Japan Basin Research Articles

High-frequency depositional sequence of the upper Quaternary deposits controlled by sediment supply: A case study of an inland basin in central Japan

Depositional sequences in inland basins are regulated by environmental changes, such as climate change, sediment supply, water input/output, and tectonics. Their mechanisms and driving factors are diverse in each geological setting. This study provides a short-term (millennial to ten thousand years timescales) overview of the sedimentologic evolution in an inland basin located at a high altitude and characterized by fast sedimentation rates (Suwa Basin, central Japan). We employed a multi-disciplinary approach combining sedimentology, paleopedology, and high-resolution sequence stratigraphic analysis of the upper Pleistocene and Holocene succession to clarify the driving factors of the basin's sedimentologic evolution. The depositional systems and systems tracts were divided into meandering fluvial (lowstand systems tract; 26.7–16.5 cal kyr BP), pond/marsh (early transgressive systems tract; 16.5–13.9 cal kyr BP), lacustrine (transgressive systems tract; 13.9–5.8 cal kyr BP), and delta systems (highstand systems tract; 5.8 cal kyr BP–). The boundary between the meandering fluvial and pond/marsh systems corresponds to a transgressive surface appearing as a rising in the lake level (ca. 16.5 cal kyr BP). The boundary between the lacustrine and delta systems corresponds to a maximum flooding surface of the lake (ca. 5.8 cal kyr BP). The lake transgression occurs simultaneously with a decrease in the sediment supply, as suggested by a decrease in the sedimentation rate, the dense development of paleosols, and high TOC and TN showing a lower decomposition rate. The timing coincides with the intensification of chemical weathering, plant succession, and the ascending alpine tree line in the source region during the Northern Hemisphere deglaciation. The depositional sequence in the interior basin might have been easily controlled by changes in the sediment supply due to vegetation/soil cover successions during the glacial-interglacial cycle. Our findings imply that low-frequency depositional sequences observed in such inland basins in the East Asian coastal region may have also been forced by global ice volume changes.

Layered Evolution of the Oceanic Lithosphere Beneath the Japan Basin, the Sea of Japan

AbstractThe formation of a new ocean basin provides a unique observational window into the structure and deformation of the oceanic lithosphere. This study offers a detailed 1‐D vertically polarized shear‐wave velocity model of the young Japan Basin by interpreting S receiver functions, seafloor Rayleigh wave ellipticities and phase velocities in a Bayesian trans‐dimensional framework. The models suggest a distinct discontinuity in the mid‐lithosphere. Additional analyses indicate that the upper layer is characterized by strong positive radial anisotropy, while the lower layer is more isotropic. We interpret that the upper layer was formed during the back‐arc opening at 20–15 Ma, where the olivine lattice‐preferred orientation (LPO) fabric induced by passive upwelling contributes to the positive radial anisotropy. The lower, less anisotropic layer may be solidified after the sudden cessation of the opening. Small‐scale convection (SSC) from such a major tectonic event could randomize the LPO enough to weaken radial anisotropy before cooling down the asthenosphere, leaving a radially anisotropic discontinuity in the present‐day mid‐lithosphere. The proposed layered evolution history of the lithosphere beneath the Japan Basin illuminates an important role of the small‐scale convection in modifying the structural fabrics of the asthenosphere in terms of radial anisotropy. Such an SSC‐induced radial anisotropy drop could also occur at the base of the lithosphere in regions with ongoing seafloor spreading and enhance the velocity drop of horizontally polarized shear waves. Thus, the role of SSC is non‐negligible in explaining the sharpness of the Gutenberg discontinuity detected by SS precursors beneath the oceanic lithosphere.

