Properties of sea ice and overlying snow in the Southern Sea of Okhotsk

The general properties of sea ice and overlying snow in the southern Sea of Okhotsk were examined during early February of 2003 to 2005 with the P/V “Soya”. Thin section analysis of crystal structure revealed that frazil ice (48% of total core length) was more prevalent than columnar ice (39%) and that stratigraphic layering was prominent with a mean layer thickness of 12 cm, indicating that dynamic processes are essential to ice growth. The mean thickness of ice blocks and visual observations suggest that ridging dominates the deformation process above thicknesses of 30 to 40 cm. As for snow, it was found that faceted crystals and depth hoar are dominant (78%), as which is also common in the Antarctic sea ice, and is indicative of the strong vertical temperature gradients within the snow. Stable isotope measurements (δ¹⁸O) indicate that snow ice occupies 9% of total core length and that the mass fraction of meteoric ice accounts for 1 to 2% of total ice volume, which is lower than the Antarctic sea ice. Associated with this, the effective fractionation coefficient during the freezing of seawater was also derived. Snow ice was characterized by lower density, higher salinity, and nearly twice the gas content of ice of seawater origin. In addition, it is shown that the surface brine volume fraction and freeboard are well correlated with ice thickness, indicating some promise for remote sensing approaches to the estimation of ice thickness.     

Isotopic anomalies of noble gases in meteorites and their origins—VII. C3V carbonaceous chondrites

Noble gases were measured in bulk samples of the C3V chondrites Grosnaja, Vigarano, and Leoville, and in HF,HCl-insoluble residues before and after etching with HNO₃. The residues were characterized by INAA and SEM. Gas components were determined, directly or by subtraction, for the following fractions: HF,HCl-solubles (?98% of the meteorite), ‘sphase Q’, a poorly characterized trace mineral that is insoluble in HCl-HF but soluble in HNO₃, and an insoluble residue, consisting of ferrichromite, carbonaceous matter, and spinel.Bulk meteorites show some correlation of the noble-gas pattern with McSween's subclasses: two ‘oxidized’ C3V's—Allende (LEWIS et al, 1975) and Grosnaja— have lower Ar/Xe but higher Ne/Xe ratios than the ‘reduced’ C3V's—Vigarano and Leoville—which are transitional to LL3's and C3O chondrites in both respects. An HCl-soluble mineral of high Ar/Xr ratio seems to be responsible. In other respects, the 3 C3V's of this study resemble Allende, with only moderate differences. Phase Q contains most of the Ar, Kr, Xe, but only small amounts of Ne; the etched residues contain planetary Ne ($\text{Ne}{}^{20}\text{Ne}{}^{22}\text{? 8.5}$) and the controversial CCFXe component, enriched in the heavy Xe isotopes ($\text{Xe136}\text{Xe132 ? 0.4–0.5}$). The CCFXe is accompanied by an ‘L-Xe’ component that is enriched in the light Xe isotopes. The proportion of the two is virtually constant in C3V's. as in all other C-chondrites. in contrast to the ~ 2-fold variation in ordinary chondrites.C3V's have systematically higher $\text{Xe}{}^{136}\text{Xe}{}^{132}$ ratios, and hence higher ratios of CCFXe to planetary Xe, than do other chondrite classes. This may reflect some peculiarity in their formation conditions, favoring uptake of CCFXe.     

Numerical study for specifying the major origin of low salinity water associated with <Emphasis Type="Italic">Chattonella</Emphasis> (Raphidophyceae) blooms in Tachibana Bay,Japan

In both 2009 and 2010, massive Chattonella blooms occurred in Tachibana Bay. Observation results show that high cell densities of Chattonella were distributed in the central area of Tachibana Bay with low salinity water. Model results indicate that the low salinity water originated from the Ariake Sea and intruded into Tachibana Bay during the northerly or weak winds. It is suggested that low salinity water was mainly discharged from the northern area of the Ariake Sea. Northerly wind enhanced the horizontal advection of the low salinity water intruding into Tachibana Bay originating from the northern area of the Ariake Sea.     

Neural Network-Based Estimation of Principal Metal Contents in the Hokuroku District,Northern Japan,for Exploring Kuroko-Type Deposits

The Hokuroku district, extending over 40 × 40 km² in northern Japan, is known to be dominated by kuroko-type massive sulfide deposits that have a genetic relation to submarine volcanic activity. The deposits are hosted in a specific stratigraphic zone of Miocene volcanic rocks. Because kuroko-type deposits are under exploration in several countries, it is important to integrate the geologic and geochemical data that have been accumulated in the Hokuroku district to characterize the distribution of deposits and produce a map of mineral potential. Thus, we collected data on multiple chemical components from 1917 rock cores at 143 drillhole sites and concentrated on components with relatively large amounts of data, which are SiO₂, Al₂O₃, and Fe₂O₃ as major elements and Cu, Pb, and Zn as trace elements. Although frequencies of these data can be approximated by normal or lognormal distributions, spatial correlation structures cannot be extracted from the semivariograms of each component nor from the cross-semivariograms between two components of the major or minor elements. To handle such complexity, a spatial method of modeling content distribution, SLANS, is developed by applying a feedforward neural network. The principle of SLANS is to train a network repeatedly to recognize the relation between the data value and the location and lithology of a sample point. One-hundred outputs for each element are obtained by changing the numbers of neurons in a middle layer from 1 to 10 and sample data used for training from 3 to 12, and finally one output is selected based on the estimation precision of the network which is restricted near the target point. After constructing a geologic distribution model from the geological column classified into 25 rock codes, three-dimensional distributions of Cu, Pb, and Zn contents are estimated over the study area. The content models are considered to be valid because high-content zones are located on the known mine sites and the margins of ancient volcanoes or calderas. Some zones are distributed along strikes of major deep-seated fractures in the district.     

