Chemical overprinting of magmatism by weathering: A practical method for evaluating the degree of chemical weathering of granitoids

Quantitative determination of the degree of chemical weathering of rocks is a fundamental task in environmental and engineering geology, and many weathering indices based on whole-rock chemistry have been proposed. However, most classical indices are of limited application to granitoids in a wide area, because these lithotypes generally exhibit wide chemical variation arising from their petrogenesis. The chemical evolution produced during rock weathering, therefore, overprints pre-existing magmatic chemical variation. This problem can cause many classical weathering indices to yield misleading results. This study proposes a method that compensates for the influence of petrogenesis on calculation of the weathering index. The method is based on a bivariate plot of the magmatic chemical variation (MCV) in granitoids, and the degree of chemical weathering (DCW). The MCV axis must be based on an element that reflects magmatic processes and is also relatively immobile during rock weathering. In this study TiO₂ contents are utilized for the MCV. The DCW axis is fundamentally defined by the ratios of more-mobile to less-mobile elements during weathering, and hence many classical indices can be applied. The improved value of the degree of chemical weathering (DCW_i) for a weathered rock is derived by:

DCWi=s×(MCVCV-MCV₁)+DCW₁${\text{DCW}}_{\text{i}}=\text{s}\times ({\text{MCV}}_{\text{CV}}-{\text{MCV}}_1)+{\text{DCW}}_1$

where MCV₁ is the measured composition (e.g. TiO₂ content) of the weathered rock. DCW₁ denotes the ratios of more-mobile to less-mobile elements of the weathered rock. The “s” parameter is the slope of the least square linear regression for fresh granitoids in the MCV–DCW relationship. MCV_CV is a correction factor which is given by the average point on the MCV axis (e.g. average TiO₂) of the fresh rocks. This method is useful for evaluating the degree of weathering of various granitoids, and enhances the practical application of many weathering indices.     

Numerical investigation of the effect of seasonal variations of depthof-closure on shoreline evolution

The current study presents the effects of seasonal variations in the depth-of-closure(Dc)on shoreline evolution using a numerical,one-line shoreline model.Beach erosion of the southern beach of the Nha Trang Coast,which is located in south central of Vietnam,is selected as the study area.This study area is immensely influenced by the tropical monsoon climate that has a clear pattern of large waves in the northeast monsoon season and calm waves in the non-monsoon season.The analysis of the long-term measured shoreline variations from a video-camera system has found a strong correlation of these variations to the monsoon-dominated wave characteristics in the Nha Trang Bay.Therefore,a new approach for determining the depth-of-closure with consideration of the seasonal wave climate changes is purposed in the current study.By implementing this new approach into a numerical,one-line shoreline model,it is found that the seasonal variations of Dc appear to better describe the periodical shoreline evolution due to the monsoon-dominated wave characteristics for the Nha Trang Coast.Such important findings are considered to commonly apply for monsoon-dominated coastal regions in general.These findings are useful information not only for scientific readers but also for the coastal authorities and managers in order to make better countermeasure plans against this kind of erosion mechanism in the future.     

Rayleigh scattering by aqueous colloidal silica as a cause for the blue color of hydrothermal water

Thermal waters in hydrothermal ponds, bathing pools and the brines of geothermal electric power plants commonly have a characteristic blue color. Although many researchers have assumed that the blue color is due to a colloidal suspension and/or absorption by dissolved ferrous iron or by water itself, there has been no specific effort to identify the physical nature of this phenomenon. We have tested, in synthetic and natural solutions, whether aqueous colloidal silica is responsible for the blue color. Aqueous colloidal silica is formed by silica polymerization in thermal waters of the neutral-chloride type which contain initially monomeric silica in concentrations up to three times above the solubilities of amorphous silica. The hue of the blue thermal waters in the pools tested agrees with that of a synthesized colloidal silica solution. Grain-size analyses of aqueous colloidal silica in the blue-colored thermal waters demonstrate that the color is caused by Rayleigh scattering from aqueous colloidal silica particles with diameters (0.1–0.45 μm) smaller than the wavelengths of visible radiation.     

