Izu Peninsula Research Articles

Turbo(Gastropoda: Turbinidae) Fossils from the Middle Miocene of Izu Peninsula, Central Japan, Including the Description of Three New Species

Three new species of Turbo (Turbo (Turbo) hosodai sp. nov., T. (Marmarostomo) yoshiharuyabei sp. nov. and T. (M.?) sanoi sp. nov.) are described herein from the middle Miocene Ena Limestone within the pale green tuffaceous sandstone of the Sakurada Formation, Yugashima Group in Matsuzaki-cho of the Izu Peninsula, Shizuoka Prefecture, central Japan. This suggests there was a diverse Turbo (Marmarostomo) assemblage in the tropical West Pacific Ocean after its first appearance in the Oligocene—Miocene.

Abrupt Late Holocene uplifts of the southern Izu Peninsula, central Japan: Evidence from emerged marine sessile assemblages

Evidence for abrupt coastal uplifts has been found in emerged sessile assemblages in a sea cave at the southern end of the Izu Peninsula, central Japan. We identified five sessile assemblage zones: Zones I to V, in ascending order. The uppermost zone (Zone I), located at an elevation of 2.7–3.5 m above the present-day mean sea level (amsl), is a hard massive shellcrust consisting mainly of the barnacles Chthamalus challenger and the tube worm Pomatoleios kraussii. Zone II, at 2.35–2.7 m amsl, is dominated by well-preserved individuals of C. challenger. Zone III, at 2.0–2.35 m amsl, is strongly eroded and consists mainly of C. challenger and P. kraussii. Zone IV, at 1.6–2.0 m amsl, is characterized by the co-occurrence of very fresh shells of C. challenger and P. kraussii. Zone V (the lowest zone), at 1.0–1.60 m amsl, is characterized by the co-occurrence of very fresh shells of Saccostrea kegaki and P. kraussii, and by the absence of C. challenger. Radiocarbon dating by accelerator mass spectrometry (AMS) and the presence of modern taxa in the sessile assemblages suggest that three episodes of coastal uplift have occurred in the area, during AD 570–820, AD 1000–1270, and AD 1430–1660, with magnitudes of 0.9–2.0 m, 0.3–0.8 m, and 1.9–2.2 m, respectively.

静岡県伊東市のボーリングコアから復元した6,300〜2,000BCの相対的海水準変動

静岡県伊東市のボーリングコアから復元した6,300〜2,000BCの相対的海水準変動

静岡平野・伊豆半島南部の中・後期完新世の古津波と古地震の地質学的記録

静岡平野・伊豆半島南部の中・後期完新世の古津波と古地震の地質学的記録

Anomalous change of groundwater radon concentration monitored at Nakaizu well in 2011

An anomalous increase in radon concentration was measured at the Nakaizu observatory on the Izu Peninsula prior to the 2011 Tohoku earthquake using a custom-made radon counter. Since the increase was more than three times the standard deviation of radon concentration variations over 35 years of recorded data, it is considered likely that it is a precursor to the earthquake. Following the earthquake, the radon concentration decreased exponentially to the background level. The anomalous increase is explained using a modified volatilization model containing three important aquifer parameters: the groundwater saturation ratio, the fracture surface area per unit volume, and the porosity. The modified model can also explain the radon concentration behavior prior to the 1978 Izu-Oshima-Kinkai earthquake.

Leakage of magmatic–hydrothermal volatiles from a crater bottom formed by a submarine eruption in 1989 at Teishi Knoll, Japan

A submarine eruption occurred off the Izu Peninsula of Japan on 13 July 1989, forming Teishi Knoll, which has a diameter of 450m and a height of ca. 10m above the surrounding 90–100m deep seafloor. Immediately after the eruption, intense gas release was observed from two vents in the crater. The gas bubbling gradually decreased and apparently ceased in 1990. Given that no survey has been undertaken to examine volatile release from the crater of Teishi Knoll, we collected seawater samples at three different sites from just above the crater bottom on 17 July 2012, in order to detect signs of magmatic volatile release. Seawater samples from the crater bottom have dissolved CH4 contents and δ13C values higher than those of shallower (50–100m deep) seawater samples. Total inorganic carbon contents from the bottom seawater samples are also higher, and δ13C and Δ14C values lower than those of shallower seawater samples. These data indicate the addition of minor CH4 and CO2 of hydrothermal or magmatic origin to the bottom seawater from the crater. 3He/4He ratios and total organic carbon data are also consistent with the leakage of magmatic fluids. The most prominent CH4 and CO2 anomalies were observed at the site located closest to one of the bubbling gas sites of the 1989 eruption. As such, volcanic gas emissions still continue today at extremely low levels, 23years after eruption of this monogenetic volcano. The monitoring of ultra-trace amounts of chemical components in seawater is a prospective method to monitor temporal changes in magmatic activity at such submarine volcanoes.

