Meander of the Kuroshio front and current variability in the East China Sea

To investigate the fluctuation of the Kuroshio front, moored current meters were deployed near the shelf break and on the continental slope in the East China Sea, northwest of Okinawa Island, during a period from 25 June to 22 July 1984. Two mooring arrays were deployed on the slope of about 800 m water depth (under the Kuroshio), about 30 km apart along the path of the Kuroshio. Another two arrays were set near the shelf break of about 300 m water depth. The fluctuation of current on the slope is found to have a predominant period of 11–14 days and a were length of 300–350 km, propagating toward the downstream direction of the Kuroshio with a phase velocity of about 30 cm sec^?1. When the Kuroshio front approaches the shelf break and the crest of the meander covers the mooring site, the current direction moves toward the downstream direction of the Kuroshio and the water temperature increases. On the other hand, when the trough of the meander covers the mooring site, the current direction changes off-shoreward across the Kuroshio or in the upstream direction of the Kuroshio, and the water temperature decreases. Three-dimensional distributions of water temperature and salinity around the mooring site were observed with a CTD twice at 5.5 days intervals, which indicate the meanders of the front is about 180° out of phase. This coincides with a period of 11–14 days obtained with the moored current meters. Wave lengths of the dominant meander of the front in the satellite thermal images were about 350 km and 100–200 km, which also coincides with results obtained with the moored current meters.     

Advances in computational morphodynamics using the International River Interface Cooperative (iRIC) software

Results from computational morphodynamics modeling of coupled flow–bed–sediment systems are described for 10 applications as a review of recent advances in the field. Each of these applications is drawn from solvers included in the public-domain International River Interface Cooperative (iRIC) software package. For mesoscale river features such as bars, predictions of alternate and higher mode river bars are shown for flows with equilibrium sediment supply and for a single case of oversupplied sediment. For microscale bed features such as bedforms, computational results are shown for the development and evolution of two-dimensional bedforms using a simple closure-based two-dimensional model, for two- and three-dimensional ripples and dunes using a three-dimensional large-eddy simulation flow model coupled to a physics-based particle transport model, and for the development of bed streaks using a three-dimensional unsteady Reynolds-averaged Navier–Stokes solver with a simple sediment-transport treatment. Finally, macroscale or channel evolution treatments are used to examine the temporal development of meandering channels, a failure model for cantilevered banks, the effect of bank vegetation on channel width, the development of channel networks in tidal systems, and the evolution of bedrock channels. In all examples, computational morphodynamics results from iRIC solvers compare well to observations of natural bed morphology. For each of the three scales investigated here, brief suggestions for future work and potential research directions are offered. © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd     

Zircon U–Pb geochronology of “Sashu mylonite”, eastern extension of Higo Plutono‐metamorphic Complex,Southwest Japan: Implication for regional tectonic evolution

Geochronological and geochemical studies reveal the possible origin of the restricted body of mylonite rocks occurring at the eastern edge of Kyushu Island, Japan, just in contact with the Sashu Fault, a part of the Paleo‐Median Tectonic Line (Paleo‐MTL). The LA‐ICP‐MS zircon U–Pb dating of the quartz diorite mylonite in this mylonitic body indicates a crystallization age of 114.0 ±1.7 Ma. Moreover, the two tonalite samples appear as thin layers within the Permian fine‐grained mafic mylonite; a part of the same body yields the age of 113.7 ±2.3 Ma and 116.9 ±1.3 Ma, with extremely low Th/U ratio. These quartz diorite mylonite and tonalite are consistent with the late Early Cretaceous magmatism and coeval metamorphism similar to those in the Higo Plutono‐metamorphic Complex in western Kyushu, Japan. This newly characterized complex occurs just south of the Cretaceous Sambagawa metamorphic rocks. The newly characterized mylonitic rocks are lying structurally above the Sambagawa Metamorphic Complex and are distributed along the Paleo‐MTL. The extension of the Higo Plutonometamorphic Complex, as well as the structural relationship between this complex and the Sambagawa Metamorphic Complex, is still controversial but holds a key to reconstruct the tectonic evolution of Southwest Japan during the Late Mesozoic to Early Cenozoic period. Hence, this article provides new insight into the reconstruction of the evolution history of East Asia as an active convergent margin.     

Origin of the early Cenozoic belt boundary thrust and Izanagi–Pacific ridge subduction in the western Pacific margin

The belt boundary thrust within the Cretaceous–Neogene accretionary complex of the Shimanto Belt, southwestern Japan, extends for more than ~ 1 000 km along the Japanese islands. A common understanding of the origin of the thrust is that it is an out of sequence thrust as a result of continuous accretion since the late Cretaceous and there is a kinematic reason for its maintaining a critically tapered wedge. The timing of the accretion gap and thrusting, however, coincides with the collision of the Paleocene–early Eocene Izanagi–Pacific spreading ridges with the trench along the western Pacific margin, which has been recently re‐hypothesized as younger than the previous assumption with respect to the Kula‐Pacific ridge subduction during the late Cretaceous. The ridge subduction hypothesis provides a consistent explanation for the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin. This is not only the case in southwestern Japan, but also along the more northern Asian margin in Hokkaido, Sakhalin, and Sikhote‐Alin. This Paleocene–early Eocene ridge subduction hypothesis is also consistent with recently acquired tomographic images beneath the Asian continent. The timing of the Izanagi–Pacific ridge subduction along the western Pacific margin allows for a revision of the classic hypothesis of a great reorganization of the Pacific Plate motion between ~ 47 Ma and 42 Ma, illustrated by the bend in the Hawaii–Emperor chain, because of the change in subduction torque balance and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin.     