Application of DInSAR for monitoring slopes around a dam basin in Japan

Monitoring the behavior of slopes around the sites of hydroelectric power plants and dam reservoirs is important from the viewpoints of their stable operation and a reduction in the risk to the local community. Since their facilities are widely located at dam sites, however, it is not easy to efficiently monitor the behavior of them. SAR can be a solution to the problem. On the other hand, there are still difficulties that must be overcome when applying DInSAR as an engineering tool for monitoring in practice, i.e., phase decorrelation, the accuracy and reliability of the results, etc. In particular, the dense vegetation that often covers mountainous areas can prevent the acquisition of precise measurement results. In this paper, DInSAR using ALOS-2 data is applied around a dam basin and the results are compared with the actual ground behavior to confirm the applicability of DInSAR. It is concluded that DInSAR using ALOS-2 data could be a useful tool for monitoring the slope stability of large dam basins, even those covered with dense vegetation.

Cadmium body burden and health effects after restoration of cadmium-polluted soils in cadmium-polluted areas in the Jinzu River basin.

Itai-itai disease is caused by environmental cadmium (Cd) pollution in the Jinzu River basin in Japan. To reduce the Cd contamination of rice, soil restoration of paddy fields was carried out. We evaluated the effect of soil restoration on the health status of residents of the former Cd-polluted area. Participants were 1,030 men and 944 women who lived in the area of restoration of Cd-polluted rice paddies. First morning urine was collected and urinary Cd, β2-microglobulin (β2MG), and N-acetyl-β-D-glucosaminidase (NAG) levels were measured. Associations among age, years of residence before and after soil restoration, and urinary Cd, β2MG, and NAG levels were evaluated by multiple regression analysis. The geometric mean (interquartile range) of urinary Cd (µg/g Cr) was 1.00 (0.58-1.68) in men and 1.67 (1.02-2.91) in women. The geometric means of urinary β2MG (µg/g Cr) and NAG (U/g Cr) were 174.6 (92.6-234.2) and 1.47 (0.72-3.14) in men, and 217.6 (115.3-28.7) and 1.48 (0.73-2.96) in women, respectively. Urinary Cd, β2MG, and NAG were significantly positively correlated (p < 0.01 all). Age and duration of residence in the Cd-polluted area before soil restoration were independently associated with urinary Cd, β2MG, and NAG. Among the 916 participants who had resided in the area before the soil restoration, urinary Cd concentrations were significantly higher, thus by 1.03-fold (95% CI, 1.01-1.04) in men and 1.03-fold (95% CI, 1.01-1.05) in women, when the years of residence before soil restoration by each 5-years increment. By contrast, urinary Cd concentrations were significantly lower, thus 0.97-fold (95% CI, 0.96-0.99) lower in men and 0.97-fold (95% CI, 0.95-0.99) lower in women, by each 5-year increment of residence after soil restoration. A similar association was observed for urinary β2MG concentration, and no significant association was observed for urinary NAG levels in men or women. Cd exposure and associated renal tubular dysfunction in residents of a former Cd-polluted area were influenced by Cd exposure from the environment prior to soil restoration. Soil restoration in Cd-polluted areas reduced the Cd exposure of local residents.

Horizontal distribution of the benthic front formed at the boundary between Japan Basin Bottom Water and Yamato Basin Bottom Water in the Japan Sea and modification processes due to mixing of both Bottom Waters

日本海の底層前線は明瞭な水温差を有し，低温かつ高酸素の日本海盆底層水（Japan Basin Bottom Water：JBBW）と高温かつ低酸素の大和海盆底層水（Yamato Basin Bottom Water：YBBW）の境界域に形成されている。その一方で，YBBW の起源はJBBW であることも知られている。そこで，2015 年春季と2016 年夏季，底層前線付近において船舶観測を実施し，これら底層水の輸送と変質過程の詳細を調べた。両底層水の水塊区分は溶存酸素ーポテンシャル水温ダイアグラムを用いて判断され，日本海盆側から大和海盆内へ流入したJBBW の先端付近では，両底層水間の顕著な混合水が形成されていることがわかった。それゆえ，この混合水が大和海盆内でほぼ閉じた水平循環流によって反時計回りに輸送されながら，地殻熱流量で加熱，酸素が消費され，高温かつ低酸素で鉛直混合されたYBBW へ変質し，そして再びJBBW と接して底層前線を形成することが推測される。底層前線のYBBW側混合水域における詳細断面観測（測点間隔が約1 マイル）では，過渡的な水塊混合状態を示唆する波長約10 km の波状構造を捉えることができた。最も興味深い特徴は，水温と溶存酸素の鉛直断面分布が一致していない点である。すなわち，水温場が示す波状構造は海底に捕捉されているようにみえるものの，その底層低温部の片側半分（約5 km）に高酸素の混合水（元々はJBBW 起源），残りの半分に低酸素のYBBW が位置していた。このように，両底層水の混合による変質過程には，波長10 km程度の海底捕捉擾乱（内部波もしくは水平渦流）の関与が示唆される。

