A neutron activation analysis of iridium concentration in Yamato carbonaceous chondrite

Iridium concentration in extra-terrestrial bodies is an important quantity in relation to Ir-rich geological layers. Ir concentration of a Yamato carbonaceous chondrite (Y-793321) has been measured by a neutron activation method. The measurement yields a value (0.57 ± 0.06) g per gramme for the chondrite.     

A BTOP model to extend TOPMODEL for distributed hydrological simulation of large basins

Topography is a dominant factor in hillslope hydrology. TOPMODEL, which uses a topographical index derived from a simplified steady state assumption of mass balance and empirical equations of motion over a hillslope, has many advantages in this respect. Its use has been demonstrated in many small basins (catchment areas of the order of 2–500 km²) but not in large basins (catchment areas of the order of 10 000–100 000 km²). The objective of this paper is to introduce the Block‐wise TOPMODEL (BTOP) as an extension of the TOPMODEL concept in a grid based framework for distributed hydrological simulation of large river basins. This extension was made by redefining the topographical index by using an effective contributing area af(a) (0?f(a)?1) per unit grid cell area instead of the upstream catchment area per unit contour length and introducing a concept of mean groundwater travel distance. Further the transmissivity parameter T₀ was replaced by a groundwater dischargeability D which can provide a link between hill slope hydrology and macro hydrology. The BTOP model uses all the original TOPMODEL equations in their basic form. The BTOP model has been used as the core hydrological module of an integrated distributed hydrological model YHyM with advanced modules of precipitation, evapotranspiration, flow routing etc. Although the model has been successfully applied to many catchments around the world since 1999, there has not been a comprehensive theoretical basis presented in such applications. In this paper, an attempt is made to address this issue highlighted with an example application using the Mekong basin. Copyright © 2007 John Wiley & Sons, Ltd.     

Interdecadal temperature variations in the North Pacific Central Mode Water simulated by an OGCM

The North Pacific Central Mode Water (CMW) is a water mass that forms in the Kuroshio-Oyashio Extension (KOE) region with characteristic low potential vorticity. Recent studies have suggested that the CMW, as low potential vorticity water, plays an important role in the adjustment of the subtropical gyre and subsurface variability on decadal to interdecadal timescales. We have forced a realistic ocean general circulation model (OGCM) with observed wind stress and sea surface temperature (SST) forcing to investigate the decadal variations of the CMW. Associated with the large atmospheric changes after the mid-1970s climate regime shift, the upper thermocline experiences a cooling as negative SST anomalies in the central North Pacific are subducted and advected southward. In addition to this thermodynamic response, the CMW’s path shifts anomalously eastward in response to anomalous Ekman pumping. This eastward shift of the core of the CMW produces a lowering of the isotherms, and a consequent warming, on the path of the CMW core. This warming partially counteracts the cooling associated with subducted surface anomalies, and it may be responsible for the reduced temperature variations at the climatological position of the CMW when both anomalous wind and heat fluxes are given. Lateral induction across the sloping bottom of the winter mixed layer in the KOE is critical to the formation of the low potential vorticity CMW. Coarse resolution models, which are widely used in climate modeling, underestimate the horizontal gradient of the mixed layer depth and form only a weak CMW or none at all. We have conducted a coarse resolution experiment with the same OGCM, showing that the subsurface response is much reduced. In particular, there is no dynamic warming in the CMW and the thermodynamic response to the SST cooling dominates. The resultant total response differs substantially from that in the finer resolution run where a strong CMW forms. This sensitivity to the model resolution corroborates the important dynamical role that the CMW may play with its distinctive low potential vorticity character and calls for its improved simulation.     

Volume Transport Variability Southeast of Okinawa Island Estimated from Satellite Altimeter Data

A nine-year-long record of the northeastward volume transport (NVT) in the region southeast of Okinawa Island from 1992 to 2001 was estimated by an empirical relation between the volume transport obtained from the ocean mooring data and the sea surface height anomaly difference across the observation line during 270 days from November 2000. The NVT had large variations ranging from −10.5 Sv (1 Sv ≡ 10⁶ m³s⁻¹) to 30.0 Sv around its mean of 4.5 Sv with a standard deviation of 5.5 Sv. This large variation was accompanied by mesoscale eddies from the east, having a pronounced period from 106 to 160 days. After removal of the eddy, NVT was found to fluctuate from 2 Sv to 12 Sv with a quasi-biennial period.     