High-pressure transitions and thermochemistry of MGeO3 (M=Mg, Zn and Sr) and Sr-silicates: systematics in enthalpies of formation of A2+B4+O3 perovskites

Phase transitions in MgGeO₃ and ZnGeO₃ were examined up to 26 GPa and 2,073 K to determine ilmenite–perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite–perovskite boundaries have negative slopes and are expressed as P(GPa)=38.4–0.0082T(K) and P(GPa)=27.4−0.0032T(K), respectively, for MgGeO₃ and ZnGeO₃. Enthalpies of SrGeO₃ polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO₃ pseudowollasonite–walstromite and walstromite–perovskite transitions at 298 K were determined to be 6.0±8.6 and 48.9±5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO₃, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (ΔH _f°) of SrGeO₃ perovskite from the constituent oxides at 298 K was determined to be −73.6±5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite–perovskite transition boundaries in MgGeO₃ and ZnGeO₃ and the boundary of formation of SrSiO₃ perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The ΔH _f° of MgGeO₃, ZnGeO₃ and SrSiO₃ perovskites from constituent oxides were 10.2±4.5, 33.8±7.2 and −3.0±2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A²⁺B⁴⁺O₃ perovskites stable at both atmospheric and high pressures to explore the relationship between ΔH _f° and ionic radii of eightfold coordinated A²⁺ (R _A) and sixfold coordinated B⁴⁺ (R _B) cations. The results show that enthalpy of formation of A²⁺B⁴⁺O₃ perovskite increases with decreasing R _A and R _B. The relationship between the enthalpy of formation and tolerance factor ( R _o: O²⁻ radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A²⁺ and B⁴⁺ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A²⁺ and B⁴⁺ radii indicates a systematic change with equienthalpy curves. These relationships of ΔH _f° with R _A and R _B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.     

非海相双壳类江西蛤（Jiangxiella）首次发现于日本

首次报道日本西南部发现的晚三叠世非海相双壳类江西蛤（Jiangxiella)，并讨论这一发现的意义。认为原产于湘、赣、粤地区的江西蛤是地理分布极有限的土著属，它在日本西南部的发现表明当时该区与华南东部的古地理关系极为密切，可能原属同一地块。     

Deterioration of eelgrass,Zostera marina L., meadows by water pollution in Seto Inland Sea,Japan

Survival of transplanted Zostera marina L. (eelgrass) and environmental conditions (water quality, bottom sediments, sedimentation on leaves and flow regime) were studied concurrently in the center, edge, and at the outside of a eelgrass meadow located in a eutrophic coastal zone in northern Hiroshima Bay, Seto Inland Sea, Japan. Eelgrass transplants at the outside of the meadow declined significantly, whereas those at the center were consistently well established. Silt content in the bottom sediments at the outside was higher than that at the center. The sediment was oxic from the surface to 2 cm deep at the center, whereas those at the edge and the outside were reductive almost from the surface. The sediment characteristics typical in eutrophic water seemed to be a factor responsible for the deterioration of eelgrass meadows. Although suspended solid concentrations in the water columns were almost the same, the amount of sediments deposited on leaves of eelgrass at the outside was higher than that at the center of the meadow. The amount of the deposition at the outside seems to be enough to inhibit photosynthesis; i.e. photosynthetic photon flux density (PPFD) available for eelgrass was only 36% of that without any deposition. The deposition in the center, however, was small enough to allow 84% of the original PPFD. Flow rates, determined at 30 cm above the bottom, a half height of average eelgrass, suggested that the rate at the outside was not enough to remove deposited sediments from the surface of eelgrass leaves. Thus, the large amount of sediment deposition caused by water pollution and/or eutrophication seemed to be another factor to inhibit the survival of eelgrass at the outside edge of the meadow.     