Deep Water PAH Cycling in the Japan Basin (the Sea of Japan)

A vertical pattern of fractionated polycyclic aromatic hydrocarbons (PAH) was studied in the Japan Basin in the Sea of Japan. The highest PAH concentration was found in the mesopelagic realm, possibly resulting from deep convection and/or subduction of intermediate water and its biogeochemical setting in the western Japan Basin. Using 226Ra and 228Ra as tracers revealed the PAH load in the open sea from the coastal polluted water. Dissolved PAHs (DPAH, fraction &lt; 0.5 µm) were significantly prevalent particulate PAHs (PPAH, fraction &gt; 0.5 µm) at all depths, associated with a predominance of dissolved organic carbon (DOC) over particulate organic carbon (POC). Hydrophobicity was more important for higher-molecular-weight PAHs to be distributed between particles and the solution, while the high Koc of low-molecular-weight PAHs indicated that their partitioning was driven by other factors, such as adsorbing of soot particles. PPAH and DPAH profiles differed from the POC and DOC profiles; nevertheless, a positive moderate correlation was found for DPAH and DOC for depths below the epipelagic, suggesting the similarity of the mechanisms of input of dissolved organic matter and DPAH into the deep interior of the Sea of Japan. The PAH flux calculations showed that biological pumps and overturning circulation contribute almost equally to removing PAHs from the bathypelagic waters of the Japan Basin.

Satellite-detected phytoplankton blooms in the Japan/East Sea during the past two decades: Magnitude and timing

The Japan/East Sea (JES) is known as a mid-latitude “Miniature Ocean” that features multiscale oceanic dynamical processes. Using principal component analysis (PCA), we investigate the variability of the sea surface chlorophyll-a concentration (SSC) and its bloom timing in the JES based on satellite remote sensing products spanning 1998–2019. The JES SSC exhibits strong seasonal variability and blooms twice annually. The spring bloom is induced under combined factors of increased photosynthetically active radiation (PAR), weakened wind speeds and sea ice melting, and terminated by the enhanced stratification. The fall bloom is induced by destratification and active dynamic processes (such as upwelling and front), and terminated by decreased PAR. The interannual variability of spring and fall bloom occur along the northwestern coast of the JES and in the deep Japan Basin, respectively. The positive SSC anomalies along the northwestern coast of the JES in spring is associated with more sea ice in the previous winter, weaker wind speed, and stronger stratification induced by the El Niño events. No significant relationship has been found between the fall bloom and the El Niño events. The bloom timing is controlled by the critical depth hypothesis. The initiation/termination timing of spring blooms has shifted earlier by 0.37/0.45 days, and the counterpart of fall blooms has shifted 0.49/1.28 days earlier per year. The duration and magnitude are independent with each other for spring bloom at interannual time scale. In contrast, they are positively correlated for fall bloom, because of both bloom timing and magnitude are dominated by active oceanic dynamical processes in fall.