Bifurcation current along the southwest coast of the Kii Peninsula

Along the southwest coast of the Kii Peninsula, a bifurcation current is regularly observed. By using ADCP data taken on board the R/V Wakayama of the Wakayama Prefectural Fisheries Experimental Station, characteristics of this bifurcation current are analyzed. The occurrence frequency of the bifurcation current reaches about 70% in the period from 1988 to 1996. The bifurcation point appears to be changeable and occurs almost evenly between Cape Ichie and Cape Shionomisaki. The current divergence in the alongshore direction was also investigated. Positive divergence values dominated in the whole analyzed area, and an onshore current appears to be dominant along the southwest coast of the Kii Peninsula, except in 1990 when the Kuroshio flowed in a large meandering path.     

Extinction curves flattened by reverse shocks in supernovae

We investigate the extinction curves of young galaxies in which dust is supplied from Type II supernovae (SNe II) and/or pair instability supernovae (PISNe). Since at high redshift ( z > 5), low-mass stars cannot be dominant sources for dust grains, SNe II and PISNe, whose progenitors are massive stars with short lifetimes, should govern the dust production. Here, we theoretically investigate the extinction curves of dust produced by SNe II and PISNe, taking into account reverse shock destruction induced by collision with ambient interstellar medium. We find that the extinction curve is sensitive to the ambient gas density around a SN, since the efficiency of reverse shock destruction strongly depends on it. The destruction is particularly efficient for small-sized grains, leading to a flat extinction curve in the optical and ultraviolet wavelengths. Such a large ambient density as   n _H≳ 1 cm⁻³  produces too flat an extinction curve to be consistent with the observed extinction curve for SDSS J1048+4637 at z = 6.2. Although the extinction curve is highly sensitive to the ambient density, the hypothesis that the dust is predominantly formed by SNe at z ∼ 6 is still allowed by the current observational constraints. For further quantification, the ambient density should be obtained by some other methods. Finally, we also discuss the importance of our results for observations of high- z galaxies, stressing a possibility of flat extinction curves.     

Numerical study of the Subtropical Front and the Subtropical Countercurrent

Using a multi-level numerical model, it is shown that the Subtropical Front and the Subtropical Countercurrent can be reproduced realistically in a highly idealized model, as a consequence of the coupling effect of wind driven gyre circulation and differential heating. In the model, the North Pacific Ocean is idealized as a rectangular flat-bottomed model ocean, and is driven by wind stress, which features the Westerlies and the Trades, and by heat flux through the sea surface formulated after Haney (1971).In the model ocean, a shallow front and an eastward current associated with the front are formed around the central latitude of the Subtropical Gyre, which show close similarities to the Subtropical Front and the Subtropical Countercurrent in the real ocean.Although the detailed mechanism of formation of the Subtropical Front and the Subtropical Countercurrent is not clarified in the present study, two factors are found inessential for the formation of the Subtropical Front and the Subtropical Countercurrent. First, the results of the model indicate that a small trough of wind stress curl in the lower latitudes of the Subtropical Gyre, which Yoshida and Kidokoro (1967a, b) attributed to the Subtropical Countercurrent, is not necessary for the formation of the Subtropical Front and the Subtropical Countercurrent, since they are reproduced well in the model without the trough. Second, using a model driven by meridional wind stress, it is shown that the meridional Ekman convergence, which many authors related to the Subtropical Front, is not essential for the formation of the Subtropical Front and the Subtropical Countercurrent.     

Long-term stability of flow-path structure in crystalline rocks distributed in an orogenic belt, Japan

Most evaluations of the contaminant retardation processes likely to be important in geological disposal (e.g. for high level radioactive waste (HLW)) consider only the present characteristics of fractures and associated mineral infills. Relatively little attention has been given to possible long-term changes in these features, and their influence on groundwater flow. The work reported here seeks to provide analogous evidence that such changes are not likely to be important and hence to improve confidence in the presently adopted evaluation methodology and its long-term applicability.

In the orogenic belt that is formed by the Japanese islands, there are wide areas of crystalline rock. The rocks in each area have a distinctive age sequence which is partly reflected in the characteristics of the fracture systems and associated mineral fillings that occur. These characteristics generally imply that groundwater and solutes can be conducted through fracture networks, except in the cases of fault zones or crushed zones. The structural and mineralogical features of these networks readily illustrate how certain contaminants might react and be retarded by the fracture fillings and open pore geometry, due to chemical sorption and/or physical retardation.