Seasonal variation of bromocarbons at Hateruma Island,Japan: implications for global sources

High-frequency measurements of dibromomethane (CH₂Br₂) and bromoform (CHBr₃) at Hateruma Island, in the subtropical East China Sea, were performed using automated preconcentration gas chromatography/mass spectrometry. Their baseline concentrations, found in air masses from the Pacific Ocean, were 0.65 and 0.26 ppt, respectively, in summer and 1.08 and 0.87 ppt, respectively, in winter. Air masses transported from Southeast Asia were rich in bromocarbons, suggesting strong emissions in this area. The passage of cold fronts from the Asian continent was associated with sharp increases in observed concentrations of bromocarbons derived from coastal regions of the continent. Comparison of the relationships between [CH₂Br₂]/[CHBr₃] and [CHBr₃] in the Hateruma Island data with those in monthly mean data from 14 globally distributed U.S. National Oceanic and Atmospheric Administration ground stations suggested that these gases are produced primarily from a common process on a global scale.     

Frequency-dependence of velocity and attenuation of elastic waves in granite under uniaxial compression

Velocity as well as attenuation factorQ ^–1 ofP-wave in a dry granitic rock sample under uniaxial compressions were measured in the range of frequency between 100 kHz and 710 kHz by using the pulse transmission technique. Above the stress of 0.5 _f , where _f is the fracture stress, theP-wave velocity decreases with increasing axial stress, whereasQ ^–1 increases. Particularly, the change ofQ ^–1 is greater for high frequency than for low frequency. At a given stress level, the higher the frequency, the higher theP-wave velocity and the largerQ ^–1. This result means that the velocity decrease with increasing stress is smaller for higher frequency. Because of this frequency-dependence of velocity decrease, theP-wave in the rock under dilatant state shows dispersion. The body wave dispersion is more remarkable at higher stress, and is not found in a homogeneous material with no cracks. Thus the disperison is attributed to the generation of cracks. When the frequency-dependence ofQ ^–1 is approximated asf ⁿ in the present frequency range, the exponentn takes a value from 0.63 to 0.77.     

The ZnSiO3 clinopyroxene-ilmenite transition: Heat capacity,enthalpy of transition,and phase equilibria

ZnSiO₃ clinopyroxene stable above 3 GPa transforms to ilmenite at 10–12 GPa, which further decomposes into ZnO (rock salt) plus stishovite at 20–30 GPa. The enthalpy of the clinopyroxene-ilmenite transition was measured by high-temperature solution calorimetry, giving ΔH⁰=51.71 ±3.18 kJ/mol at 298 K. The heat capacities of clinopyroxene and ilmenite were measured by differential scanning calorimetry at 343–733 and 343–633 K, respectively. The C _p of ilmenite is 3–5% smaller than that of clinopyroxene. The entropy of transition was calculated using the measured enthalpy and the free energy calculated from the phase equilibrium data. The enthalpy, entropy and volume changes of the pyroxene-ilmenite transition in ZnSiO₃ are similar in magnitude to those in MgSiO₃. The present thermochemical data are used to calculate the phase boundary of the ZnSiO₃ clinopyroxene-ilmenite transition. The calculated boundary,

Wind-dependent formation of phytoplankton spring bloom in Otsuchi Bay,a ria in Sanriku,Japan

Spring blooms of phytoplankton composed of centric diatoms developed in late February, March, and April in Otsuchi Bay on Sanriku ria coast, Japan. During this period, associated with prolonged seasonal west wind (>1 day), intense exchange of waters occurred between inside and outside the bay: outflow of nearsurface brackish water over inflow of oceanic water at depth. This circulation interrupted formation of the blooms, and transported phytoplankton populations seaward. By such water movements, a significant amount of nutrients in the bay was carried out, otherwise replenished into the bay, depending on water masses located outside the bay. Owing to irregular features of wind events, a bloom lasted from several days to a week. From February to April, supply of nutrients seemed to be replete except for the latter half of the bloom period, and estimates of the critical depth exceeded the depth of the bottom consistently. Thus, net growth of phytoplankton was expected throughout the observation period, and potentially blooms could be formed. However, the blooms were only formed under calm weather. We hypothesize that the exchange of waters dilutes populations in the bay, and that formation of the bloom, that is, accumulation of biomass depends on a balance between the growth of phytoplankton and the dilution of bay water.