Influence of Data Length on the Determination of Data Adjustment Parameters in Conceptual Hydrological Modeling: A Case Study Using the Xinanjiang Model

Minimum data length is vital to guarantee accuracy in hydrological analysis. In practice, it is sometimes determined by the experiences of hydrologists, leading the selection of the acceptable minimum data length to an arguable issue among hydrologists. Therefore, this study aims to investigate the impact of data length on parameter estimation and hydrological model performance, especially for data-scarce regions. Using four primary datasets from river basins in Japan and USA, subsets were generated from a 28-year dataset and used to estimate data adjustment parameters based on the aridity index approach to improve the parameter estimation. The influence of their length on hydrological analysis is evaluated using the Xinanjiang (XAJ) model; also, the effectiveness of outlier removal on the parameter estimation is checked using regression analysis. Here, we present the estimation of the most acceptable minimum data length in parameter estimation for assessing the XAJ model and the effectiveness of parameter adjustment by removing the outliers in observed datasets. The results show that between 10-year to 13-year datasets are generally sufficient for the robust estimate of the most acceptable minimum data length in the XAJ model. Moreover, removing outliers can improve parameter estimation in all study basins.

STUDY OF METHANE CONCENTRATION VARIABILITY IN THE SURFACE LAYER OF THE SEA OF JAPAN IN THE CONTEXT OF SEISMIC EVENTS (BASED ON THE RESULTS OF EXPEDITION STUDIES IN 2017–2018)

A spatial distribution of methane dissolved in sea water is a critical but poorly understood factor in the context of seismic activity. Based on the results of the RV AKADEMIK OPARIN integrated geological-geophysical expedition (September 21 – October 31, 2017), this paper deals with the regularities of methane concentration variability in the surface layer of the Sea of Japan: the average growth and the average growth period were 70 % and 10 h, respectively, after each earthquake whereas a decrease in methane concentration in the sea water was 10–30 % 2–4 h before a seismic event. A decrease in methane concentration occurs irrespectively of the depth of an earthquake. The results obtained show good agreement with the published data and gaseous-geochemical monitoring materials, thus making it possible to associate seismic-related gaseous-geochemical regime not only with gas-saturated sediments but also with the water column of the Japan Basin (Sea of Japan).

Improving Snow‐Process Modeling by Evaluating Reanalysis Vertical Temperature Profiles Using a Distributed Hydrological Model

AbstractThe use of the Japanese 55‐year Reanalysis (JRA55) vertical temperature profile (VTP) has been suggested in previous studies for estimating gridded air temperatures at high elevations in data‐scarce regions to conduct hydrological simulations. In this study, we investigate and further enhance the applicability of VTP for hydrological simulations through several experimental sets using a distributed hydrological model with improved snow physics (WEB‐DHM‐S) based on water and energy budget to simulate the hydrology of a snowfed river basin in Japan, Kurobe Basin, while simultaneously correcting the snow precipitation estimates through optimization. We examined the simulated discharge, point snow depth, and snow cover, finding that the snow processes were highly sensitive to the individual station used with VTP for extrapolating air temperature to higher elevations, with corrected snowfall estimates differing by as much as 18%. We further compared this with simulations based on linear regression‐based lapse rates derived from station observations (scarce and often distributed at low elevations). It was found that the use of VTP improved discharge simulations but deteriorated point snow depth simulations at low elevations, compared with linear regression‐based lapse rates. A hybrid of the two methods led to better discharge and point snow depth simulations with reasonable snow cover simulations. The study advances the application of snow‐process models in data‐scarce regions by enhancing our understanding about the usage of VTP.

Western End of Late Cretaceous Fore-arc Basin and Provenance in SW Japan: Detrital Zircon U–Pb Dating of Upper Cretaceous–Paleogene Sandstones in Nagasaki-Nishisonogi Peninsula, Western Kyushu

