Climatic and environmental changes at southeastern coast of Lake Biwa over past 3000 years, inferred from borehole temperature data

In order to infer past climatic change in central Japan, we measured temperatures in a borehole at the Karasuma site, on the southeastern coast of Lake Biwa, and reconstructed sediment surface temperature history during the last 3000 years. The reconstructed temperature history shows apparent Medieval Warm Period, Little Ice Age, and contemporary temperature warming. However, the large amplitude of the temperature changes up to 4-5 K cannot be explained by past climatic change only, suggesting that there was some other cause of the larger amplitude temperature changes. The onsets of temperature decrease in the late 12th century a.d. and temperature increase in the mid 17th century a.d. appear to coincide with occurrences of two destructive earthquakes (1185 and 1662 a.d.) that caused water level changes of Lake Biwa. It suggests that the reconstructed sediment surface temperature history reflects the environmental change due to tectonically induced water level changes of the lake. If the annual mean of the ground surface temperature was higher than that of the bottom water temperature in a shallow part of the lake, which is consistent with the present-day data, the large amplitude of the sediment surface temperature change may be attributed to a combined effect of past climatic and environmental changes. Thus, we suggest that the borehole temperature at the Karasuma site preserves information not only on past climate changes but also on environmental changes due to tectonically induced water level changes.     

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data

The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s⁻¹) P-wave velocities, while the eastern has higher (6.3–6.9 km s⁻¹) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjørnøya. South of Bjørnøya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s⁻¹ is significantly lower than the high (7 km s⁻¹) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.     

GEMS: Geothermal Expert Modeling System

In carrying out geothermal-energy development projects, an exhaustive study has to be conducted on the local subsurface geothermal system based on extensive geothermal surveys and well data. The New Energy and Industrial Development Organization has had an extensive geothermal research program under way since 1987, known as the Nationwide Geothermal Resources Exploration Project. In this program, the total analysis system, which is called the Geothermal Expert Modelling System (GEMS), has been developed to systematize all processes of building models of geothermal systems based on the various regional exploration data. The aim of the system is to select narrow promising areas (about 50–60 square kilometers) for more elaborate exploration from within a region extending over hundreds of square kilometers, by analyzing the various explorations, and by building a geothermal system model of the area. This scheme includes studies to consolidate individual geothermal survey data into an integrated data base and to work out a computer-aided support system for building geothermal models. It calls for the integration of these processes into a system on a user-friendly, engineering work-station that has sufficient graphic functions. An attempt was made to actually identify promising areas in regions covered by wide-area surveys on hydrothermal systems, using the proposed total analysis system.     

Anomalous Large Scavenging of 230Th and 231Pa Controlled by Particle Composition in the Northwestern North Pacific

We report the role of particle composition and lateral particle movement that influences the oceanic distribution of ²³¹Pa and ²³⁰Th. Settling particles were collected during sediment trap experiments. These and surface sediments were obtained from five stations along 38 to 44°N in the northwestern North Pacific. The high total mass flux and seasonal variations in the marginal area of the western North Pacific are controlled by the supply of lithogenic materials and primary productivity. The high content of the lithogenic material in the settling particles in this area contributes to ²³⁰Th_ex fluxes that exceed the local rate of supply. The lithogenic materials are important as a carrier of ²³⁰Th and contribute to the fractionation between ²³⁰Th and ²³¹Pa in the ocean, as the ²³¹Pa_ex/²³⁰Th_ex ratio in the settling particles decreases with increasing ²³²Th concentration. The ²³¹Pa_ex/²³⁰Th_ex ratio in the settling particles collected in the abyssal basin decreases with water depth, which indicates that lateral transport of the lithogenic particles from the marginal area and/or shallower depth plays an important role in determining the ²³¹Pa_ex/²³⁰Th_ex ratio in a population of settling particles and remineralization. This indicates that lateral redistribution of particles and sediment focusing influence the ²³¹Pa_ex/²³⁰Th_ex ratios in surface sediments. Thus, the observations reported here mean that the use of the sediment ²³¹Pa_ex/²³⁰Th_ex ratio as a paleoproductivity proxy will be problematic in the northwestern North Pacific. This revised version was published online in July 2006 with corrections to the Cover Date.     

A tidal front in winter influenced by river discharge

Water quality of Osaka Bay is greatly influenced by freshwater discharge from rivers, to the effect of salinity playing a major role in forming the stratification. The tidal front is expected to appear in such an area even in winter considering the theory of the formation of a tidal front. From the field observation, we recognized a tidal front in winter for the first time in Osaka Bay. The critical value of a parameter log(H/U ³) for generation of a tidal front is obtained as a function of river discharge and cooling effect through sea surface. Differences of nutrients (NH₄–N, PO₄–P) concentration across the front are not clear, probably because they are utilized by phytoplankton easily. But in (NO₂–N)+(NO₃–N), DIN, PP and T–P, there is a discontinuity structure across the front, even if the absolute value of concentration difference is small.     