Indigenous genetic lineages of Fagus crenata found in the Izu Peninsula suggest that there was one of refugia for the species during the last glacial maximum

Regions believed to be refugia are known to have lineages indigenous to the regions. The Izu Peninsula, where genus Fagus existed during the last glacial period, was focused on to survey chloroplast DNA of Japanese beech (Fagus crenata Blume) that grew there. DNA sequence analysis of chloroplast genome regions of about 14 kb revealed two new haplotypes, O and E-1, in F. crenata populations in the Izu Peninsula and adjacent regions. Both haplotypes were conceivably derived from haplotype E. The result of the genetic analysis in this study in combination with the result of pollen analyses performed in the past clearly showed that refugia for F. crenata existed in the Izu Peninsula, and that F. crenata now in growth are lineages indigenous to the region.

First record of Euborlasia nigrocincta Coe, 1940 (Nemertea: Heteronemertea) from the western Pacific

The heteronemertean Euborlasia nigrocincta Coe, 1940 was previously known exclusively from the eastern Pacific. A specimen collected on the Izu Peninsula, Honshu, Japan, herein identified as E. nigrocincta, represents the first record of the species from the western Pacific, increasing the western extent of the species’ known range by more than 8300 km and indicating an amphi-Pacific distribution

Fossil tropical Reef-building coral assemblage from the Middl Miocene Yugashima Group of Izu Peninsula,central Japan

Fossil tropical Reef-building coral assemblage from the Middl Miocene Yugashima Group of Izu Peninsula,central Japan

The molluscan assemblage from Shirahama Group of Izu Peninsula

The molluscan assemblage from Shirahama Group of Izu Peninsula

Complex slab structure and arc magmatism beneath the Japanese Islands

A dense nationwide seismic network recently constructed in Japan has resulted in the production of a large amount of high-quality data that have enabled the high-resolution imaging of deep seismic structures in the Japanese subduction zone. Seismic tomography, precise locations of earthquakes, and focal mechanism research have allowed the identification of the complex structure of subducting slabs beneath Japan, revealing that the subducting Philippine Sea slab underneath southwestern Japan has an undulatory configuration down to a depth of 60–200km, and is continuous from Kanto to Kyushu without disruption or splitting, even within areas north of the Izu Peninsula. Analysis of the geometry of the Pacific and Philippine Sea slabs identified a broad contact zone beneath the Kanto Plain that causes anomalously deep interplate and intraslab earthquake activity. Seismic tomographic inversions using both teleseismic and local events provide a clear image of the deep aseismic portion of the Philippine Sea slab beneath the Japan Sea north of Chugoku and Kyushu, and beneath the East China Sea west of Kyushu down to a depth of ∼450km. Seismic tomography also allowed the identification of an inclined sheet-like seismic low-velocity zone in the mantle wedge beneath Tohoku. A recent seismic tomography work further revealed clear images of similar inclined low-velocity zones in the mantle wedge for almost all other areas of Japan. The presence of the inclined low-velocity zones in the mantle wedge across the entirety of Japan suggests that it is a common feature to all subduction zones. These low-velocity zones may correspond to the upwelling flow portion of subduction-induced convection systems. These upwelling flows reach the Moho directly beneath active volcanic areas, suggesting a link between volcanism and upwelling.

過去40万年間の関東平野の地形発達史

The Kanto Plain, the hinterland of the Tokyo metropolitan area, is the largest plain in Japan and is characterized by marked marine and fluvial terrace levels that developed during Marine oxygen Isotope Stage (MIS) 5. Late Quaternary topographical changes to the plain have been controlled by concurrent tectonic activity and glacio-eustatic sea-level changes. The shoreline at the maximum transgression of MIS 11, 9, 7, 5 and 1 is reconstructed based on the distribution of marine sediments revealed by many geologic columnar sections and marine terrace surfaces. A comparison of the magnitudes of the last five full-interglacial transgressions above shows that magnitude decreased over the long term. This is due probably to changes in the tectonic regime in the Kanto basin, from subsidence to uplift along with the northward migration of the depositional center, probably associated with changes in the motion of the Philippine Sea Plate and the collision with the Izu peninsula. The marine transgression has also been controlled by fluvial processes, especially in the north-western part of the plain because of high sediment inputs from the Tone, Ara, and Watarase rivers. Aggradation coupled with regional uplift since MIS 5.4 limited the MIS 1 marine transgression within the incised valley formed during MIS 2. As a result, the Paleo Tokyo bay, which was connected directly with the Pacific Ocean, disappeared. Instead, a large shallow submarine area of about 10,000km2 emerged. The northern part of the present Tokyo bay is still subsiding and large volumes of water and sediments have been concentrated in the bay area during the Holocene. Such natural environmental conditions enable supplies of natural resources, such as fresh water, fertile soil, and flat land for the development of greater Tokyo.