The Holocene development of Kushu Lake on Rebun Island in Hokkaido, Japan

Diatom assemblages and sulfur content in sediments were analyzed to clarify changes in the sedimentary environment of Kushu Lake, a coastal lake on Rebun Island in Hokkaido, Japan. Salinity variations were assessed by means of a diatom-based index of paleosalinity and the sedimentary sulfur content. This paper discusses the Holocene development of the lake, in relation to Holocene relative sea-level change. For paleoenvironmental interpretation of the lake development, the rationale of the threshold method (Anundsen et al., 1994) was applied.At ca. 8000 yr BP, a coastal embayment (paleo-Kushu Bay) resulted from marine ingression. The threshold elevation at the mouth of the paleo-Kushu Bay kept pace with the rising sea-level, resulting in its enclosure at the culmination of Holocene marine transgression (ca. 6500–5000 yr BP). From predicted relative sea-level at ca. 6000 yr BP for Rebun Island (Nakada et al., 1991), the threshold may have been at least above –3 to –5 m altitude. A freshwater lake environment with strongly anoxic bottom conditions may have occurred from ca. 5500 to 5100 yr BP. After an important episode of marine ingression, the lake was isolated completely from the open sea at ca. 4900 yr BP. The diatom record suggests that the maximum lacustrine extent occurred at ca. 4900–3100 yr BP. Thereafter, water depth decreased at the lake margins.In Kushu Lake, the threshold elevation, due to a build-up of a coastal barrier, prevents us from determining the amplitude of sea-level changes, even though the age of isolation contacts corresponds to periods of regression and climatic deterioration. In spite of isostatic subsidence, the effective protection provided by the well-developed barrier did not allow registration of any relative sea-level fluctuations since its isolation.     

New Indices for Characterizing Spatial Models of Ore Deposits by the Use of a Sensitivity Vector and an Influence Factor

Precise spatial estimation of ore grades and impurity contents from sample data limited in amount and location is indispensable to metallic and nonmetallic resource exploration. One of the advantages of using geostatistics for this purpose is that it can incorporate multivariate data into spatial estimation of one variable. However, there are two weak points concerning technical and post-processing problems. First is the difficulty in application to geologic data in which spatial correlations are not clear because of intrinsic nonlinear behavior. Second is the absence of indices to interpret the mechanisms and factors which govern the spatial distribution. To address these problems, a spatial method of modeling based on a feedforward neural network, SLANS, which recognizes the relationship between the data value and location by considering supplementary attributes such as lithology and biostratigraphy, and a sensitivity analysis using this network were developed. These methods were applied to two case studies, genetic mechanisms of kuroko deposits and quality assessment of a limestone mine. The first case study is a spatial analysis of principal metals of kuroko deposits (volcanogenic massive sulfide deposits) in the Hokuroku district, northern Japan. It was clarified that upward and downward sensitivity vectors were distinguished near the deposits inside and outside the tectonic basin, respectively. Sensitivity analysis for the second case study showed a strong effect of crystalline limestone on the important impurity, P₂O₅ contents. Hydrothermal alteration, which could cause leaching and secondary concentration of phosphorus, is considered to have produced this effect.     

Two- and three-dimensional numerical simulations of accretion discs in a close binary system

We perform 2D and 3D numerical simulations of an accretion disc in a close binary system using the simplified flux vector splitting (SFS) finite volume method. In our calculations, the gas is assumed to be ideal with γ =1.01, 1.05, 1.1 and 1.2 . The mass ratio of the mass-losing star to the mass-accreting star is unity. Our results show that spiral shocks are formed on the accretion disc in all cases. In 2D calculations we find that the smaller γ is, the more tightly the spiral winds. We observe this trend in 3D calculations as well in a somewhat weaker sense. Mach numbers in our discs are less than 10. These values are lower than the values in observed accretion discs in close binary systems.
Recently, Steeghs, Harlaftis & Horne found the first convincing evidence for spiral structure in the accretion disc of the eclipsing dwarf nova binary IP Pegasi, using the technique known as Doppler tomography. Although the Mach numbers in present calculations are rather low, we may claim that the spiral structure that we discovered in earlier numerical simulations is now found observationally.     

Noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite

Abstract— We measured abundances and isotopic compositions of noble gases in metal and schreibersite of the Acuña (IIIAB) iron meteorite. The concentrations of noble gases in Acuña metal are very low compared to those reported so far for other iron meteorites. The isotopic ratios of He, Ne and Ar indicate that they are mostly of cosmogenic origin. Cosmogenic components are even present in Kr and Xe, which could not have been produced from Fe, Ni and P and are probably due to the spallation of trace elements of higher masses. The high ⁴He/²¹Ne ratio of 420 in Acuña metal indicates that the samples were at a deep position within a very large meteoroid. The exposure ages of Acuña were estimated to be 50–200 Ma from ³He, ²¹Ne and ³⁸Ar abundances and by utilizing the diagrams of production rates vs. the ⁴He/²¹Ne ratio based on the Signer-Nier model. The low exposure age of Acuña may indicate a history different from that of other IIIAB irons whose exposure ages cluster at ～670 Ma. Otherwise, Acuña may be one of the samples with the low production rate, which can not be estimated from the diagrams of the Signer-Nier model.