Abstract The polar region is sensitive to climate change, with concerns about the effects on ecosystems and human society. In particular, the thawing of permafrost associated with rising temperatures accelerates the microbial decomposition of organic carbon in the soil, leading to greenhouse gas emissions. Thermokarst is a landform process formed by thawing ice-rich permafrost and subsidence of the ground surface. This landform is an indicator of permafrost degradation; thus, evaluating the distribution of thermokarst is essential for understanding the impact on Arctic regions. Although assessing the thermokarst has been a labor-intensive task because of its widespread occurrence in the Arctic, automatic detection using deep learning and remote sensing techniques has been applied. However, the cost of creating training data for the specific area was challenging because thermokarst size and shape varied by region. Here, we classified thermokarst from satellite images using a recently developed method, the chopped picture method, which is suitable for identifying ambiguous and amorphous objects such as the thermokarst. This study uses high-resolution panchromatic and pan-sharpened images in eastern Siberia to evaluate the effects of differences in satellite images on classification accuracy. The training and test images were divided into 60 pixels in height and width, and each cell was classified into two categories: thermokarst topography or others. In addition, we used Global Map Data to calculate the percentage of thermokarst topography for each slope orientation (south or north) to identify the environmental conditions that facilitate the development of this topography. Results showed that our approach could clearly and automatically distinguish developed thermokarst from other landforms such as forests, lakes, and urban. Classification of thermokarst topography in panchromatic and pan-sharpened images indicated that automatic detection was possible in both images. Additionally, thermokarst topography was distributed on south-oriented slopes rather than north. This method will achieve low-cost automatic detection of thermokarst through the use of satellite data and AI. With the increase of small satellites, opportunities to utilize satellite images for observations in Arctic research will expand. Our approach will contribute to environmental monitoring in the Arctic by enabling the automatic mapping of thermokarst.

Abstract Evaluating the hydraulic properties of fractured rock masses in the subsurface is crucial for geoengineering applications and mitigating geohazards. Understanding these properties around faults is particularly important due to their impact on fluid behavior. Various methods have been developed to evaluate formation permeability, including in situ tests on boreholes, laboratory tests on rock samples, and evaluations based on borehole logging data. This paper reviews previous studies on the permeability of fractured rock and classifies these studies into three approaches: in situ permeability tests conducted using boreholes, laboratory permeability tests performed on samples, and hydraulic property evaluations based on well logging data. It also presents research focused on analytical evaluations of fault permeability, with particular emphasis on fracture geometry. Additionally, permeability values obtained by different methods at identical well sites are compared, and the challenges involved are discussed. Results show that permeability estimates differ significantly among methods, reflecting both methodological characteristics and the influence of mesoscale fractures on the effective evaluation scale. These findings highlight the strong scale dependence of hydraulic properties in fault zones and underscore the need to select evaluation methods appropriate to the target scale and purpose. Furthermore, where technically feasible, combining multiple approaches provides a more robust framework for characterizing permeability in fractured rock masses, particularly in fault zones containing fractures of various sizes.

Abstract Stable isotope analysis is useful in elucidating the flow of elements among organisms and environments. Understanding the movement of organisms is important for the conservation of ecosystems, and radiogenic strontium stable isotope ratios (⁸⁷Sr/⁸⁶Sr) are used as a tool to elucidate the relationship between fish and their habitat water and to investigate the movements and habitat ranges of fish. In this study, we compared ⁸⁷Sr/⁸⁶Sr ratios of whole otoliths of sticklebacks with those of ambient water to examine the habitat range of freshwater-resident ecotype of three-spined sticklebacks in the estuary of Otsuchi town, Iwate Prefecture, Japan. Using cluster analysis, we classified the sites in the Otsuchi water areas into four freshwater strontium isotope groups (SIGs) with non-overlapping ⁸⁷Sr/⁸⁶Sr ratios except for the tidal sites, which were identified based on ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations. Another cluster consisting only of the tidal sites and seawater was formed, distinct from that of freshwater. When ⁸⁷Sr/⁸⁶Sr ratio of the incoming freshwater was close to that of the tidal cluster, the influence of the tides on otolith classification was small. We calculated the range of ⁸⁷Sr/⁸⁶Sr ratios for each SIG from water ⁸⁷Sr/⁸⁶Sr ratios and compared otolith ⁸⁷Sr/⁸⁶Sr ratios with those ranges. All 17 sticklebacks in one tributary were classified into the same SIG as that of the water at the sampling site, suggesting a narrow habitat range, whereas six out of 24 or one out of 38 sticklebacks in two other habitats were classified into different SIGs from those of the water at the sampling sites, suggesting movement from other habitat groups. The remaining habitat, which was a tributary include the tidal zone, had a point of change in classification results between the upper (all 40 sticklebacks were classified into the same group as that of the water of sampling sties) and lower reaches (sticklebacks were classified into multiple SIGs), suggesting the presence of a movement barrier. Our study showed that dividing study area into SIGs based on water ⁸⁷Sr/⁸⁶Sr ratios and comparing ⁸⁷Sr/⁸⁶Sr ratios of otoliths with those of SIGs were useful to elucidate the fish habitat use.

