Numerical Simulation of Residual Circulation due to Bottom Rouglmess Variability Under Tidal Flows in A Semi-Enclosed Bay

Nowadays there are some chronic serious environmental problems, such as eutrophication, blue tide and so on, in a complicated coastal zone or a semi-enclosed bay, because the water exchanges between an inner bay and an outer sea is weak compared with the supply of contaminant. Under this situation, a method to improve the water quality by 3-dimensional small unsymmetrical structures has been proposed by Komatsu et al. In this paper, several numerical simulations of the tidal current and concentration for various arrangements of bottom roughness in a semi-enclosed model bay are carfled out with a depth-averaged 2-D numerical model. The model is solved by the hybrid finite analytic method with nonstaggered grid. And the SIMPLES algorithm with Rhie and Chow＇ s momentum interpolation technique is used for the simulation. The effect of Komatsu＇ s method for water purification is examined by numerical simulation. The result of numerical experiment indicates that it is possible to generate a new tidal residual current and to activate a tidal exchange by bottom roughness arrangement only.     

Numerical simulation of Pacific water intrusions into Otsuchi Bay,northeast of Japan,with a nested-grid OGCM

A numerical simulation of Otsuchi Bay located on the northeast coast of the Honshu, the largest island of Japan, is conducted, using an ocean general circulation model (OGCM) with a nested-grid system in order to illustrate seasonal variability of the circulation in the bay. Through a year, an anticlockwise circulation is dominant in the bay, as observational studies have implied, although it is modified in the bay-mouth-half of the bay in winter. In addition, there is an intense outflow at the surface layer during spring to autumn, influenced by river water discharge. Intrusion of the Pacific water into the bay is influened by mean circulations, but it is also influenced by baroclinic tides from spring to autumn. Pacific water intrusions affected by baroclinic tides may have an impact on the environment in Otsuchi Bay.     

Harmakhis Vallis Source Region,Mars: Insights into the recent geothermal history based on geological mapping

The mapped area of Harmakhis Vallis, at the eastern Hellas Planitia region (35°30–42°50′S; 91°00–97°30′E), covers the surface area of about 212,000 km². The region displays an enhanced modification of the initial topography formed by the Hellas impact. The long and complex history of degradation and alteration involves mass-wasting processes, volcanism and fluvial activity, confronting effects of climate-induced slow mass-wasting processes to effects caused by temporary, catastrophic events (impact cratering, volcanism, etc.). Geological mapping at scale of 1:1,500,000 (full scale at 1:540,000) have been carried out on multiple co-registered data sets available from the past and ongoing orbiter missions to Mars. The mapped geomorphic features of small- and medium-scales reveal in detail events that shaped the topography of the region throughout history, providing specific constraints on the geologic and climatic history of the region. This study highlights events from the most recent Martian history, including fluvial activity recorded in relation to a debris apron flanking Centauri Montes, and evidence of recent positive geothermal anomalies of a high heat-flux with relatively small spatial extents, on the timescale of several million years ago.     

Outflow channel sources, reactivation, and chaos formation, Xanthe Terra, Mars

The undulating, warped, and densely fractured surfaces of highland regions east of Valles Marineris (located north of the eastern Aureum Chaos, east of the Hydraotes Chaos, and south of the Hydaspis Chaos) resulted from extensional surface warping related to ground subsidence, caused when pressurized water confined in subterranean caverns was released to the surface. Water emanations formed crater lakes and resulted in channeling episodes involved in the excavation of Ares, Tiu, and Simud Valles of the eastern part of the circum-Chryse outflow channel system. Progressive surface subsidence and associated reduction of the subsurface cavernous volume, and/or episodes of magmatic-driven activity, led to increases of the hydrostatic pressure, resulting in reactivation of both catastrophic and non-catastrophic outflow activity. Ancient cratered highland and basin materials that underwent large-scale subsidence grade into densely fractured terrains. Collapse of rock materials in these regions resulted in the formation of chaotic terrains, which occur in and near the headwaters of the eastern circum-Chryse outflow channels. The deepest chaotic terrain in the Hydaspis Chaos region resulted from the collapse of pre-existing outflow channel floors. The release of volatiles and related collapse may have included water emanations not necessarily linked to catastrophic outflow. Basal warming related to dike intrusions, thermokarst activity involving wet sediments and/or dissected ice-enriched country rock, permafrost exposed to the atmosphere by extensional tectonism and channel incision, and/or the injection of water into porous floor material, may have enhanced outflow channel floor instability and subsequent collapse. In addition to the possible genetic linkage to outflow channel development dating back to at least the Late Noachian, clear disruption of impact craters with pristine ejecta blankets and rims, as well as preservation of fine tectonic fabrics, suggest that plateau subsidence and chaos formation may have continued well into the Amazonian Period. The geologic and paleohydrologic histories presented here have important implications, as new mechanisms for outflow channel formation and other fluvial activity are described, and new reactivation mechanisms are proposed for the origin of chaotic terrain as contributors to flooding. Detailed geomorphic analysis indicates that subterranean caverns may have been exposed during chaos formation, and thus chaotic terrains mark prime locations for future geologic, hydrologic, and possible astrobiologic exploration.     

