ブータンは，ヒマラヤ山脈東部の南側に位置し，始新世にはじまったインド亜大陸とユーラシア大陸の衝突により生じた造山帯における広域変成作用や火成活動を理解するうえで重要な地域である．国全体の地質構造については概略が明らかになりつつあるが，とくに国境沿いの北部山岳地帯や東部の地質には，依然として多くの不明点が残されている．

 国立科学博物館では，2021年度より五か年計画で分野横断型の総合研究「極限環境の科学」を実施しており，2024年8月にブータン北西部において地質および植物に関する調査を行った．この地域は寒冷かつ乾燥し，紫外線も強い，動植物にとって過酷な「極限環境」であり，高山域における植物と地質の関係を解明することを目的としている．

 本写真は，ジョモラリ山（標高7,314 m）のベースキャンプ付近（標高約4,300 m）で撮影したものである．手前に写っている植物は，キク科のボンボリトウヒレンである．淡黄色のぼんぼり状の構造は花ではなく，花を包む「苞葉（ほうよう）」と呼ばれる葉である．苞葉は，花を覆うことで温室のような空間を作り出して保護しており，寒冷な高山環境で生き抜くための工夫となっている．

（写真・解説：谷　健一郎　2024年8月17日撮影）

 Four epochal stages of faunal change in the evolutionary history of fishes are recognized. The first epochal stage is recognized in the middle Cambrian fossil record as the small benthic dweller, Millokunmingia. Hagfishes and lampreys, also benthic dwellers, diverged in ca. 520-505 Ma, (near the end of the Cambrian) according to a molecular analysis. The second epochal stage saw ostracoderms (arandaspids, astraspids, and Eriptychius as agnathans) and gnathostomes (some “chondrichthyan-like scales”), characterized by exoskeletons, emerging in the Ordovician; ostracoderms (heterostracans, therodonts, osteostracans, and galeaspids) and gnathostomes (placoderms, osteichthyans, and chondrichthyans) diversified greatly in the Silurian and Devonian. The third epochal stage, after the extinction of all ostracoderms and placoderms, saw actinopterygians reduced in size, durophagous dipnoans, and durophagous holocephalans in the Carboniferous and Permian. The fourth epochal stage has seen new forms of both osteichthyans and chondrichthyans being found in all waters globally, from Paleozoic types since the beginning of the Triassic. Generally, the diversification of fishes reached the maximum level during the Silurian and Devonian of the second epochal stage with five classes (Cyclostomi, Ostracodermi, Placodermi, Osteichthyes, and Chondrichthyes); fish fauna have become restricted since the Carboniferous and are represented by only three classes (Cyclostomi, Osteichthyes and Chondrichthyes).

 The Izu Islands are located south of the Nankai Trough and the Sagami Trough, and are at a different location from mainland Japan with respect to these plate boundaries. Thus, the tsunami history of these islands is important for earthquake and tsunami research along these plate boundaries. Hachijo Island is the only island in the area with a rich historical record. Hachijo Island is known to have been affected by three large tsunamis during the Edo period: the 1605 Keicho, the 1677 Enpo, and the 1703 Genroku events. Runup heights are estimated to have been approximately 8-10 m for the 1677 Enpo tsunami and > 10 m for the 1703 Genroku tsunami. However, the runup height for the 1605 Keicho tsunami is controversial: some studies estimate it to be about 8-10 m, while others suggest it to be between 10 and 20 m. The former interpretation is based on the historic text “Kaisho” in the archive “Hachijo Jikki.” However, it is revealed that the description in this text is most likely unreliable. The runup height referred to in “Hachijo Jikki” was obtained from “Hachijo Nendaiki.” Reference to the original “Hachijo Jikki” indicates that the original description of the tsunami-affected area was probably mistakenly copied in “Hachijo Jikki.” The Kanji character meaning “not” looks similar to that meaning “under,” and it is very likely that the description was altered from “the whole village was damaged” to “the area below the village was damaged.” This part of “Kaisho” in “Hachijo Jikki” is the key source of information for previous research that stated the runup height to be less than 10 m. When the description “the whole village was damaged” is adopted, the runup height needs to be at least 10 m. This interpretation is supported by the geographical characteristics of Hachijo Island, which possesses a sea cliff with an elevation of about 5 m. Furthermore, it is known that the coastal area up to an elevation of about 10 m is frequently damaged by typhoons to the extent that boulders are often washed up. This suggests that land below 10 m in elevation is not suitable for daily settlement, so it is unlikely that there was a village at such a low elevation. From this it is concluded that the runup height caused by the 1605 Keicho tsunami was more than 10 m at west coast of Hachijo Island. This reassessment of runup height is important for understanding the tsunami source of the 1605 Keicho event.

