Geological Survey of Japan

A systematic laboratory work on high pressure experimentation at the Geological Survey has revealed that compaction (consolidation) is the most important factor to control mechanical properties of the clastic sedimentary rocks, although there are a great variety in terms of geological age, grain size, and texture in it ( Hoshino et al, 1972; Hoshino, 1974; Hoshino and Inami, 1977). The specimens have been collected from all sedimentary basins in Japanese archipelago, most of which are composed of considerably thick layers of clastic rocks such as mudstone and sandstone only. The apparatus and method of experimentation was described in Hoshino et al (1972).

Volcanic hazard assessments are in part constrained by understanding the past behaviour of a volcano (e.g., eruptive frequency and magnitude), this is largely reconstructed using tephra deposits preserved proximal to source. However, these near-vent eruption records are often fragmentary and incomplete owing to burial and erosion processes, thus hampering the accuracy of hazard assessments. Here, we capitalise on the potential of long, undisturbed records of ash fall events preserved in East Asian marine and lacustrine sedimentary archives, typically positioned >100 km from volcanic sources, to plug the gaps in near-source eruption records. The extraction and identification of microscopic ash layers (cryptotephra) from sedimentary archives is adopted to provide important constraints on the timing of mid-intensity explosive eruptions, which are frequently under-reported at source.Following detailed cryptotephra investigations, we present a new eruption record captured by high-resolution sediment cores collected from the Sea of Japan spanning approximately the last 200,000 years. Detailed geochemical fingerprinting is used to assign tephra and cryptotephra deposits to volcanic source, and where possible to known eruption units, some of which are the target of zircon double-dating (ZDD). Furthermore, the chemical signatures are used to link the Sea of Japan tephra layers to those preserved in the precisely dated sediments of Lake Suigetsu (Honshu Island), providing important chronological constraints on our newly developed eruption record. Our investigations provide evidence of near-vent under-reporting of explosive eruptions and new insights into the repose periods between pre-historic eruptions at specific volcanoes.

Triple junctions between fundamental tectonic units are where different compositions and magmatic processes interact. We investigated the intersection of a spreading center with an island arc within the Philippine Sea Plate, i.e. an arc-ridge-arc triple junction, using a combination of submersible sampling and shipboard geophysics. We use Ar Ar dating, elemental concentrations and Sr-Nd-Pb radiogenic isotope ratios to constrain the structure, age and origin of magmatism as spreading interacted with the arc. Between 33 and 31 Ma the tip of the West Philippine Basin spreading center was in the back-arc west of the Kyushu-Palau arc. Spreading volcanism had mantle-like Ce/Pb and Th/Ce indicating a lack of slab flux. Radiogenic isotopes form a trajectory towards low 143 Nd/ 144 Nd and high 20x Pb/ 204 Pb, but extend to Δ 208 Pb/ 204 Pb > 85 which corresponds to an EM-2-like mantle component rather than pelagic sediment input from the slab. The final stage of spreading-related activity occurred between 31 and 28.5 Ma when the spreading axis rotated from NW-SE to E -W and propagated eastwards into the arc. It resulted in magmatism contemporaneous with the adjacent frontal arc. This final magmatism extended the radiogenic isotopic trajectories observed in the earlier spreading towards less radiogenic Pb and higher 143 Nd/ 144 Nd, i.e. towards MORB from the EM-2 component. However, during the final stage magmatism La/Sm, Nb/Zr increased while Pb and Sr isotopes became progressively more radiogenic, demonstrating a progressive decrease in the degree of partial melting and preferential tapping of the EM-2 component. This change in melt generation and composition appears a common process at the end of spreading. • Magmatism at arc-ridge-arc triple junction in the Philippine Sea Plate was studied. • CBF Rift spreading center intersected KPR arc, caused overlapping MORB magmatism. • Interaction between a volcanic arc and a spreading centre caused excess magmatism. • Spreading center at wanning stage preferentially consumes more enriched mantle.