地震波に対する被圧地下水の応答

Frequency response of water level fluctuation to seismic wave was determined at a confined water well in the Izu Peninsula, Japan. The purpose of this study is to detect the enhancement of hydraulic conductivity of the aquifer in this area that was suggested associated with nearby seismic swarm activity due to removal of deposits in the aquifer caused by strong seismic motion. In the analysis, depression in the peak value of the response function of water level to seismic wave was found as the amplitude of seismic wave increases. The observed depression was explained by the theoretical estimate of the depression due to friction associated with the water movement as a turbulent flow in the pipe of the well. After correcting this dependency, temporal variation in the response function was investigated. However, expected increase of the response function associated with the seismic swarm activities was not detected. The response function showed a decreasing trend with time, although the variation did not exceed the estimated errors; this trend is interpreted by decreasing hydraulic conductivity due to formation of barrier by deposition in the aquifer.

伊豆半島下田周辺海域におけるドチザメの年齢・成長と繁殖について

伊豆半島下田周辺海域におけるドチザメの年齢・成長と繁殖について

Stable isotope signature and pigment biomarker evidence of the diet sources of Gaetice depressus (Crustacea: Eubrachyura: Varunidae) in a boulder shore ecosystem

Abstract: Gaetice depressus , a varunid crab common on intertidal boulder shores, is a potential key organism for monitoring organic matter flow through the food web. In order to elucidate its biogeochemical role, the diet source and trophic position of this crab on the boulder shore of an island off the Izu Peninsula, Japan, were estimated using three approaches: foregut content examination, stable isotope signature (δ 13 C and δ 15 N) and pigment biomarkers. The results suggest that G. depressus utilizes green macroalgae ( Ulva sp., Ulvaria sp.) as its main diet source together with red macroalgae ( Gloiopeltis complanata, Gl. furcata ). This crab also utilizes periphytic and planktonic microau-totrophs (occasionally tissues of heterotrophs) when macroalgae prove insufficient due to seasonal change. Therefore, G. depressus can be considered to be an omnivore since it consumes both autotrophs and heterotrophs, although it obtains organic matter mostly from autotrophs. Key words: diet source, foregut content,

Connecting the dots: confirmation and description of Paranamixis misakiensis Thomas, 1997 leucomorphs (Amphipoda, Leucothoidae)

Leucothoid amphipods in the genus Paranamixis Schellenberg, 1938 exhibit two morphologies: pre-transformational leucomorphs and post-transformational anamorphs. The previously unknown leucomorph stage of Paranamixis misakiensis Thomas, 1997 is described. The connection of the leucomorph lifestage to the anamorph lifestage is confirmed via collection from a sponge host in Nabeta Bay, Shimoda, Izu Peninsula, Japan and with nuclear 18S ribosomal DNA sequences.

1930年北伊豆地震で発生した伊豆市城地区の川越え地すべり

1930年北伊豆地震で発生した伊豆市城地区の川越え地すべり

伊豆半島松崎町における桜葉畑景観の成立過程

The leaves of Oshima cherry (Prunus Wilson var. speciosa Makino) are edible and used to wrap sakuramochi. Seventy percent of leaves of the Oshima cherry are produced in Matsuzaki town, Izu Peninsula. In this town, cherry trees are cultivated for harvesting their leaves. These trees are closely planted and pruned, enabling them to grow many tillers, which spread out and form the distinctive landscape in this area. In this study, we investigated the actual distribution, forming process of leaves in cherry tree fields, and change in the production process in Matsuzaki town. From the results, we assumed that the number of cherry trees planted corresponded to the amount of charcoal produced. However, the field cultivation method of cherry trees was devised only by the end of the 1960s, after the production of charcoal decreased due to an energy revolution, which resulted in a crisis in the cherry tree leaf production. It was considered that cherry tree fields were distributed throughout Matsuzaki town. However, our results show that these fields are unevenly distributed.

Evaluation of chilling sensitivity of three species of Dipterocarpaceae by experimental methods and planting trial in southern part of Izu Peninsula, Japan

Evaluation of chilling sensitivity of three species of Dipterocarpaceae by experimental methods and planting trial in southern part of Izu Peninsula, Japan

Age Determination of the Kawagodaira Volcanic Eruption in Japan by 14C Wiggle-Matching

The Kawagodaira Volcano is located on the northwest slope of Amagi Volcano on the Izu Peninsula of central Japan. The fine pumice that erupted from Kawagodaira Volcano is widely distributed in the central and western parts of the Japanese mainland. Here, we report an accurate eruption age determined by radiocarbon wiggle-matching of a Japanese cedar timber excavated from the pyroclastic deposit at the pumice flow terminal of Kawagodaira Volcano. The 14C wiggle-match estimate for the Kawagodaira Volcano eruption is 1210–1187 cal BC (95.4% confidence level).