 The stratigraphic relationship of Kanayama limestone which is approximately 20 m thick and is narrowly distributed 700 m long in the strike direction within the Akazawa Formation of the Cambrian Hitachi volcano-plutonic complex, is examined. The complex including the Akazawa Formation was metamorphosed under conditions with greenschist transitioning to amphibolite facies. Five lithologic units are recognized based on the lithology, grain size, and sedimentary structure of the Kanayama limestone exposed along the first ridge trail. Because all units are mainly composed of very fine- to fine-grained calcite detrital grains with well-developed parallel- and wave-ripple lamination, Kanayama limestone appears to have been deposited as calcarenite under a shallow-water environment dominated by waves and water currents. It unconformably overlies the Akazawa Formation and seems to be in fault contact with the Akazawa Formation above the uppermost unit. Upper Devonian to Carboniferous crinoid columnal fossils (Cyclocion sp.) are scattered from the uppermost unit. U–Pb radiometric ages of detrital zircons ranging from 545 to 338 Ma, with the youngest at 338 Ma from a zircon grain margin, suggest that the Kanayama limestone is probably early Carboniferous and later in age. The two age assignments support that the proposition that Kanayama limestone was deposited during the Carboniferous. On the other hand, zircon ages of 1480-395 Ma obtained from sandy schist intercalated in limestone of the Carboniferous Daioin Formation indicate that the formation has a different provenance from Kanayama limestone. This difference suggests that each limestone, in spite of its current proximity, was initially deposited as a different formation.

 At Kujukuri Beach in Chiba, Japan, the Holocene series unconformably overlies the Pleistocene Kazusa Group, which was deposited in a deep-sea environment; notably, the strata contain natural gas dissolved in water. The region is known particularly for gas seeps (from underground plains and coasts) observed in paddy fields, rivers, and beach zones in the area. Off Kujukuri Beach, gas bubbles emerging from the seafloor are observed on the sea surface; gas plumes can be confirmed using a fish-finder echo sounder. The distribution of gas seeps across the coastal and offshore Kujukuri is investigated. In the southern region of Kujukuri Beach, numerous gas seeps are observed; these seeps are not distributed regularly, but appear sporadically. Their distribution pattern extends from north-northeast to south-southwest, and north-to-south. Offshore Kujukuri, there are two gas-plume concentration zones that extend northeastward from the shore to offshore areas. The southern gas-plume concentration zone is influenced by the strike and faulting of the Otadai Formation of the Kazusa Group. The central gas-plume concentration zone is influenced by the strike of the Umegase Formation of the Kazusa Group and the morphology of a buried valley formed during the postglacial age. A few notable effects of gas seepage in the region are observed. High concentrations of ammonia nitrogen (derived from natural gas brine discharged into the Kujukuri coast) are observed in the study area. Furthermore, high concentrations of phosphate ions are detected in groundwater at gas-seep locations on the beach. The release of PO₄³⁻-P from sediments under strongly reducing conditions induced by gas seepage may contribute to biological productivity. Satellite images reveal high chlorophyll-a concentrations on Offshore Kujukuri.