Abstract The Nankai Trough, offshore of Southwest Japan, represents a plate subduction zone where a long history of devastating mega-earthquake and tsunami events has been best documented. Because of this societal relevance, the area has been extensively studied and monitored for several decades. Among the Nankai Trough region, the Eastern Nankai Trough and the Enshu Forearc Basin remain a focal point for ongoing geological and geophysical research due to their complex morpho-structural framework compared with Central and Western Nankai Troughs. In this study, twenty-five migrated Multi-Channel Seismic (MCS) profiles parallel to the subduction direction and combined bathymetric data were used to investigate the morpho-structural framework of the outer accretionary prism of the Eastern Nankai Trough. Our results show three domains (Domain 1 ~ 3) of the accretionary prism from west to east of the study area. Domain 1 is characterized by the development of an outer accretionary prism separated by a boundary thrust into an upper prism and lower prism, named Outer Accretionary Prism Boundary Thrust (OPBT). This domain also shows close similarity with the Central Nankai Trough region morpho-structural framework, where the Tokai Thrust is an equivalent of the megasplay fault. Domain 2 shows that a large part of the outer accretionary prism to the Kumano Basin edge has been deformed and eroded. Piggy-back style upper slope basins are recognized between the OPBT and the Tokai Thrust. Domain 3 represents the development of backthrusts seaward of the upper slope basin. The overall morpho-structural framework of the Eastern Nankai Trough, characterized by frontal thrust, OPBT, backthrust system, slope basins, Tokai Thrust, and forearc basin deformation caused by the Kodaiba Fault, could be best explained by subduction of a trough-parallel oceanic basement high complex (OBHC), possibly with some seamounts similar to the present-day Zenisu Ridge. We therefore confirm that the previous proposal of the subduction of the Paleo-Zenisu Ridge, which took place since around 1 Ma, is likely the most reasonable scenario for the morpho-structural evolution of this region.

Abstract Symbiotic relationships of microorganisms including eukaryotic algae and prokaryotes affect holobiont nitrogen metabolism and provide survival advantages in extreme environments; however, the influences of different symbionts on nitrogen metabolism in host organisms remain unclear. By tracing the nitrogen isotopic composition (δ 15 N) of amino acids (AAs), it is possible to constrain the biosynthetic sources and trophic interactions. We targeted reef-dwelling large benthic foraminifers hosting species-specific symbiotic algae to identify the trophic positions of multiple species with distinct feeding strategies. We measured δ 15 N in AAs of bulk organic matter and of foraminiferal shells and compared the feeding strategy of each species with its estimated trophic position (TP). Estimated TPs based on δ 15 N in AAs were inconsistent with respect to species-specific feeding strategies. In particular, Amphisorus kudakajimensis , which depends on heterotrophic feeding, showed light δ 15 N Glu and δ 15 N Phe and TP of ~ 1. A shared nutrient source for both host and symbionts and an interactive supply of trophic compounds may explain the low trophic position. The mixotrophic or heterotrophic TPs of Calcarinidae hosting endosymbiotic diatom with limited feeding (TP = 1.5–2.3) may be affected by bacterial heterotrophic processes. Differences in symbiotic algae (zooxanthellae and diatoms) may have influenced host’s nutritional strategies through functional variations in nitrogen metabolism. Large variations in δ 15 N Glu and δ 15 N Phe within single species suggested that large benthic foraminifers exploit multiple sources of nitrogen. Additional culture studies using different nitrogen sources could provide insights into detailed nitrogen metabolism of organisms with endosymbionts and role of associated organisms.