Cryptosporidium and Giardia in Natural,Drinking, and Recreational Water of Northwestern Greece

An investigation about distribution of Giardia cysts and Cryptosporidium oocysts in natural, drinking, and recreational water in Northwestern Greece was performed. Five rivers (Aoos, Arachthos, Kalamas, Louros, and Voidomatis) and one lake (Pamvotis Ioannina Lake) in Northwestern Greece were investigated during a 10‐month period. Drinking and recreational water (swimming pools) from the area were also examined. Samples were collected from prefixed sampling stations and processed following a modification of standard methods for the microbiological examination of water, as suggested by the APHA/AWWA/WEF. Both Giardia cysts and Cryptosporidium oocysts were isolated from Pamvotis Ioannina Lake (15 positive/27 examined samples). Significantly lower numbers of Cryptosporidium oocysts were detected in Arachthos River (1/5), Voidomatis River (1/5), drinking water (1/7), and pool water samples (1/9). No Giardia cysts were detected, neither in river water, nor in drinking, and pool water samples. The results clearly show that, with the exception of Pamvotis Ioannina Lake, where contamination of high level was observed, natural water sources of the investigated area have low pollution, resulting in low contamination with parasites.     

Oxygen isotope systematics of chondrule olivine,pyroxene, and plagioclase in one of the most pristine CV3Red chondrites (Northwest Africa 8613)

We performed in situ oxygen three‐isotope measurements of chondrule olivine, pyroxenes, and plagioclase from the newly described CV_Red chondrite NWA 8613. Additionally, oxygen isotope ratios of plagioclase in chondrules from the Kaba CV3_OxB chondrite were determined to enable comparisons of isotope ratios and degree of alteration of chondrules in both CV lithologies. NWA 8613 was affected by only mild thermal metamorphism. The majority of oxygen isotope ratios of olivine and pyroxenes plot along a slope‐1 line in the oxygen three‐isotope diagram, except for a type II and a remolten barred olivine chondrule. When isotopic relict olivine is excluded, olivine, and low‐ and high‐Ca pyroxenes are indistinguishable regarding Δ¹⁷O values. Conversely, plagioclase in chondrules from NWA 8613 and Kaba plot along mass‐dependent fractionation lines. Oxygen isotopic disequilibrium between phenocrysts and plagioclase was caused probably by exchange of plagioclase with ¹⁶O‐poor fluids on the CV parent body. Based on an existing oxygen isotope mass balance model, possible dust enrichment and ice enhancement factors were estimated. Type I chondrules from NWA 8613 possibly formed at moderately high dust enrichment factors (50× to 150× CI dust relative to solar abundances); estimates for water ice in the chondrule precursors range from 0.2× to 0.6× the nominal amount of ice in dust of CI composition. Findings agree with results from an earlier study on oxygen isotopes in chondrules of the Kaba CV chondrite, providing further evidence for a relatively dry and only moderately high dust‐enriched disk in the CV chondrule‐forming region.     

Primordial,thermal, and shock features of ordinary chondrites: Emulating bulk X‐ray diffraction using in‐plane rotation of polished thin sections

Using an X‐ray diffractometer, powder‐like diffraction patterns were acquired from in‐plane rotation of polished thin sections (PTSs) of 60 ordinary chondrites (23 H, 21 L, and 16 LL), in order to explore the thermal and shock metamorphism and its modifications of primordial features. The olivine (Ol) 130 peak position shown as Bragg indices clearly correlates with the chemical group for equilibrated ordinary chondrites (EOCs), while the peak is split or broad for unequilibrated ordinary chondrites (UOCs). The intensity ratio of kamacite may be useful for distinguishing the chemical group between H and L‐LL, but it is not definite because of heterogeneous terrestrial weathering of kamacite, especially in H chondrites. The summed intensities of the orthoenstatite (Oen) 511 and 421 peaks positively correlates with the metamorphic sequence from 3 to 6, while that of clinoenstatite (Cen) 22 is inversely correlated. The shock stage positively correlates with the summed full width of half maximum values of the Oen 511 and 421 peaks and the FWHM of Ol 130 peak for each class. Significant amount of Oen (Pbca) transformed through Cen (C2/c) finally to Cen (P2₁/c) is stable at high pressure for shock stage S6 (Tenham and NWA 4719). The shock melted LL chondrite is characterized by the occurrence of Cen and abundant homogeneous olivine. The effects of both thermal and shock metamorphism are thus incorporated into the bulk X‐ray diffraction (XRD) data. The bulk XRD method is useful for determining the bulk mineralogy, resulting in the classification of ordinary chondrites. The method is also applicable to samples other than PTS.     

