The southern Okinawa Trough provides an opportunity to investigate the early stage of back-arc rifting before the onset of oceanic spreading. We examined the Yaeyama and Yonaguni Rifts using multichannel seismic reflection data, swath bathymetry, heat flow measurements, and geomagnetic observations. The seismic profiles reveal widespread seafloor-cutting normal faults and thick trough-fill sediments in both rift segments. In the Yaeyama Rift, a localized acoustically transparent body is identified beneath the rift axis and may represent a shallow intrusive feature. However, higher-resolution seismic data or additional geophysical constraints would be required to confirm its origin. Heat flow values are generally low (40–63 mW/m2) except for one localized anomaly, consistent with previous observations of subdued heat flow in the southern Okinawa Trough. The geomagnetic data do not indicate extensive shallow volcanic construction, although localized magnetized bodies (e.g., small intrusive or volcanic units) may be present. These observations suggest that early rifting in the southern Okinawa Trough is dominated by distributed normal faulting under thermally subdued conditions and with only limited evidence for substantial magmatic contribution, with possible localized magmatic activity in the Yaeyama Rift. Our results provide structural and thermal constraints on the diversity of early-stage rifting processes in continental back-arc basins.
Two new genera and species of tubular foraminifera (Shimenawa okuyamae gen. et sp. nov. and Yorishiro morii gen. et sp. nov.) are described from approximately 4000-year-old nuclei of deep-water acervulinid macroids collected from the shelf off Kikai Jima, Ryukyu Islands (Japan). Macroids form extensive beds on mobile sandy and gravelly bioclastic carbonate sediments at water depths of 75–97 m. SEM and Raman analyses show that Shimenawa okuyamae is characterized by a porcelaneous texture with randomly oriented, needle-shaped calcite crystals, whereas Yorishiro morii possesses a three-layered shell composed of rhombohedral, plate-shaped, and rod-shaped calcite crystals. These shell textures have not previously been recognized or described in detail in foraminifera. Shimenawa okuyamae is tentatively assigned to the Squamulinacea based on its porcelaneous, imperforate, unilocular shell. Due to its novel shell texture among tubular foraminifera, Yorishiro morii cannot be assigned with certainty to any family or higher taxonomic rank.
Abstract Upland forest soils represent the second-largest global sink for atmospheric methane (CH 4 ), but accurate estimation of this sink is hindered by the high spatial heterogeneity of CH 4 fluxes, particularly in complex mountainous terrain. This study investigated the mechanisms underlying CH 4 uptake and emissions across slope positions in a temperate secondary forest in Kyoto, Japan, over three years (2023–2025). We integrated closed-chamber measurements, soil CH 4 concentration gradient analysis, and CH 4 oxidation potential assays to examine surface fluxes and subsurface oxidation processes across ridges, mid-slopes, valley bottoms, and riparian positions. CH 4 uptake showed pronounced spatial variation, with the uptake rates in the drier ridge and mid-slope areas approximately twice those at the valley bottom. This pattern was primarily associated with lower soil moisture at upslope positions, which facilitated CH 4 diffusion and methanotrophic activity. Although deep soil layers retained CH 4 oxidation potentials comparable to those of surface soils, CH 4 consumption per unit soil volume was greatest in the uppermost 5 cm. These findings indicate that the surface layer acts as an efficient CH 4 biofilter, creating a substrate-limited regime in which atmospheric CH 4 is largely consumed before it reaches deeper horizons. In contrast, the saturated riparian zone acted as a strong CH 4 source, with summer emissions reaching magnitudes up to approximately 100 times greater than upslope uptake. These findings demonstrate that topographically controlled soil moisture regimes govern the spatial heterogeneity of CH 4 fluxes and should be considered in watershed-scale CH 4 budget assessments.
Abstract Quaternary granitic bodies are considered to host high-temperature geothermal reservoirs containing supercritical fluids exceeding 400 °C. However, both the micro- and reservoir-scale geometry and connectivity of pore networks within granite under such extreme conditions remain poorly understood. In this study, we performed micro X-ray computed tomography (µCT) analyses on drill core samples from well WD-1a, which penetrates the Kakkonda granite in northeast Japan. To characterize the geometry and spacing of micropores, we conducted topological data analysis (TDA) using persistent homology. Using 0th-dimensional persistent homology (PH0), we quantified the geometry and spacing of pores as sets of birth and death values {( b i , d i )}. Their statistical characteristics were further analyzed using principal component analysis (PCA). Three types of pores were identified: (1) alteration-type pores formed at feldspar replacement fronts, (2) relatively large facet-type pores bounded by crystal faces, and (3) cracks occurring along grain boundaries. Integrated PH0 and PCA results revealed variations in micropore topology among samples collected across the hydrological boundary and deeper thermal conduction zone. In the weakly metamorphosed granodiorite sample from the hydrothermal convection zone, abundant alteration-type pores exhibited fine-grained textures and close spacing. The sample from near the hydrological boundary showed both alteration and facet-type pores, but had low µCT-derived connected porosity. In the deeper sample, located within the thermal conduction zone, large facet-type pores were interconnected by grain-boundary cracks, forming extensive networks. The observed differences in pore topology are broadly consistent with the permeability variations measured among samples in the laboratory, highlighting the importance of pore generation mechanisms in evaluating geothermal reservoir potential.
Abstract Decadal modulation of the Kuroshio Extension (KE) between stable and unstable states influences phytoplankton and nitrate concentrations in the KE region. Decadal variability of phytoplankton and nitrate concentrations is examined using the outputs of an eddy-resolving coupled physical–biological model. The model captures decadal variations in phytoplankton and nitrate concentrations south and north of the KE jet. During unstable states, south of the KE jet, vertical nitrate movement of the isopycnal surfaces, vertical nitrate mixing, and meridional shifts in the KE jet contribute to a large spring phytoplankton bloom. The winter MLD becomes shallower earlier than during stable states, leading to an earlier supply of high nitrate concentrations into the euphotic layer and a phytoplankton bloom occurring one month earlier. The nutricline also becomes shallower throughout the year, and the shallow nutricline maintains the high subsurface phytoplankton concentrations. During stable states, the winter MLD and nutricline become shallower north of the KE jet, leading to a large and earlier spring phytoplankton bloom. The offline analysis of the nitrate budget over the top 100 m suggests that the horizontal nitrate flux south of the KE jet during unstable states enhances phytoplankton biomass in spring. Conversely, vertical nitrate flux north of the KE jet during stable states enhances phytoplankton biomass in spring. Decadal modulation of the KE induces distinct surface and subsurface distributions of phytoplankton and nitrate on the southern and northern flanks of the KE jet.
