthquakes

Simultaneous monitoring of He, H2, Ar, N2, CH4 and Rn in gas bubbles of groundwater from the 1,500 m deep Nagashima well in the active Yoro fault zone, central Japan, reveals that three microearthquakes with M=1.6–2.6 in the fault zone are accompanied and preceded by increases of H2, as is a moderately large event with M=5.4 in the same fault zone. All the focal distances of the earthquakes are within 25 km. No comparable anomalies were seen in the gas species other than H2. We suggest that the selective and preseimic increases of H2 in groundwater gas bubbles are geochemical signals of the earthquake nucleation process of stress-corrosion mechanism. Aseismic slips preceding earthquakes in the active fault zone are likely to have enhanced H2 production by mechanochemical reaction of fractured rock surface with water in pre-existing fault planes as groundwater conduits.

Estimation of areal flux of atmospheric methane in an urban area of Nagoya, Japan, inferred from atmospheric radon-222 data

Abstract To clarify the emission of methane (CH4) from area sources, we have adopted atmospheric radon-222 (222Rn) as a natural tracer. The diurnal variation in CH4 concentration is predominantly affected by local meteorological conditions as is the case of 222Rn. Positive correlations are recognized between atmospheric CH4 and 222Rn concentrations. From these correlations, the CH4 flux from urban area in Nagoya, Japan, was estimated to be almost constant at 0.02 gCH4m−2d−1 over a year.

Carbon isotopic analysis of atmospheric methane in urban and suburban areas : Fossil and non-fossil methane from local sources

The carbon isotopic composition (12C, 13C and 14C) of atmospheric CH4 was studied to estimate CH4 sources in local areas. In urban areas of Nagoya, Japan (35°10′N, 136°55′E), where there is much anthropogenic influence, evidence of methane release into the atmosphere has been reported. Variations in the carbon isotopic composition were analyzed with diurnal fluctuation of atmospheric CH4 concentration, since the diurnal increase in the CH4 concentration is attributed mainly to release from local sources. In November and December 1993, the contribution of fossil CH4 to local CH4 released from the urban area was calculated to be 102±8%, and its δ13C was −40.8±3.0‰. Leakage of supplied natural gas and exhaust gas from automobiles would be important CH4 sources in the urban area in late autumn to winter. In suburban areas in Japan there are many rice paddy fields, and these are one kind of major anthropogenic source of atmospheric CH4. In a suburban area of Nagoya fossil, CH4 contributed to less than 10% of local release in July and August 1994. The calculated value of δ13C for non-fossil CH4 was approximately −65‰, which is within the range of reported values of δ13C for CH4 derived from bacterial CH4 sources such as irrigated rice paddies.

Evolution of light hydrocarbon gases in subsurface processes: Constraints from chemical equilibrium

Abstract The behaviour of CH 4 , C 2 H 6 and C 3 H 8 in subsurface processes such as magma intrusion, volcanic gas discharge and natural gas generation have been examined from the viewpoint of chemical equilibrium. It seems that equilibrium among these three hydrocarbons is attainable at about 200°C. When a system at high temperatures is cooled, re-equilibration is continued until a low temperature is reached. The rate at which re-equilibration is achieved, however, steadily diminishes and, below 200°C, the reaction between the hydrocarbons stops and the gas composition at this time is frozen in, and it remains unchanged in a metastable state for a long period of geological time. Natural gas compositions from various fields have shown that, when a hydrocarbon system out of chemical equilibrium is heated, it gradually approaches equilibrium above 150°C. On the way towards equilibration, compositions of thermogenic gases apparently temporarily show a thermodynamic equilibrium constant at a temperature that is higher than the real equilibrium temperature expected from the ambient temperature of the samples; in contrast, biogenic gases indicate a lower temperature. In lower temperature regions, kinetic effects probably control the gas composition; the compositions are essentially subjected to genetic processes operating on the gases (such as pyrolysis of organic material and bacterial activity) and they fluctuate substantially. Examination of volcanic gases and pyrolysis experimental data, however, have suggested that the equilibration rate of these hydrocarbons is sluggish in comparison with that of reactive inorganic species such as H 2 S and SO 2 . The view presented in this study will be helpful in understanding the genetic processes that create oil and gas and the migration of these hydrocarbons and in interpreting the origins of magmatic gases.

Origin and coseismic behavior of mineral spring gas at Byakko, Japan, studied by automated gas chromatographic analyses

Abstract The origin and coseismic behavior of mineral spring gas were examined on the basis of continual monitoring data for several gas species. The automatic monitoring system used in this study consists of two gas chromatographs and a personal computer. Several gas species can simultaneously be analyzed with these analyzers. The system was so stable and sensitive that minute coseismic anomalies in these species could be detected. A positive correlation between N 2 Ar and CH 4 Ar implies that the mineral spring gas is a mixture of atmospheric air and lithospheric gas. The lithospheric gas is mainly composed of N 2 and CH 4 , and also contains trace amounts of He. It is inferred from isotopic data that lithospheric N 2 and CH 4 are thermogenic and He is mostly radiogenic. In contrast, O 2 and H 2 , in trace amounts, show large diurnal fluctuations, suggesting that these species should be mostly biogenic. Ten earthquakes felt around the station occurred during the monitoring period, and four of them coincided with coseismic anomalies in the He/Ar ratio. The anomalies may be interpreted as follows: by the ground motion at these felt quakes, He accumulated in the ground was additionally released into the spring water. The continual monitoring of these gas species enables us to discuss in detail the origin and behavior of the spring gas and to evaluate background noise of the data.

Coseismic changes of subsurface gas compositions disclosed by an improved seismo‐geochemical system

An automatic system for seismo-geochemical observation has been improved to monitor small changes in subsurface gas composition. This system is computer-based and consists of an on-site monitoring unit for gas analyses and a telemetry unit for transmitting the observation data to our laboratory. The analytical data with this system showed three coseismic anomalies in spring gas compositions at a monitoring station for these 11 months. These anomalies were associated with a far (and large) earthquake or two small (and near) ones, respectively. The gas anomaly as a spike-like signal appears only for a short period and can be detected on the basis of temporally dense data provided by the new system. The result shows that the behavior of subsurface gases is sensitive to crustal movements.