Uranium–Pb ages of detrital zircons taken from seven sandstone samples collected at what were previously called “Paleogene strata” in the Nagasaki and Nishisonogi Peninsula, northwestern Kyushu, SW Japan are measured. Depositional ages of three sandstones in the Nagasaki peninsula are revealed to be e Campanian–Maastrichtian (Late Cretaceous) or younger. Depositional ages of two sandstones in Terashima Island off the west coast of the Nishisonogi Peninsula are likewise dated as Campanian or younger. By contrast, two sandstones in western Nagasaki Peninsula are confirmed as the Ypresian–Lutetian (Eocene) or younger. These new ages are concordant with sporadically reported mollusk fossils, basically supporting the previous stratigraphic scheme, except for a distinct Upper Cretaceous strata called the Mitsuze Formation. The age spectra of detrital zircons from the four Campanian sandstones share the same pattern; i.e., ranging in the Aptian–Campanian with a peak around the Cenomanian. The Maastrichtian sandstone has grains ranging in the Albian–Maastrichtian with a peak around the Turonian, which is slightly younger than that of Campanian sandstones. Judging from the dominance of Middle Cretaceous zircon grains, the main provenances of the sandstones are probably the Ryoke/San-yo and San-in granite belts in SW Japan, where Middle to Late Cretaceous felsic igneous rocks are extensively exposed. The age spectra of detrital zircons in the Campanian–Maastrichtian sandstones are almost identical to those of the Himenoura and Mifune groups in central Kyushu, and the Izumi Group in Shikoku/western Kii peninsula. All of these units were deposited unconformably over the Ryoke granitoids. Within the elongated arc-trench system extending for over 1,300 km from western Kyushu to northern Kanto, Mitsuze Formation sandstones in the Nagasaki-Nishionogi Peninsula mark the known western end of the large-scale Upper Cretaceous fore-arc basin in Japan.

Basin effect quantitation of four deep basins in Japan to explain the gap between empirical and numerical basin‐depth effect models

AbstractTo incorporate basin effects into seismic hazard assessment of urban areas and infrastructure, basin‐depth effect models that are the dependence of basin amplification factor on basin depth have been derived via numerical and empirical approaches. However, commonly observed quantitative and qualitative differences between numerical and empirical models remain unresolved. In particular, empirical models tend to predict smaller basin amplification factors. In this study, a modified empirical approach from Choi et al. was applied to the derivation of basin‐depth effect models for four deep basins in Japan using a database consisting of 71MJ &gt; 6 (Mw &gt; 5.7) earthquakes and 13,562 records from strong‐motion seismograph networks (K‐NET and KiK‐net). The obtained basin‐depth models vary among basins and crustal‐subduction earthquake pairs, which confirms the recent trend of accounting for regional‐seismotectonic basin amplification in seismic hazard analysis. The basin amplification observed for crustal earthquakes is generally larger than that for subduction earthquakes. Moreover, the differences between the numerical and empirical models can be explained by changing the options of basin or outside‐basin stations for computing the inter‐event residuals. These results bridge a knowledge gap in seismic hazard analysis between ground‐motion prediction equations and those using large‐scale seismic wave simulations, which are now the main tools for regions lacking seismic records.

Temporal variability of plutonium in surface waters of the Sea of Japan

Long-term temporal variations of plutonium in Sea of Japan (SOJ) surface waters have been examined with the aim to better understand its behavior during several decades. The first observation is that 239,240Pu activity concentrations in surface waters of the SOJ during 1977–2019 were 6.5 ± 4.7 mBq m−3 in average, and 5.1 mBq m−3 as the median, whereas 137Cs and 90Sr activity concentrations decreased with time, except of the perturbation due to the 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Another observation is that sporadic high 239,240Pu activity concentrations occurred in the east Japan Basin, ranging from 1 to 39 mBq m−3. The spatial distribution of 239,240Pu activity concentrations in surface waters revealed that high 239,240Pu levels (>20 mBq m−3) occurred in 1994 in the northern SOJ, which was considered to be due to winter convection. To elucidate factors controlling the temporal variability of surface 239,240Pu levels in the SOJ, a relationship between surface 239,240Pu activity concentrations and vertical diffusion coefficients was examined. The results revealed that this relationship could be classified into two groups: one group did not show a change with increasing diffusion coefficient, while the other group showed a positive correlation. The vertical 239,240Pu distribution in SOJ waters suggests that the high surface 239,240Pu levels occurred due to the upwelling of cyclonic eddy. The rapid recycling of deeper plutonium occurred in the SOJ due to deep winter convection and upwelling associated with cyclonic eddy. The plutonium levels in the SOJ have been found to be sensitive to climate changes. Warming of the SOJ may cause a reduction of winter convection and eddy activity as a result of increasing sea surface temperature. This leads to a decline of recirculation of plutonium and other bioavailable elements from Japan Sea Proper Water (JSPW) to surface water layers. Plutonium would be, therefore, an important indicator of biogeochemical processes in the marine environment, helping to assess climate change impacts on marine ecological systems.