Four climate regimes on a land planet with wet surface: Effects of obliquity change and implications for ancient Mars

Series of numerical experiments are performed using a general circulation model to gain insights on the hydrologic cycle on ancient Mars. Since the state of the ancient Mars atmosphere is not well constrained, we did not try to simulate an ancient Mars climate under warm and wet condition. In stead, we used an idealized model and tried to extract general features of the hydrologic cycle by modeling an ideal land planet that has no ocean on its surface. Four different climate regimes, “warm-upright,” “warm-oblique,” “frozen-upright,” and “frozen-oblique” regimes, are recognized depending on the inclination of the spin axis (obliquity) and average surface temperature. The period of active hydrologic cycle suggested from the geomorphology on Mars seems to be consistent with that at the “warm-oblique” regime, which appears at warm (above-freezing) environment with high-obliquity (higher than about 30°) condition.     

Effects of Mg and Si ions on the symmetry of δ-AlOOH

We conducted powder neutron diffraction for δ-AlOOH samples with and without Mg and Si ions under ambient conditions in order to investigate the long-standing problem of the symmetry of this phase. The observed reflection conditions clearly show that the space group of pure δ-AlOOH is P2₁ nm with ordered hydrogen bonds, whereas that of δ-(Al_0.86Mg_0.07Si_0.07)OOH is Pnnm or Pnn2 with disordered hydrogen bonds. It is more likely that the space group of δ-(Al_0.86Mg_0.07Si_0.07)OOH is Pnnm, because cation or hydrogen ordering that breaks the mirror plane perpendicular to c axis in the Pnnm structure would not occur. The previously reported inconsistency for the space group of this phase was caused by the substitution of Mg and Si ions to Al site, i.e., the disordered cations with different valences may fluctuate hydrogen positions, and the disordered hydrogen causes the symmetry change.     

Tectonic evolution of lower crustal rocks in an exposed magmatic arc section in the Hidaka metamorphic belt, Hokkaido, northern Japan

Abstract : The Hidaka metamorphic belt consists of an island-arc assembly of lower to upper crustal rocks formed during early to middle Paleogene time and exhumed during middle Paleogene to Miocene time. The tectonic evolution of the belt is divided into four stages, D₀rs, D₁, D₂rs, and D₃, based on their characteristic deformation, metamorphism, and igneous activity. The premetamorphic and igneous stage (D₀) involves tectonic thickening of an uppermost Cretaceous and earliest Tertiary accretionary complex, including oceanic materials in the lower part of the complex. D₁ is the stage of prograde metamorphism with increasing temperatures at a constant pressure during an early phase, and with a slight decrease of pressure at the peak metamorphic phase, accompanying flattening of metamorphic rocks and intrusions of mafic to intermediate igneous rocks. At the peak, incipient partial melting of pelitic and psammitic gneisses took place in the amphibolite–granulite facies transition zone, the melt and residuals cutting the foliations formed by flattening. In the deep crust, large amounts of S-type tonalite magma formed by crustal anatexis, intruded into the granulite facies gneiss zone and also into the upper levels of the metamorphic sequence during the subsequent stage. During D₁ stage, mafic and intermediate magmas supplied and transported heat to form the arc-type crust and at the same time, the magmatic underplating caused extensional doming of the crust, giving rise to flattening and vertical uplifting of the crustal rocks. D₂ stage is characterized by subhorizontal top-to-the-south displacement and thrusting of lower to upper crustal rocks, forming a basal detachment surface (décollement) and duplex structures associated with intrusions of S-type tonalite. Deformation structures and textures of high-temperature mylonites formed along the décollement, as well as the duplex structures, show that the D₂ stage movement occurred under a N-S trending compressional tectonic regime. The depth of intra-crustal décollement in the Hidaka belt was defined by the effect of multiplication of two factors, the fraction of partial melt which increases downward, and the fluid flux which decreases downward. The crustal décollement, however, might have extended to the crust-mantle boundary and/or to the lithosphere and asthenosphere boundary. The subhorizontal movement was transitional to a dextral-reverse-slip (dextral transpression) movement accompanied by low-temperature mylonitization with retrograde metamorphism, the stage defined as D₃. The crustal rocks from the basal décollement to the upper were tilted eastward on the N–S axis and exhumed during the D₃ stage. During D₂ and D₃ stages, the intrusion of crustal acidic magmas enhanced the crustal deformation and exhumation in the compressional and subsequent transpressional tectonic regime.     

