Molecular and isotopic composition of hydrate-bound and dissolved gases in the southern basin of Lake Baikal, based on an improved headspace gas method

Assessments of the molecular and isotopic composition of hydrate-bound and dissolved gases in pore water were conducted during the multi-phase gas hydrate project (MHP-09) cruise VER09-03 to the southern basin of Lake Baikal in September 2009. To avoid changes in gas composition during core sampling and transport, various headspace methods were investigated aimed at preserving the dissolved gases in pore water. When distilled water was added to the sediment samples, the concentrations of carbon dioxide and oxygen decreased because of dissolution into the water and/or microbial consumption. When the headspace was not flushed with inert gases, trace levels of hydrogen and ethylene were detected. The findings suggest that best preparation is achieved by flushing the headspace with helium, and adding a saturated aqueous solution of sodium chloride. This improved headspace method served to examine the molecular and isotopic compositions of gas samples retrieved at several new sites in the southern basin. Methane was the major component, and the proportion of ethane ranged widely from 0.0009 to 1.67?mol% of the total hydrocarbon gases. The proportions of propane and higher hydrocarbons were small or less than their detection limits. The carbon isotope signatures suggest that microbial-sourced methane and ethane were dominant in the Peschanka study area, whereas ethane was of thermogenic origin at all other study sites in the southern basin of Lake Baikal.     

Simplification of strong ground motion considering inelastic responses of structures

Simplification of strong ground motions to 1 cycle sine waves was investigated from the elastic and inelastic earthquake response analyses and response analyses under sine wave input using single‐degree‐of‐freedom systems. Strong ground motions could be simplified to 1 cycle sine waves if large plastic deformations, with ductility factor more than 2, were assumed. This is because the approximate maximum responses from input sine waves are determined by the initial response cycle, due to period elongation and plastic energy dissipation of the systems. A sine wave whose acceleration amplitude is the peak ground acceleration (PGA) and whose period is that of an equivalent 1 cycle sine wave is proposed. The period of an equivalent sine wave is easily obtained from the elastic response acceleration spectrum of a seismic record. This means that the inelastic responses are approximately determined by the PGA and an equivalent 1 cycle sine wave period. Therefore, an equivalent 1 cycle sine wave period provides a single index to express the frequency characteristics of a strong ground motion. Copyright © 2000 John Wiley & Sons, Ltd.     

Rate-dependent strength of porous ice–silica mixtures and its implications for the shape of small to middle-sized icy satellites

Porosity is one of the most important physical properties in the rheology of small icy satellites composed of ice–silicate mixtures. Deformation experiments involving ice and 1 μm silica bead mixtures were conducted to clarify the effect of porosity on the flow law of ice–silica mixtures. Mixtures with silica mass contents of 0, 30, and 50 wt.% were used for the experiments, and the porosity was changed from 0% to 25% in each mixture. The temperature ranged from −10 to −20 °C, and the strain rate was changed from 1.2 × 10⁻⁶ to 4.2 × 10⁻⁴ s⁻¹. As a result, it was found that the ice–silica mixtures deformed plastically, and that the relationship between the maximum stress, σ_max, on the stress–strain curve and the applied strain rate, , could be described by the following flow law: . The mixture became softer as the porosity or silica mass content increased, and the stress exponent n and activation energy Q were independent of porosity, depending only on the silica mass content. Furthermore, the parameter A₀ could be written as A₀ = B(1 − ?)^−α, where ? is the porosity. The constants B and α also depended only on the silica mass content, and they increased with the increase in this content. The Maxwell relaxation time was calculated in order to estimate the conditions for topographic relaxation of icy satellites, and it was found that topographic relaxation occurred at temperatures higher than 160 K in the case of icy satellites with mean radii of 200 km.     

Elastic-plastic response spectra for different hysteretic rules

The general features of elastic-plastic response spectra for several accelerograms that are widely used for the design buildings in Japan were studied in terms of the effects of the different hysteretic models used in the analyses. Lare fluctuations existed in the spectra for both input ground motion and the different models. The models were classified according to their strain energy-absorbing capacities in three groups, within each of which the relevant nature of elastic-plastic responses of the structures were similar. Finally, empirical formulae with which to estimate two measures structural damage, the ductility factors and cumulative plastic displacement normalized by the yielding displacement, wer developed for each group of hysteretic models.     

Diurnal variation of CO2 concentration, Δ14C and δ13C in an urban forest: estimate of the anthropogenic and biogenic CO2 contributions

Diurnal variation in the atmospheric CO₂ concentration and the carbon isotopic composition (Δ¹⁴C and δ¹³C) was measured in a forest in an urban area on 9 February 1999. The carbon isotope approach used in the present study differentiated between the quantitative contributions from anthropogenic and biogenic CO₂ sources in the urban atmosphere. The anthropogenic (fossil fuel) and biogenic (soil respiration) contributions was estimated, and they ranged from 1 to 16% and from 2 to 8% of the total atmospheric CO₂. The diurnal variation of the anthropogenic CO₂ was the major cause of the total atmospheric CO₂ variation, while the biogenic CO₂ remained relatively constant throughout the day. Estimating the contribution of soil respired CO₂ provided the mean residence time of soil respired CO₂ within the forest atmosphere.     