Imaging the Kanto Basin seismic basement with earthquake and noise autocorrelation functions

SUMMARYSedimentary basins can strongly amplify seismic waves from earthquakes. To better predict future ground motions, detailed knowledge of the sediment thickness and internal structure of basins is required. We image the sediment-to-bedrock interface of the Kanto Basin in Japan using the P-wave reflectivity response from earthquake and ambient seismic noise autocorrelation functions (ACFs) at 286 shallow borehole stations. Earthquake ACFs are computed using P-wave records from 50 Mw 6+ teleseismic events. Noise ACFs are obtained using 1 month of continuous data. Both methods are used to retrieve P-wave traveltimes between the surface and the bedrock interface and map the basin basement geometry. Our prediction of the basement depth agrees generally well with that from a reference velocity model, except for smoother variations in the central part of the basin. Using full-wavefield simulations, we show that the nature of the autocorrelated wavefield has a significant impact on the shape of the ACF waveforms and that earthquake ACFs yield more accurate results in the Kanto Basin.

Ground-Motion Modeling as an Image Processing Task: Introducing a Neural Network Based, Fully Data-Driven, and Nonergodic Approach

ABSTRACT We construct and examine the prototype of a deep learning-based ground-motion model (GMM) that is both fully data driven and nonergodic. We formulate ground-motion modeling as an image processing task, in which a specific type of neural network, the U-Net, relates continuous, horizontal maps of earthquake predictive parameters to sparse observations of a ground-motion intensity measure (IM). The processing of map-shaped data allows the natural incorporation of absolute earthquake source and observation site coordinates, and is, therefore, well suited to include site-, source-, and path-specific amplification effects in a nonergodic GMM. Data-driven interpolation of the IM between observation points is an inherent feature of the U-Net and requires no a priori assumptions. We evaluate our model using both a synthetic dataset and a subset of observations from the KiK-net strong motion network in the Kanto basin in Japan. We find that the U-Net model is capable of learning the magnitude–distance scaling, as well as site-, source-, and path-specific amplification effects from a strong motion dataset. The interpolation scheme is evaluated using a fivefold cross validation and is found to provide on average unbiased predictions. The magnitude–distance scaling as well as the site amplification of response spectral acceleration at a period of 1 s obtained for the Kanto basin are comparable to previous regional studies.

Estimation of baseflow recession constant and regression of low flow indices in eastern Japan

ABSTRACT We propose a convenient procedure for the matching strip method in determining the recession constant of the master recession curve using the ratio of flow over successive days (Qn+1/Qn). The method is applied to 29 basins (6.1–740 km2) in eastern Japan to develop linear regression models that estimate low flow indices through the recession constant (λ), and either mean annual precipitation or mean annual runoff. Significant models were developed for Q710 (seven-day 10-year low flow), and Q9710 (10-year low flow exceeded 97% of the time) in the case of all basins, and in particular for basins classified into sedimentary or igneous geology (adjusted R2 up to 0.784). When basins were not classified by geology, models based on mean annual runoff as the second independent variable performed better than those based on mean annual precipitation, with adjusted R2 values of 0.705 and 0.717 for the Q710 and Q9710 models, respectively.