Abstract Salt marshes, which sustain high productivity, serve as essential living environments for a wide range of organisms. The aquatic food web in salt marshes is basically supported by the high primary production of aquatic algae within the intertidal zone and the organic matter contributed from the terrestrial ecosystems, including the adjacent supratidal zone. As salt marshes facilitate the incorporation of terrestrially fixed energy into estuarine and marine food webs, they represent a key environment for understanding energy transfer from land to marine ecosystems. However, the energy contribution from terrestrial ecosystems to salt marsh food webs remains unquantified. Stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) recorded in organisms have been widely used for identifying dietary carbon and nitrogen sources in food webs and for estimating trophic positions (TPs). Compound-specific isotope analysis (CSIA) of nitrogen within amino acids (AAs) provides a highly precise approach for identifying the dietary nitrogen sources and TPs. The δ 15 N values of phenylalanine (δ 15 N Phe ) in organisms reflect those in their diets, while differences in the δ 15 N values between glutamic acid (δ 15 N Glu ) and phenylalanine (Δ 15 N Glu-Phe ) reflect the TP of the organism (TP Glu/Phe ). Since the conversion from Δ 15 N Glu-Phe to TP Glu/Phe varies depending on the contribution of nitrogen from vascular plants, comparing the ecologically estimated TP value (TP est ), assigned based on feeding ecology, with TP Glu/Phe allows for the back-calculation of the dietary nitrogen contribution from terrestrial ecosystems sustained by vascular plants. In this study, we analyzed δ 13 C and δ 15 N, in particular, conducted CSIA of nitrogen within AAs in animals inhabiting the salt marsh of the Obitsu River estuary located in Tokyo Bay. The results showed that bulk δ 13 C ranged from − 25.1 to − 13.9‰ and δ 15 N Phe from 3.8 to 10.2‰, indicating that both aquatic algae and terrestrial vascular plants serve as ultimate dietary carbon and nitrogen sources. Furthermore, the comparison between TP Glu/Phe and TP est clearly demonstrated a substantial nitrogen contribution from terrestrial ecosystems to various salt marsh animals, and the overall dietary nitrogen contribution was estimated to be at least 30%. This study demonstrates the advantages of CSIA of nitrogen within AAs in elucidating the complex salt marsh food webs.

Abstract The Arctic's rapid transformation due to climate change significantly impacts Northern Eurasia. Eastern Siberia experienced increased summer precipitation and permafrost thaw in the mid-2000s, leading to wetter surfaces and higher river runoff. Furthermore, Arctic warming is linked to winter cooling in Eurasia, indicating a major disruption in the interconnected Arctic Ocean–atmosphere–vegetation–permafrost–river system. Research on these changes in Northern Eurasia focuses on the water cycle, particularly summer rainfall and winter snowfall, which are crucial for water resources and climate feedback. Japanese research institutions have played a vital role since the 1990s, collaborating with Russian and Mongolian counterparts through projects, integrating field observations, remote sensing, and modeling. Understanding changes in Eurasian precipitation and atmospheric water vapor transport is crucial for assessing the impacts of Arctic climate change, particularly considering westerly, poleward, and southward transport. Summer precipitation is influenced by the recirculation of water vapor resulting from repeated cycles of precipitation and evapotranspiration over land areas, and potentially "Siberian Atmospheric Rivers." Decadal atmospheric circulation shifts, possibly amplified by warming, have contributed to events like the East Siberian wet period. In winter, Arctic warming paradoxically links to both less snow cover and extreme cold snaps with heavy snowfall in Eurasia due to increased evaporation from reduced Arctic sea ice along the Eurasian side. The "Warm Arctic, Cold Eurasia" (WACE) pattern is debated, with models suggesting that it may be part of a larger atmospheric variability. Eastern Siberian boreal forests, adapted to permafrost, utilize both rainwater and meltwater within the soil active layer. The wet period of 2004–2010 significantly altered surface water dynamics, initially increasing evapotranspiration but eventually causing waterlogging and shifts in vegetation and permafrost near the surface. Major Siberian rivers significantly contribute to the Arctic Ocean's freshwater inflow. Satellite data revealed an increase in terrestrial water storage in the Lena River basin during the wet period. These changes, along with permafrost dynamics, directly influence river runoff, with the wet conditions leading to summer flood peaks. Future research should consider multi-scale interactions, long-term climate change, and feedback processes to understand these complex and interconnected environmental changes in Northern Eurasia.

Abstract Serpentinization-associated geofluid systems are thought to facilitate the abiotic synthesis of organic compounds, which may have played a key role in the prebiotic chemical evolution on the early Earth. The Mariana convergent margin hosts one of the serpentinization-associated systems, which has evolved into serpentinite mud volcanoes at the forearc region. Previous investigations have revealed abundant hydrocarbons and other organic compounds in the Mariana geofluid systems, but their origin, whether abiogenic or not, remains controversial owing to the potential input of organic-rich materials from the subducting slab. In this study, we collected serpentinite mud samples from two Mariana mud volcanoes, the South-Chamorro and Asùt Tesoru seamounts, using a deep-sea boring machine system, with the deepest core from 33.7 m below the seafloor. Consistent with prior scientific-drilling-based observations from the Ocean Drilling Program (ODP) Expedition 195 and the International Ocean Discovery Program (IODP) Expeditions 366, the high pH and negligible abundance of Mg coupled with patterns of major ions species in the deep parts of cores indicate that the upwelling pristine fluids were recovered successfully. To elucidate the origin of organics in the muds, the doubly-substituted (clumped) isotopologue of ethane ( 13 C 2 H 6 ) and conventional isotopic compositions of various hydrocarbons were analyzed. In addition, the characteristic patterns of long-chain alkanes, fatty acids, and amino acids were examined to assess the origin of organic compounds other than ethane. The clumped isotopologue signatures of ethane indicated the biotic source with thermal diagenesis. Similar signatures, indicating the thermal maturation of biotic organic matter, are further observed in the polyphasic analyzes: the high concentrations of ammonium and the predominance of even-numbered carbon chains in both aliphatic hydrocarbons with the unresolved complex mixture and long-chain fatty acids. These results indicate that a major part of the indigenous organic compounds in the Mariana serpentinite mud volcanoes is derived from thermogenic alteration of subducting organic matter rather than from abiotic synthesis. Although distinguishing the source of methane, the most abundant hydrocarbon, is challenging, its thermogenic provenance seems to be very likely because all other abundant organic matter represents the biotic origin that have undergone thermal maturation. Our study shows that multiple proxies are necessary critically to distinguish organic molecule sources; otherwise, the abiotic synthesis of organic matter in the Earth’s crust may be overestimated.