Grain size,La/Yb and Th/Sc of settling particles in the western North Pacific: Evidence for lateral transport of small Asian loess

To understand the transport process of lithogenic particles in the ocean, we measured the grain size distributions of lithogenic particles and measured the opal, La, Yb, Th, and Sc concentrations of the settling particles collected from time-series sediment traps at Sta. KNOT (44°N, 155°E, water depth 5320 m) from June 2002 to May 2004. The annual mean lithogenic particle flux observed at the lower sediment trap (5100 m) was twice as high as that at the upper sediment trap (770 m). The contribution of Asian loess estimated by the La/Yb and the Th/Sc ratios in the lower layer was greater than that in the upper layer. The fluxes of small lithogenic particles with sizes of 3–4 μm at the lower layer (5 to 65 mg/m²/day) were approximately four times larger than that at the upper layer (0.6 to 27 mg/m²/day). These results indicate that the horizontal addition of small particle sizes of Asian loess is a main factor in the increase of lithogenic particles at the lower layer. The temporal variations in the small lithogenic particle flux at the lower layer had a positive correlation with those at the upper layer (r = 0.71). The small lithogenic particle fluxes showed a strong positive correlation with the opal fluxes (r = 0.9). We therefore conclude that the small lithogenic particles were laterally transported and scavenged by the formation of aggregates with opal.     

Vibrations of corner point supported shallow shells of rectangular planform

The first known solution of the title problem is presented. The Ritz method is used, with algebraic polynomials forming the set of trial functions. The condition that all three components of displacement be zero at the four corners is straightforwardly enforced. Numerical studies show that the convergence is relatively slow, requiring more terms than for shells which are completely free. The class of problems studied includes independent, constant curvature in each of the directions parallel to the edges, yielding vibration modes which fall into one of four symmetry classes, with symmetry or antisymmetry of the displacements existing with respect to each of the two symmetry axes of the problem. Detailed results are given for the frequencies and mode shapes of the first two modes of each symmetry class for shells having square planform and circular cylindrical, spherical and hyperbolic paraboloidal curvatures. Accuracy of the results is partially established by comparison with other previously published, accurate results for the corner supported flat square plate.     

Apparent velocity measurements on an oceanic lithosphere

A direct measurement of apparent velocities for oceanic paths was made with an array of sensitive ocean bottom seismographs. The measurement was performed by recording waves from shallow earthquakes which occurred in the area close to trench axes and which were accurately located by the land seismological network in Japan. The range of epicentral distances is from 500 to 1,800 km.The observed P travel times are less than those in the Jeffreys-Bullen tables by 6–10 s for the range of distances.Since the dimension of the OBS array is about 400 km, the apparent velocities are determined quite precisely and show little dependence on the epicentral distances. The average value of the apparent velocities for the range 500–1,700 km is 8.64 ± 0.13 km/s.An offset of travel times, which is thought to be associated with a low-velocity layer underneath the oceanic lithosphere, has been observed.These results indicate that a high-velocity layer with a velocity of 8.6 km/s exists in the lower part of the oceanic lithosphere. Beneath the 8.6-km/s layer there is a thin low-velocity layer under which the velocity of the P wave is again 8.6 km/s.     

Effect of Mine Water on the Stability of Underground Coal Mine Roadways

The basic objectives of mine roadways are to provide sufficient cross sections to accommodate equipment, transport, personnel travel and ventilation. However, many roadways become damaged to the extent of needing maintenance, generally dinting, and in some cases requiring re-ripping. Strata conditions play important effects on stability of roadways and other mining activities. Weak rocks cause excessive roadway closures, and water softens some rocks and worsens the closure problem. Therefore, study on effect of mine water on the stability of underground coal mine roadways is important for underground coal mine development. A scientific discussion on the effect of water on the stability of roadways is given on the basis of results obtained by means of field investigation and laboratory tests. Based on analysis, rock in saturated condition has only between 0.19 to 0.49 of its compressive strength and 0.17 to 0.59 of its tensile strength in dry condition. Among the coal measures rock in research area, water has the most obvious effect on the strength reduction of shale. The slaking behavior of shale has also been the worst among the other. Field investigation of roadway driven in shale shown vertical closure in the wet and dry condition area reached 40–60 and 5–15 cm respectively, in 30 days after the drivage. Among the measures, drainage is considered to be the most economical and simplest method to reduce the water content in the rocks or the rock masses.