Connectivity between the interannual salinity variation in the western channel of the Tsushima Strait and hydrographic conditions in the Cheju Strait

The connectivity between the interannual salinity variations in the Tsushima and Cheju Straits has been investigated on the basis of historical hydrographic data. Salinity in the Cheju Strait correlates positively with that in the western channel of the Tsushima Strait, but does not show a significant correlation with that in the eastern channel. Empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses of temperature and salinity in the Cheju Strait revealed that salinity in the strait is associated with the cold bottom water in summer. Drastic freshening in the Cheju Strait occurs in a period when the Cheju Current intensifies. The results allow us to hypothesize that the mechanism of interannual salinity variations in the Cheju Strait and western channel of the Tsushima Strait is as follows. The intrusion of cold bottom water into the Cheju Strait in summer intensifies the Cheju Current by increasing the baroclinicity. Since colder bottom water develops a stronger eastward surface current, the larger volume of the Changjiang diluted water is drawn into the strait, which results in a lower salinity condition in the Cheju Strait. As the water in the Cheju Strait flows into the western channel of the Tsushima Strait, salinity in the western channel varies synchronously. This hypothesis is supported by SVD analysis of temperature in the Cheju Strait and salinity in the Tsushima Strait. The salinity condition in the East China Sea is suggested to be another important influence on salinity in the western channel of the Tsushima Strait.     

Intrusion events of the intermediate Oyashio Water into Sagami Bay, Japan

Intermediate intrusion of low salinity water (LSW) into Sagami Bay was investigated on the basis of CTD data taken in Sagami Bay and off the Boso Peninsula in 1993–1994. In October 1993, water of low temperature (<7.0°C), low salinity (<34.20 psu) and high dissolved oxygen concentration (>3.5 ml I⁻¹) intruded along the isopycnal surface of {ie29-1} at depths of 320–500 m from the Oshima East Channel to the center of the bay. On the other hand, the LSW was absent in Sagami Bay in the period of September–November 1994, though it was always found to the south off the Boso Peninsula. Salinity and dissolved oxygen distributions on relevant isopycnal surfaces and water characteristics of LSW cores revealed that the LSW intruded from the south off the Boso Peninsula to Sagami Bay through the Oshima East Channel. The LSW cores were distributed on the continental slope along 500–1000 m isobaths and its onshore-offshore scales were two to three times the internal deformation radius. Initial phosphate concentrations in the LSW revealed its origin in the northern seas. These facts suggest that the observed LSW is the submerged Oyashio Water and it flows southwestward along the continental slope as a density current in the rotating fluid. The variation of the LSW near the center of Sagami Bay is closely related to the Kuroshio flow path. The duration of LSW in Sagami Bay is 0.5 to 1.5 months.     

Using sulfur isotopes to determine the sources of vermillion in ancient burial mounds in Japan

This study represents an attempt to determine the sources of vermillion found in ancient Japanese burial mounds of the 1st–6th centuries A.D., by comparing their ratios of sulfur isotopes with those of cinnabar ore samples collected in Japan and China. Cinnabar ore samples were taken from three mines in central Japan (Niu in Mie, Yamato in Nara, and Sui in Tokushima prefectures), and from Wanshan in China, where mining activity has been recorded back to the 6th century A.D. and earlier. When the ratios of a ³⁴S and ³²S were compared with the Canyon Diablo meteorite standard, a high δ³⁴S value of +22.6 ± 3.6‰ was found for the Wanshan mine, as opposed to low values ranging from −7.3 ± 1.9 to −2.1 ± 1.6‰ for the Japanese mines. The ratios of sulfur isotopes in vermillion collected from ancient Japanese burial mounds also divided into two groups. High ratios (+11.1 to +22.8‰) were found in 1st‐ and 2nd‐ century burials in the western regions of northern Kyushu and San'in, suggesting that local, powerful chiefs obtained vermillion through relations with China. Lower ratios (−8.4 to −2.0‰) were found in burials of the 2nd through 6th centuries in central Japan, where the ancient Yamato dynasty emerged as the first unified polity around the end of the 2nd century A.D. We, therefore, conclude that the Yamato dynasty exploited local sources of vermillion, rather than depending solely on China. The present study demonstrates the feasibility of determining sources of vermillion using sulfur isotope ratios, and the relevance of such findings for archaeological research. © 2005 Wiley Periodicals, Inc.     

Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan: A Hypothesis on the Oxygen-Deficient Water Mass Formation

A hydrographic survey and a 25-hour stationary observation were carried out in the western part of Suo-Nada in the summer of 1998 to elucidate the formation mechanism of the oxygen-deficient water mass. A steep thermocline and halocline separated the upper layer water from the bottom water over the observational area except for near the Kanmon Strait. The bottom water, in comparison with the upper layer water, indicated lower temperature, higher salinity, lower dissolved oxygen, higher turbidity, and higher chlorophyll a. Turbidity in the upper layer water changed with semi-diurnal period while the bottom water turbidity showed a quarter-diurnal variation, though the M₂ tidal current prevailed in both waters. From the turbidity distribution and the current variation, it is revealed that the turbidity in the upper layer water is controlled by the advection due to the M₂ tidal current. On the other hand, the quarter-diurnal variation in the bottom water turbidity is caused by the resuspension of bottom sediments due to the M₂ tidal current. The steep thermocline and halocline were maintained throughout the observation period in spite of the rather strong tidal currents. This implies an active intrusion of the low temperature and high salinity water from the east to the bottom of Suo-Nada. Based on the observational results, a hypothesis on the oxygen-deficient water mass formation was proposed; the periodical turbidity variation in the bottom water quickly modifies the oxygen-rich water in the east to the oxygen-deficient bottom water in Suo-Nada in a course of circulation.