Simulation of Formation and Spreading of Salinity Minimum Associated with NPIW Using a High-Resolution Model

A series of numerical experiments were conducted with a high-resolution (eddy-permitting) North Pacific model to simulate the formation and spreading of the salinity minimum associated with the North Pacific Intermediate Water (NPIW). It was found that two factors are required to simulate a realistic configuration of the salinity minimum: a realistic wind stress field and small-scale disturbances. The NCEP reanalyzed wind stress data lead to better results than the Hellerman and Rosenstein wind stress data, due to the closer location of the simulated Oyashio and Kuroshio at the western boundary. Small-scale disturbances formed by relaxing computational diffusivity included in the advection scheme promote the large-scale isopycnal mixing between the Oyashio and Kuroshio waters, simulating a realistic configuration of the salinity minimum. A detailed analysis of the Oyashio water transport was carried out on the final three-year data of the experiment with reduced computational diffusivity. Simulated transport of the Kuroshio Extension in the intermediate layer is generally smaller than the observed value, while those of the Oyashio and the flow at the subarctic front are comparable to the observed levels. In the Oyashio-Kuroshio interfrontal zone the zonally integrated southward transport of the Oyashio water (140–155°E) is borne by the eddy activity, though the time-mean flow reveals the existence of a coastal Oyashio intrusion. In the eastern part (155°E–180°) the zonally integrated transport of the Oyashio water indicates a southward peak at the southern edge of the Kuroshio Extension, which corresponds to the branching of the recirculating flow from the Kuroshio Extension. This revised version was published online in July 2006 with corrections to the Cover Date.     

Geological and geomorphological characteristics of landslides triggered by the 2004 Mid Niigta prefecture earthquake in Japan

The 2004 Mid Niigta prefecture earthquake (M_JMA 6.8) triggered more than one thousand landslides in the Miocene to Quaternary sedimentary rocks in Japan. The most common landslides were shallow disrupted landslides on steep slopes, which has been common in many previous disastrous earthquakes in the world. The Mid Niigta prefecture earthquake also triggered more than one hundred deep landslides, providing valuable information on the conditions for their occurrence. A field investigation and the interpretation of aerial photographs taken before and after the earthquake suggest that reactivation of existing landslides and undercutting of slopes are the most important factors for deep landslides to be triggered by earthquakes. In addition, planar sliding surfaces seem to be essential for the generation of catastrophic landslides triggered by this earthquake. Planar bedding–parallel sliding surfaces were formed at the boundary between the overlying permeable sandstone and underlying siltstone or along the bedding planes of alternating beds of sandstone and siltstone. Sliding surfaces along the slope-parallel oxidation front were formed in the area of black mudstone. New landslides (rockslide-avalanches) occurred with the sliding surfaces in a several-cm thick tuff interbedded in siltstone. One rockslide-avalanche occurred on a slope where buckling deformation preceded the earthquake. Gentle valley bottom sediments were mobilized in many locations, probably because they were saturated and partial liquefaction had occurred due to the earthquake shaking.     

Neodymium isotopic variations in Northwest Pacific waters

Four vertical profiles of the concentration and isotopic composition of Nd in seawater were obtained in the western North Pacific. Two profiles from the Kuroshio Current regime showed congruently that although the Nd concentration increases gradually with depth, its isotopic composition varies significantly with depth depending upon the water mass occupying the water column. The high-salinity Kuroshio waters originating from the North Pacific Tropical Water (NPTW) carry the least radiogenic Nd (?_Nd = −7.4 to −8.7) to this region at ∼250 m from the western margin continental shelves, most likely from the East China Sea. The Nd isotopic compositions in the North Pacific Intermediate Water (NPIW) that occurs at 600 to 1000 m in the subtropical region are fairly uniform at ?_Nd = −3.7. The profile data from the ∼38° to 40°N Kuroshio/Oyashio mixed water region off Sanriku of Honshu, Japan, also suggest that the newest NPIW with ?_Nd = −3.2 is formed there by the mixing of various source waters, and the radiogenic component of Nd is derived mainly from the Oyashio waters.In the Pacific Deep Water (PDW) below ∼1000 m, the Nd isotopic composition is neither vertically nor horizontally homogeneous, suggesting that it serves as a useful tracer for sluggish deep water circulation as well. Two profiles from the Izu-Ogasawara Trench showed a minimum ?_Nd value at ∼2000 m, suggesting that there exists a horizontal advective flow in the vicinity of Honshu, Japan. There is some evidence from other chemical properties to support this observation. The waters below 4000 m including those within the trench in the subtropical region have ?_Nd values of around −5, suggesting that the deep waters are fed from the south along the western boundary, ultimately from the Antarctic Bottom Water (AABW) in the South Pacific. This extends up to ∼40°N along the Japanese Islands. In the subarctic region (>∼42°N), the waters have more radiogenic Nd with ?_Nd > −4.0 throughout the water column, presumably due to the supply of Nd by weathering in such igneous provinces as the Kuril-Kamchatska-Aleutian Island chain. The lateral inhomogeneity of the Nd isotopic composition in PDW suggests that there may be different circulation and mixing regimes in the North Pacific Basin.     

