Paleoenvironmental reconstructions using sediments from isolation basins including lakes are useful approaches to obtain evidence of sea-level and ice-sheet changes in Antarctica. However, a detailed study using multiple sediment cores of how isolation basins respond to external forcings is needed to improve the accuracy of paleoenvironmental reconstructions. We reconstructed how the water mass structure of Lake Nurume, located in Langhovde, Lützow-Holm Bay, East Antarctica, changed in response to sea-level and ice-sheet changes during the Holocene using sediments from different water depths and nearby trenches. Sedimentary observations, diatom analyses, and X-ray fluorescence core scanner analyses were conducted to reconstruct the three-dimensional characterization of water mass structure combined with the paleotopographic reconstruction using glacial isostatic adjustment modelling. Sedimentation resumed at approximately 6.6 cal kyr BP when the lake became ice-free and marine conditions, as evidenced by the presence of marine diatoms and shell fossils. At approximately 2.0 cal kyr BP, laminated structures appeared in sediment cores, accompanied by a decrease in benthic diatoms. This transition is interpreted as a response to ice-sheet readvance, whereby the ice margin approached the lake, increasing freshwater input and reducing vertical mixing. The resulting stratification caused anoxic conditions in the bottom water. Finally, at approximately 0.5 cal kyr BP, the lake became completely isolated from the sea, establishing its current water mass structure. These findings contribute to more accurate paleoenvironmental reconstructions such as sea-level and ice-sheet changes using sediments from isolation basins in Antarctica.
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.