Here, we describe the fracture systems developed in crystalline rocks with different ages that are intruded into the Japanese orogenic belt. The aim is to build a model for the long-term fracturing process and hence to evaluate fracture ‘stability’. In particular, the comparisons are made between the fracture geometries and the frequencies observed in the 1.9–0.8 Ma Takidani Granodiorite (the youngest exposed pluton in the world), the ca. 67 Ma Toki Granite and the ca. 117 Ma Kurihashi Granodiorite located in central to northwest Japan. The observations show that all these crystalline rocks have similar fracture frequencies, with 1 to 2 fractures per meter in the massive part of rock bodies. Mineralogical studies and dating analyses of fracture fillings also suggest that fractures are relatively physically stable. Major new fractures tend not to be created in the massive part of rock bodies even when a pluton has been subjected to the regional stresses of plate movements with a duration of about 100 Ma. The results show the unique characteristics of the fracture forming process and the relatively stable geometries of fracture network systems in crystalline rocks distributed within the orogenic belt. This analogue also enables us to provide a model to build confidence in a technical approach applicable for modeling of hydrogeology and geology over long time scales under the orogenic stress field present in Japan. The model may also be useful for other stable tectonic settings as well as for a characterizing sites in crystalline rocks for the possible geological disposal of HLW and other toxic wastes.     

A quadratic element method for evaluating groundwater flow by the inversion of surface tilt with application to the Tono Area, Japan

By modifying a previous method with constant elements, we developed a quadratic element method for more accurately estimating groundwater flow by the inversion of tilt data. In this method: (1) a region of groundwater flow is divided into quadratic elements in which the change in groundwater volume per unit volume of rock (Δv) and the Skempton coefficient (B) vary in a quadratic manner with the coordinates, (2) the values of Δv are set to zero at the boundaries of the region of groundwater flow and (3) the sum of the squared second derivatives of Δv is adopted as a constraining condition that is weighted and added to the sum of the squared errors in tilt. First, analyses were performed for a flow model to determine the accuracy of this method for estimating groundwater flow and also to clarify the effect of the assumed size of a region of groundwater flow. These analyses showed that the quadratic element method proposed in this study gives a much better estimation of Δv than the constant element method and that a large region of groundwater flow should be assumed, rather than a small region, since the values of Δv at points outside of the actual region of groundwater flow are estimated to be nearly zero when a large region is assumed while these values are greatly overestimated when an excessively small region is assumed. Finally, the quadratic element method was applied to the site of the Mizunami Underground Research Laboratory in the Tono area, Japan. Inverse analyses were performed for tilt data measured by four tiltmeters with a resolution of 10⁻⁹ radians during the excavation of two shafts under the assumption that the rock mass is an isotropic and homogeneous half- space. The results showed that the method proposed in this study reproduced the tilt data very accurately. Thus, the distribution of Δv was estimated without sacrificing the reproducibility of the tilt data. The contour maps of B(1 + ν)Δv (ν: Poisson’s ratio) showed that the heterogeneous flow of groundwater occurred at the site and that groundwater volume decreased mainly in the area surrounded by two faults. The latter result is consistent with the finding obtained by previous investigations that these faults have low permeability in the direction perpendicular to the strike and may act as a flow barrier.     

Permeability and pore-connectivity variation of pumices from a single pyroclastic flow eruption: Implications for partial fragmentation

The relationship between permeability and vesicularity in volcanic rocks has been used to infer the degassing behavior of hydrous magma. Recent data on natural samples from various eruptions show a wide variation, fitting a power–law relationship of the percolation models with low (< 30%) critical vesicularity (Ф_C). In this study, we present data on permeability and pore-connectivity of juvenile rhyolitic pumice clasts in a pyroclastic flow around Onikobe volcano, NE Japan, and investigate their relationship with vesicularity developed in a single eruption event. The permeability of the pumices having a relatively low abundance of microlites and microphenocrysts shows a trend increasing by 4 orders of magnitude (from 10^− 13.8 to 10^− 10.1 m²) in a high and narrow vesicularity range (from 72 to 80%). This trend intersects at a high angle with the fit to the permeability–vesicularity data in the previous studies that has a low Ф_C, and is located on the extension of the trend for the products of isotropic decompression experiments. The two-dimensional (2D) connectivities of pores for the pumices were also measured from thin sections. From the point of view of percolation theory, connectivity provides information about the probability of percolation. They showed a steep increase from ca. 0 to 0.7 in an almost similar vesicularity range, as compared to their permeabilities. We attribute the increase in 2D connectivity to the increasing amount of ruptured bubble walls, which might have provided less-tortuous paths through larger apertures for gas flow. This, in turn, would cause an effective increase in the permeability. Aggregates of bubble-wall-shaped glass shards were found in the pumices, and their amount and degree of welding are higher in the pumices that have a higher abundance of microlites and microphenocrysts. These pumices have relatively high permeability and 2D connectivity at low vesicularity, which is accounted for by the existence of large irregularly shaped pores. These textural characteristics suggest that a series of partial fragmentation processes, including local rupturing of bubble walls and subsequent foam-collapse with permeable gas flow, might have occurred before the ultimate bulk fragmentation, thus resulting in the increase in permeability. We suggest that the 2D connectivity of pores is a useful parameter to quantify the degree of fragmentation of bubble walls and has the potential for use to assess their permeability.