U-Pb and Pb-Pb ages of zircons from basaltic eucrites: Implications for early basaltic volcanism on the eucrite parent body

We have undertaken petrologic and SHRIMP U-Th-Pb isotopic studies on zircons from basaltic eucrites (Yamato [Y]-75011, Y-792510, Asuka [A]-881388, A-881467 and Padvarninkai) with different thermal and shock histories. Eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites. Some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene, and typically have rounded to subrounded shapes due to recrystallization. Superchondritic Zr/Hf ratios of eucritic zircons indicate that they crystallized from incompatible element-rich melts after crystallization of ilmenite. Concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite Y-75011 are higher than those in metamorphosed eucrites.The U-Pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance. Radiogenic lead-loss up to 48% from zircons is observed in the shock-melted eucrite Padvarninkai. The ²⁰⁷Pb-²⁰⁶Pb ages of zircon in Y-75011 (4550 ± 9 Ma, n = 5) are nearly identical, within analytical uncertainty, to the ages of zircons from the metamorphosed eucrite Y-792510 (4545 ± 15 Ma, n = 13), the highly metamorphosed eucrites A-881388 (4555 ± 54 Ma, n = 5) and A-881467 (4558 ± 13 Ma, n = 8), and the shock-melted eucrite Padvarninkai (4555 ± 13 Ma, n = 18). The averaged ²⁰⁷Pb-²⁰⁶Pb age of zircon from five eucrites analyzed in this study is 4554 ± 7 Ma (95% confidence limits, n = 49), indistinguishable from the averaged U-Pb age (4552 ± 9 Ma) of the same samples. Because of the high closure temperature of lead in zircon (T_closure = ∼1050°C with a cooling rate of 0.2°C/yr), the ²⁰⁷Pb-²⁰⁶Pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas.This fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the Solar System history, 7-20 Ma after CAI formation (4567.2 ± 0.6 Ma), thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body. The U-Pb ages of eucritic zircons are older than the U-Pb, Sm-Nd and Rb-Sr ages of some basaltic eucrites, which is consistent with differences in closure temperatures of each isotopic system, and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene, plagioclase and phosphates.     

Pumpellyite in Low-grade Metamorphism

The regional metamorphism of the Kant? Mountains in Japan producedrocks of the following facies, with rising temperature: chloritefacies, pumpellyite-chlorite facies, glauco-phane-schist facies,and greenschist facies. This relationship is compared with theprogressive metamorphic zones in other regions where pumpellyitehas been found. Almost similar relations appear to hold in manymetamorphic terrains. Pumpellyites in glaucophanitic metamorphicterrains have, generally, low Fe'/R'R ratios. Physical andchemical conditions responsible for the formation of pumpellyiteare also discussed.     

Fluid inclusion studies of the Abawso gold prospect,near the Ashanti Belt,Ghana

The major Ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the Kumasi Basin with the Ashanti and Sefwi Belts, respectively. The investigated area of the Abawso small-scale concession, covering the workings of the old Ettadom mine, is situated 3 km west of the lithological contact of the Birimian metavolcanic rocks of the Akropong Belt in the east with the Birimian metasedimentary rocks of the Kumasi Basin in the west. The rocks of the Abawso concession represent a steeply NW-dipping limb of a SE-verging anticline with an axis plunging to the SW. Quartz veining occurs predominantly in the form of en échelon dilatational veins along NNE–SSW-striking shear zones of a few metres width and shows evidence of brittle and ductile deformation. Also stockwork-style quartz veining occurs in the vicinity of the main shaft of the old Ettadom mine. Hydrothermal alteration includes sericitisation, sulphidation and locally carbonatisation. The auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins; however, some occur in stockwork. Fluid inclusion studies reveal a large number of H₂O inclusions along intragranular trails in auriferous quartz vein samples, as well as an overall dominance of H₂O and H₂O-CO₂ inclusions over CO₂ inclusions. Textural observations and physico-chemical fluid inclusion properties indicate post-entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation. This process is interpreted to be responsible for the generation of the CO₂ inclusions from a H₂O-CO₂ parent fluid. In comparison with mineralisation at the Ashanti and Prestea deposits, which are characterised by CO₂±N₂ inclusions, the observed inclusion assemblage may be due to a shallower crustal level of mineralisation, or different degrees and styles of recrystallisation, or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations. Furthermore, the microthermometric observations allow the reconstruction of a possible retrograde P-T path, depicting near-isothermal decompression in the P-T range of the brittle/ductile transition.Editorial handling: E. Frimmel     

Kinetics and Equilibrium Sorption Studies of 4-Nitrophenol on pyrolyzed and activated oil shale residue

Sorption of the organic pollutant 4-nitrophenol (4-NP) by pyrolyzed and activated Jordanian oil-shale was studied. Pyrolyzed oil shale was prepared using a fluidized bed reactor at 520 °C in the presence of nitrogen. Physical activation was carried out by treating the pyrolyzed oil shale with CO₂ at 830 °C, while chemical activation was achieved by using KOH and ZnCl₂ as impregnating agents. Batch kinetics and isotherm studies were conducted to evaluate the sorption process. Effects of contact time, initial sorbate concentration, sorbent concentration, temperature, pH and inorganic salts (NaCl and KCl) on the sorption process by the different sorbents were considered. Chemically activated oil shale, pretreated with ZnCl₂, gave the highest uptake of 4-NP. The isotherm experimental data fit reasonably well to Langmuir, Freundlich and Redlich-Paterson models. Three kinetic models, namely the Morris-Weber, Lagergren, and Pseudo-Second-Order model (PSOM), were applied to represent the experimental results for both pyrolyzed and ZnCl₂-oil shale sorbents.