Mantle diapir-induced arc volcanism: The Ueno Basalts, Nomugi-Toge and Hida volcanic suites, central Japan

Stratigraphic and geochronological data show that the late Cenozoic Ueno Basalts and related Nomugi-Toge and Hida volcanic suites of the Norikura Volcanic Chain, Japan, were active for ~ 1 million years. Temporal and spatial variations of the volcanic activity and chemistry of the volcanic products suggest that it was induced by a common mantle diapir. The Ueno Basalts are small monogenetic volcanoes scattered over an area 50 km in diameter, and comprise a small volcanic province. The Ueno Basalts are almost all subalkalic basalt to basaltic andesite, erupted through the late Pliocene to the earliest Pleistocene (2.7–1.5 Ma). Andesite to dacite of the Nomugi-Toge volcanic rocks were concurrently active in the back arc side, and two eruption stages (2.6–2.2 and 2.1–1.7 Ma) are recognizable. Two voluminous dacite and rhyolite ignimbrites, the Hida Volcanic Rocks, were erupted deeper in the back-arc region, at ca 1.75 and 1.7 Ma. Both the Nomugi-Toge and Hida suites are also subalkalic, except for the last ignimbrite. In the Ueno Basalts, alkali olivine basalt was erupted in the earliest stage, and was followed by subalkalic basalt, showing that the magma segregation depth ascended with time. This coincided with uplift of the volcanic province and with quasi-concentric expansion of the eruption centers, suggesting that an upwelling mantle diapir was the cause of the volcanism. The Nomugi-Toge andesite–dacite lavas and the Hida dacite and rhyolite ignimbrites are considered to have originated from the same mantle diapir, because of their close proximity to the Ueno Basalts and their near-contemporaneous activity. Mantle diapirs have a significant role in the origin of subalkalic volcanic rocks in the island arcs.     

Magma plumbing system beneath Ontake Volcano, central Japan

Ontake Volcano in central Japan was last active from ~ 100–35 Ka. The eruptions contained rhyodacite pumice and lavas in the first stage (stage O1, > 33 km³), followed by eruptions of andesite lavas and pyroclastics (stages O2 and O3, > 16 km³). Modeling of major and incompatible elements with Sr isotope ratios suggests that the primary magma was a high-alumina basalt. One andesite magma type appears to have evolved from the basalt in a closed system magma chamber, in part by fractional crystallization, and its generation included crustal assimilation. The other andesite magma type is considered to have evolved in an open system magma chamber in which repeated input of primary magma occurred together with wall-rock assimilation and fractional crystallization. The rhyodacite is inferred to have evolved in a closed system magma chamber by fractional crystallization of the second type of andesite. These genetic relationships require that the magma chamber functioned alternately as an open and a closed system. Geobarometry indicates that there may have been multiple magma chambers, located in the upper crust for the rhyodacite, near the upper–lower crust interface for the andesite and in the mid-lower crust for the basalt. These chambers were stacked to form the magma plumbing system of Ontake. Incompatible element compositions of the basalt are considered to have changed during the eruptions, suggesting that two different plumbing systems for stage O1 magma and for stages O2, O3 magmas existed during the 65 Ka of activity. Evolutionary history of the systems implies that the primary magma was introduced into the magma plumbing system each for ~ 17 500 years and that the life span of a magma plumbing system was shorter than 40 Ka.     

Subinertial and seasonal variations in the Soya Warm Current revealed by HF ocean radars,coastal tide gauges,and bottom-mounted ADCP

Subinertial and seasonal variations in the Soya Warm Current (SWC) are investigated using data obtained by high frequency (HF) ocean radars, coastal tide gauges, and a bottom-mounted acoustic Doppler current profiler (ADCP). The HF radars clearly captured the seasonal variations in the surface current fields of the SWC. Almost the same seasonal cycle was repeated in the period from August 2003 to March 2007, although interannual variations were also discernible. In addition to the annual and interannual variations, the SWC exhibited subinertial variations with a period of 5–20 days. The surface transport by the SWC was significantly correlated with the sea level difference between the Sea of Japan and Sea of Okhotsk for both the seasonal and subinertial variations, indicating that the SWC is driven by the sea level difference between the two seas. The generation mechanism of the subinertial variation is discussed using wind data from the European Centre for Medium-range Weather Forecasts (ECMWF) analyses. The subinertial variations in the SWC were significantly correlated with the meridional wind stress component over the region. The subinertial variations in the sea level difference and surface current delay from the meridional wind stress variations by one or two days. Sea level difference through the strait caused by wind-generated coastally trapped waves (CTWs) along the east coast of Sakhalin and west coast of Hokkaido is considered to be a possible mechanism causing the subinertial variations in the SWC.