The trace fossil Skolithos is a simple, sub-vertical, unbranched, cylindrical burrow that characterizes the trace fossil from early Cambrian to recent times. The Skolithos ichnofacies is an indicator of high-energy coastal marine facies, such as the sandy beaches of wave-dominated coasts; however, to date, neoichnological studies have only reported its occurrence in low-energy settings, where it is produced by a variety of annelids, phoronids, and priapulids. In this paper, we describe the burrow produced by the Australian giant beach worm ( Australonuphis , family Onuphidae) at Bengello Beach, southeastern Australia, where they occur in a high-energy, wave-dominated sandy coastal setting. The burrows of this omnivorous scavenger show a straight or slightly bent vertical shaft without branching or chambers, and may be regarded as a modern analog of the trace fossil Skolithos. The burrows are distributed only below the lower foreshore, and therefore we infer that the uppermost level of Skolithos occurrences in the Phanerozoic fossil record may be a useful indicator of ancient sea-level in the Skolithos ichnofacies. As such, our findings improve knowledge of the palaeoecology of the Skolithos ichnofacies, including feeding ecology, and enhance its palaeoenvironmental utility. • The Australian giant beach worm produces a simple but long vertical burrow. • The burrow has the same feature as the trace fossil Skolithos . • The burrow occurs only below the lower foreshore. • The trace fossil Skolithos can be used as an indicator of ancient sea level.

This study investigates the Holocene evolution of the upper Kiso River delta in central Japan, focusing on sedimentary processes and accumulation patterns in the upper delta plain near the upstream limit, an area directly influenced by transgressive and regressive phases driven primarily by eustatic sea-level change and less explored than the lower delta plain. The analysis is based primarily on a 23-m sediment core (UK) retrieved from a floodbasin approximately 40 km inland from the present mouth of the Nagara River. The core was analyzed for sedimentary facies, grain size distribution, colour, electrical conductivity, loss on ignition and AMS radiocarbon dating. Six depositional units were identified, ranging from basal river gravels to marine-influenced sands and overlying floodbasin muds and peats. An upward-coarsening succession from tidal river to distributary channel deposits along with the presence of gravels marks the onset of regression at approximately 8 ka. Radiocarbon data indicate that sediment accumulation initially almost kept pace with eustatic sea-level rise but slowed to approximately 1.6 m/kyr during the highstand phase, exceeding the contemporaneous sea-level rise. Stratigraphic correlation with nearby boreholes along the Kiso and Nagara rivers reveals consistent depositional patterns and a lack of prodelta mud in upstream areas. A turning point in N -values within the sand layer may represent the maximum flooding surface. Floodplain aggradation has persisted since approximately 6 ka, likely accompanied by vertical accretion within the channel belt. Despite the slowdown in sea-level rise and regional crustal uplift associated with glacio-hydro isostatic adjustment, tectonic subsidence appears to have maintained sufficient accommodation. Peat accumulation, which became increasingly prominent after 4 ka, reflects reduced clastic input and associated hydrological changes. Comparative stratigraphic and geomorphological analyses suggest that floodbasin deposits have been less frequently replaced by channel migration and avulsion along the Ibi and Nagara rivers than along the Kiso River.

Abstract Observations of exoplanet transits by small satellites have gained increasing attention for reducing biases in the detection of long-period planets. However, no unambiguous detection of an exoplanet has yet been demonstrated using optics with apertures smaller than 60 mm. Here, we investigated the detectability of exoplanet transits using the telescopic Optical Navigation Camera (ONC-T) on board the Hayabusa2 spacecraft, which has an effective aperture of only 15 mm. We conducted transit observations of the hot Jupiters WASP-189 b and MASCARA-1 b, collecting data for 10 and four events, respectively. The transit signal was detected with a signal-to-noise ratio (SNR) of 13 for WASP-189 b and 8 for MASCARA-1 b for each event. Stacking all events improved the SNR to 40 and 16, respectively. The transit midtimes of each event were measured with a precision of 6 minutes and were consistent with Transiting Exoplanet Survey Satellite (TESS) data to within 2 minutes. The planet-to-star radius ratio was determined with an absolute precision of 0.004 (6% relative) and agreed with TESS results to within 0.002 (3% relative). The recent ONC-T and TESS data enabled an update to the planetary ephemerides. We report a 4 σ discrepancy between the updated orbital period of MASCARA-1 b and previously reported values. ONC-T sets a new record for the smallest-aperture instrument to detect an exoplanet transit from space, advancing the frontier of exoplanet science with miniature instrumentation. Our results suggest that optics as small as ONC-T may be capable of detecting transiting long-period Jupiters: a population that remains underrepresented in current surveys.