Pn Anisotropic Tomography of Northeast Asia: New Insight Into Subduction Dynamics and Volcanism

AbstractWe present a new high‐resolution model of P‐wave anisotropic tomography of the uppermost mantle beneath NE Asia obtained by inverting 88,303 high‐quality manually picked Pn arrival times. Our model reveals strong lateral heterogeneities in both isotropic velocity and azimuthal anisotropy. Distinct high Pn velocities are visible under the basins in NE China, and eastern and western Japan basins. Very high Pn velocities with trench‐normal fast Pn anisotropy are revealed near the Kurile‐Japan trench, representing the incoming Pacific plate. Prominent low Pn velocity anomalies are revealed under active intraplate volcanoes in NE China and arc volcanoes on the Japan Islands. Very low Pn velocities with trench‐parallel fast Pn anisotropy are observed along the volcanic front in Japan, reflecting compressed mantle flows. An NW‐SE oriented low‐velocity (low‐V) zone from the Wudalianchi volcano to the Changbaishan volcano meets the E‐W oriented low‐V zone around the Changbaishan volcano and the N‐S low‐V zone east of the Sanjiang basin, which converge to a broad N‐S low‐V zone. This zone passes through the central Japan basin to the junction of the Japan and Izu‐Bonin trenches, where it meets the nearly E‐W low‐V zone from the Changle volcano. These low‐V anomalies exhibit zone‐parallel fast Pn anisotropy and may indicate channels for material and energy exchanges among the volcanoes, driven by mantle flows in the big mantle wedge. These results provide new insights into interplate and intraplate volcanism and subduction dynamics in the study region.

High-Order Depositional Sequence of the Upper Quaternary Deposits Controlled by Sediment Supply: A Case Study of an Inland Basin in Central Japan

High-Order Depositional Sequence of the Upper Quaternary Deposits Controlled by Sediment Supply: A Case Study of an Inland Basin in Central Japan

КОРРЕЛЯЦИЯ АКУСТИЧЕСКИХ ХАРАКТЕРИСТИК И ВОЗРАСТА БАЗАЛЬНЫХ ГОРИЗОНТОВ КАЙНОЗОЙСКОГО ОСАДОЧНОГО ЧЕХЛА ЯПОНСКОГО МОРЯ

Velocity sections and acoustic characteristics of the sedimentary cover of the Central (Japan), Yamato (Honshu), Tsushima basins and the Kita-Yamato trough of the Sea of Japan, as well as the rocks of the basement and Cenozoic cover of the Yamato Upland were studied. A comparative analysis of similar characteristics of the Cenozoic basins of the South China and East China marginal seas was carried out. The relationship between the acoustic properties (P-wave velocity - Vp) of the basal horizons and the age of the Cenozoic cover in the above basins were analyzed. It has been substantiated that the values of Vp = 3.0-3.6 km/s are an acoustic characteristic of Paleocene-Eocene sedimentary strata. According to the studies performed, the basal layers in the inferred depocenters of the Sea of Japan basins (Central, Yamato, Tsushima) have Vp = 3.0-3.3 km/s, which indicates the Paleocene-Eocene age of the beginning of their formation.

Utilizing environmental DNA for wide-range distributions of reproductive area of an invasive terrestrial toad in Ishikari river basin in Japan

Understanding the distribution of invasive species and their reproductive area is crucial for their managements after invasion. While catch and observation surveys are still embraced, environmental DNA (eDNA) has been increasingly utilized as an efficient tool for identifying these species in the wild. In this study, we developed a Bufo-specific eDNA assay for detecting an invasive, toxic, and terrestrial toad species Bufo japonicus formosus in Hokkaido, Japan, and applied it to their reproductive area at watershed scale. The eDNA assay was field-validated in ponds where B. japonicus were observed, as well as in rivers downstream of the reproductive ponds. Thus, the assay provided us an opportunity to screen watersheds that include their reproductive area by collecting downstream water samples. Applying it to the Ishikari river basin, the largest river basin in Hokkaido (c.a., 14,330 km2), we detected toad eDNA at 32 out of 73 sampling sites. They are composed of eleven sites with species observation records nearby (all the sites with observation records within a 500 m radius) and 21 sites without such records. And those eDNA detections were from twelve out of 31 river systems in the entire river basin. A Bayesian, multiscale occupancy model supported high eDNA detectability among those sites. These results suggest that the eDNA assay can efficiently estimate the presence of reproductive area of the terrestrial toad even from a distant downstream of the watershed, and that it provides a powerful means of detecting new reproductive area and monitoring further spread of invasive species.