The influence of local topography for wind direction on Mars: two examples of dune fields in crater basins

In this work, we perform an analysis of large dark dunes within Moreux Crater and Herschel Crater on Mars using High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) data sets. These data allow us to conduct a detailed analysis of dune morphology and slip faces, concluding that the studied dune fields are influenced by topographically‐controlled complex wind directions. Our morphological analysis reveals that inside Moreux Crater in particular, the topographic setting dominates the wind flow direction, leading to the development of a sand transport pathway encircling the central peak of the crater. The dune fields in Herschel Crater are also affected by winds controlled by variable topography as suggested by the presence of complex dunes and dune fields. Our analysis indicate that the studied dune systems is not the result of paleo‐wind regimes. Furthermore, we perform thermal inertia measurements using thermal emission spectrometer (TES) data, which indicate that the studied dune fields consist of medium sand 250–500 µm in diameter. Copyright © 2012 John Wiley & Sons, Ltd.     

Late Triassic ammonoid Sirenites from the Sabudani Formation in Tokushima,Southwest Japan,and its biostratigraphic and paleobiogeographic implications

Discovery of Sirenites senticosus (Dittmar) in the upper part of the Sabudani Formation of the Kurosegawa Belt, Kito area, Tokushima Prefecture, Japan, establishes a late Early Carnian age for this part of the stratigraphic unit. Because S. senticosus was mainly distributed in the Tethyan region, its occurrence provides evidence that Late Triassic ammonoids of Japan had strong affinities with those of the Tethyan faunas. This finding clearly differs from the biogeographic distribution of contemporary bivalves in the region, which are referred to as the Kochigatani bivalve faunas, and show strong affinities to faunas of the Boreal region.     

The paleobiogeographical significance of the Silurian and Devonian trilobites of Japan

Six major groups of trilobites from the Silurian and Devonian of Japan are evaluated for their paleobiogeographical signature. Silurian illaenids and scutelluids show four generic‐level and at least two species‐level links with the Australian segment of the Gondwana paleocontinent; encrinurids also indicate two generic‐level links with Australia and also the South China paleocontinent; whilst Devonian phacopids, and possibly proetids, suggest at least two generic‐level links with the North China paleocontinent. These different patterns may reflect the fragmentary biostratigraphical record of Japanese trilobites, but they also appear to reflect paleoenvironmental parameters associated with lithofacies, and paleoecology. Thus, Japanese assemblages of proetids and phacopids occurring in deep‐water clastic lithofacies have counterparts in similar settings in North China, and Japanese scutelluids and illaenids are strongly associated with shallow marine carbonate lithofacies that are similar to those of their occurrences in Australia. Japanese encrinurids occur in carbonate rocks indicative of shallow marine settings in the Kurosegawa Terrane, and they demonstrate a consistent paleobiogeographical affinity with Australia and South China. Larval ecology cannot be directly assessed for Japanese trilobite groups. However, proetids have consistently been shown to have planktonic protaspides, whereas illaenids, scutelluids, and encrinurids have benthic protaspides. Planktonic protaspides would have a greater propensity for distribution in ocean currents than benthic ones, and therefore may be of more limited paleobiogeographical utility. The combined data from the six different groups indicates that the complex paleobiogeographical patterns of the Japanese trilobite assemblages need to be interpreted with caution, and similarity of taxa does not necessarily denote paleogeographical proximity to other regions.