 Predicting radiocaesium activity concentrations in standing trees over a period of decades or more in the radioactively contaminated mountain forests of Fukushima is one of the most important topics for resuming the use of forestry products after the TEPCO Fukushima Daiichi Nuclear Power Plant accident. The transport processes of ¹³⁷Cs within Japanese cedar (Cryptomeria japonica) based on the detailed distribution of ¹³⁷Cs from the outermost part to the center of the tree (outer bark, inner bark, sapwood, and heartwood) are discussed. In a cedar plantation in the Abukuma Mountains of Fukushima Prefecture, located approximately 14 km south-west from the TEPCO Fukushima Daiichi Nuclear Power Plant, three standing trees were felled in September 2017, and bark and wood samples were collected at cross-section intervals of approximately 1 cm. Besides, bark and wood samples were collected from living trees using an increment borer in the same forest stand every summer from 2015 to 2019. The transport process of ¹³⁷Cs within the Japanese cedar is considered to have been influenced by the chemical form of ¹³⁷Cs deposited on the bark surface and by the suppressive effects of potassium within the tree. Specifically, immediately after the accident, ¹³⁷Cs deposited on the outermost layer of the outer bark migrated into the tree trunk through passive diffusion, with insoluble components remaining on the outermost outer bark. Subsequently, as root uptake of ¹³⁷Cs began, the transport of ¹³⁷Cs progressed from the inner bark towards the outer bark, while simultaneously being transported into the heartwood. As the ¹³⁷Cs activity concentration in the heartwood increased, it is assumed that it eventually competed with potassium stored in the heartwood, resulting in a suppression of the further transport of ¹³⁷Cs into the heartwood, causing a plateau in its activity concentration. In order to clarify the timing of the transition in the transport process within the tree body, long-term transport experiments over more than ten years using stable caesium are required.

 On the Nobi Plain, all currently recognized riverine dunes are limited to the Kiso River and its former distributary, the Saya River; no riverine dunes have been reported along the Nagara and Ibi Rivers. Grain-size analyses of sediments from alluvial fans, riverine dunes, and natural levees in the plain are conducted and compared with results for riverine dune sediments from the Nakagawa lowland and coastal dune sand from Otake beach, Kanto Plain. Sediment mineral and chemical composition are also analyzed using stereomicroscope and X-ray fluorescence to estimate the provenance of small elevations at Nakasu, Suka, and Suwaki along the Nagara and Ibi Rivers. The small elevations are considered to be riverine dunes because their sediments are distinguishable from those of coastal dunes and natural levees in terms of mean grain size and skewness. Combined analyses of grain size, mineral composition, and chemical composition suggest that Suwaki dune sediments are a mixture of sediments from the Kiso, Ibi, and Nagara Rivers, and that the Suka dunes are mainly composed of sediments from the Nagara and Ibi Rivers. Nakasu dune sediments located near the Ibi River are similar to Neo River sediments, suggesting a change in flow path after dune formation. Combined analyses of grain size, mineral composition, and chemical composition of riverine dune sediments are, in short, useful for estimating their provenance and river channel avulsion histories. Although typical riverine dunes rarely develop in Japan, finding small riverine dunes and applying the above-mentioned analyses can be used to evaluate channel stability and sediment transport systems on alluvial plains.

 Even in the digital era of the mid-2020s, bathymetry using actual soundings has left areas largely unmapped, especially in open waters. Recent advances in Autonomous Underwater Vehicles (AUVs) could change this situation, especially in the case of vehicles capable of operating down to a water depth of 8000 m. Among the various potential targets for bathymetric surveys with AUVs, fracture zone walls, intra-plate volcanism, internal structures of leaky fracture zones, slopes of trenches, and ocean floor crater chronology are regarded to be especially valuable survey targets for ultra-high resolution morphological studies.