Abstract The existence of missing geomagnetic reversals has been proposed, with potential for new magnetostratigraphic age controls. We estimate geomagnetic reversal frequency from 0 to 155 Ma using adaptive‐bandwidth kernel density estimation (AKDE) to evaluate data sparseness and to assess how reversal frequency changes when recently identified geomagnetic reversals are incorporated into the geomagnetic polarity time scale (GPTS) data set. AKDE is a two‐stage procedure that uses an initial density estimator based on an initial (pilot) bandwidth. We found that the pilot bandwidth determined using cross‐validation is stable with respect to data set length. The AKDE results obtained based on the cross‐validated pilot bandwidth reveal four troughs after the Cretaceous Normal Superchron, spaced 13.5–15.0 Myr apart and corresponding to relatively long chrons (>0.8 Myr). One trough near 32 Ma becomes less distinct after the four recently identified reversals are added to the data set. This sensitivity suggests that troughs in the frequency curve may indicate missing geomagnetic reversals.

紀伊半島東部，三重県朝柄地域の中央構造線沿いに分布する珪長質火山砕屑岩からなる朝柄層と付随する黒雲母トーナル岩のジルコンU–Pb年代測定を行った．朝柄層の層序を再検討し，下位より，松葉谷流紋岩質凝灰岩部層，上出流紋岩質凝灰質砂岩泥岩互層部層，不動谷流紋岩質凝灰角礫岩部層，中出流紋岩質凝灰岩部層に層序区分した．松葉谷流紋岩質凝灰岩部層からは96.1±0.9 Ma，上部の中出流紋岩質凝灰岩部層からは91.7±1.3 Maの年代を得た．これらの年代から，朝柄層は紀伊半島西部から淡路島に分布する泉南流紋岩類に対比される．一方，付随する黒雲母トーナル岩からは79.9±8.9 Maの年代が得られ，この年代は松葉谷流紋岩質凝灰岩部層の約78 Maの小クラスター年代に類似する．このことは黒雲母トーナル岩が松葉谷流紋岩質凝灰岩部層の近傍で約78 Maに貫入し，熱水変質作用を伴った弱い接触変成作用を起こしたと考えられる．

Abstract We performed spectral measurements on mixtures of ilmenite powder (<48 μ m) and clinopyroxene or gabbro powder (<155 μ m) to investigate the effect of ilmenite content on the spectral characteristics in the ultraviolet (UV), visible (Vis), and near-infrared (NIR) ranges. We found that increasing ilmenite content leads to (1) a darkening of Vis and NIR reflectance, (2) a decrease of the UV slope, (3) a weakening of band depths around 950–1050 nm and 1250–1300 nm, and (4) a neutralization of the Vis–NIR slope. For lunar mare regions, which have ∼7wt% ilmenite, it was estimated that the albedo in the Vis–NIR reflectance would decrease by 25%–29%, the Vis–NIR slope would decrease by 9%–26%, and the UV slope would decrease by 4%–5%. We also examined the effect of the addition of clinopyroxenes and gabbro on the spectral characteristics of ilmenite. The results indicate that most diagnostic features of ilmenite were masked, except for a distinct peak near 900 nm, which can thus serve as a reliable indicator of the presence of ilmenite in mixed compositions. Furthermore, we evaluated existing methods for estimating TiO 2 content in lunar mare regions using spectral band ratios. The results indicate that ratios that incorporate UV reflectance are more sensitive to ilmenite content than those that incorporate reflectance only at Vis and/or NIR wavelengths longer than 415 nm. This suggests that UV reflectance is more reliable for the estimation of ilmenite abundance on the lunar surface.