Abstract Long-range transported dust across the North Pacific influences the climate by altering radiative forcing, affecting cloud properties as it acts as an ice nucleus, and impacting surface ocean biogeochemistry and carbon cycling through the supply of iron to the subarctic Pacific, a region characterized by high nutrients and low chlorophyll. Asian dust has for a long time been considered the dominant source of trans-Pacific dust; yet recent numerical simulations have revealed the frequent presence of Saharan dust in the upper troposphere over East Asia, highlighting its potential importance in North Pacific environments. However, owing to technical limitations in detecting diluted trans-Pacific Saharan dust and the lack of annual records identifying it over North America, the amount, frequency, and seasonality of Saharan dust transport across the North Pacific—as well as its climatic impact—remain unclear. To address this issue, we determined the provenance of trans-Pacific dust during 1985–1989 using scanning electron microscope–cathodoluminescence analysis of single quartz particles within an ice core from Mount Logan, southern Yukon Territory, Canada, combined with numerical simulations using the Integrated Massively Parallel Atmospheric Chemical Transport model. Ice core records showed that Asian dust is predominant during most seasons, whereas Saharan dust becomes dominant in winter. Numerical simulations showed that from spring to fall (1985–1989 averages), Asian dust accounted for 49–73% of the total deposition at Mount Logan, with notable Saharan contributions of 16–18%. In contrast, during winter, Saharan dust accounted for 44% of the deposition, primarily owing to a decrease in Asian dust deposition while Saharan dust deposition remained relatively high, thereby supporting the findings from Mount Logan records. Our results for the first time provide evidence of frequent transport of the Saharan dust across the North Pacific. Combined with the simulated vertical concentration of Saharan dust over the North Pacific, trans-Pacific Saharan dust may substantially influence high-altitude cirrus cloud formation and potentially affect North Pacific ecosystems through iron deposition. These findings indicate that Saharan dust over the North Pacific is considerably more significant than previously recognized.

Abstract Changes in the physical and biogeochemical conditions of the ocean over time can affect marine ecosystems. In this study, we use biogeochemical observational data for the past 25 years (1999–2023) to investigate ocean acidification and changes in biological production at site K2 (47˚ N, 160˚ E) in the western subarctic region of the North Pacific Ocean. During this period, satellite-derived sea surface temperatures increased at a rate of 0.056 °C yr –1 , while the surface mixed-layer salinity decreased by 0.004 yr −1 . As a result of the oceanic uptake of anthropogenic CO 2 from the atmosphere, the deseasonalized annual mean surface mixed-layer pH and saturation states of calcium carbonate minerals of calcite and aragonite decreased at rates of 0.0013 ± 0.0004, 0.007 ± 0.003, and 0.004 ± 0.002 yr −1 , respectively. These rates are consistent with those calculated for winter. Under these acidification conditions, no significant trends were observed in either the annual mean or winter concentrations of nutrients (phosphate, nitrate, and silicate), or in total alkalinity in the surface mixed layer. However, the decadal trends in nutrient concentrations show a significant increase in May and decrease in July. Net community production (NCP), which is an index of biological production, was estimated from differences in nutrient concentrations between winter and May or July. This analysis revealed significant decreasing trends in NCP from winter to May, followed by increasing trends from winter to July. The stoichiometric molar ratio of Si associated with the July NCP increase (P:N:Si = 1:15:55) is higher than the previously reported ratio (1:16:40). A significant decreasing trend in satellite-derived photosynthetically active radiation (PAR) was observed in May (0.20 ± 0.08 yr −1 ), which may be linked to reduced biological production during that month. This decrease may be offset by increased production in summer that is likely due to a shift in the timing of the diatom bloom. These findings highlight the effects of long-term changes of potential drivers of both atmospheric and deep oceanic origin on oceanic biological production.