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Journal Articles

$$^{10}$$Be analysis of the rock samples from the northeastern shore of Lake Pumoyum Co in south Tibetan Plateau

Nara, Fumiko*; Watanabe, Takahiro; Kokubu, Yoko; Zhu, L.*

Nuclear Instruments and Methods in Physics Research B, 539, p.28 - 32, 2023/06

 Times Cited Count:0

Lake Pumoyum Co is located on the south Tibetan Plateau. The lake terraces are developed on the eastern lake shore, and it supposed that the large lake level changes would have happened in Pumoyum Co. The in-situ terrestrial cosmogenic adionuclides can be used to estimate the earth surface processes, such as the erosion rate and exposure age dating of rocks. Here we report the results of $$^{10}$$Be values of the rock samples from the lake terraces around Pumoyum Co. The concentrations of $$^{10}$$Be were measured by the JAEA-AMS-TONO-5MV in the Tono Geoscience Center, Japan Atomic Energy Agency. The $$^{10}$$Be concentrations ranged from 3.78 to 10.8$$times$$10$$^{6}$$ (atoms/g), but the $$^{10}$$Be values showed the decreasing trend following to the distance from the lake shore. This result indicates that $$^{10}$$Be values of the rocks at the shore of Pumoyum Co could be influenced from the erosion rate or tectonic process rather than the exposure date resulting from the lake level changes.

Journal Articles

Late-Holocene salinity changes in Lake Ogawara, Pacific coast of northeast Japan, related to sea-level fall inferred from sedimentary geochemical signatures

Nara, Fumiko*; Watanabe, Takahiro; Matsunaka, Tetsuya*; Yamasaki, Shinichi*; Tsuchiya, Noriyoshi*; Seto, Koji*; Yamada, Kazuyoshi*; Yasuda, Yoshinori*

Palaeogeography, Palaeoclimatology, Palaeoecology, 592, p.110907_1 - 110907_11, 2022/04

 Times Cited Count:0 Percentile:0.03(Geography, Physical)

Radiocarbon dating, tephrochronology, and geochemical signatures such as bromine (Br), iodine (I), total sulfur (TS), total organic carbon (TOC), and total nitrogen (TN) in a continuous sediment core (OG12-2) were applied to estimate past salinity change on the Lake Ogawara, Pacific coast of northeast Japan. Since the Lake Ogawara was the inner bay of the Pacific Ocean and became the brackish lake in the late Holocene, clarifying the past salinity change on the lake Ogawara could give us important insight into the past sea level changes of the Pacific Ocean. The core OG12-2 has two tephra layers, the Towada eruption and the Changbaishan eruption (B-Tm: AD 946). The age model for the core OG12-2 was established using the radiocarbon measurements of the plant residues in the core, also was constrained by the B-Tm tephra. Depletions of marine-derived elements (Br, I, and TS), from 88.4 to 64.2 mg/kg, 20.0 to 14.1 mg/kg, and 3.1 to 1.1 mass %, respectively, were observed in the layers with 2200-2000 cal BP. These results indicate paleosalinity changes from saline (inner bay) to brackish conditions within Lake Ogawara caused by sea level fluctuations during the late Holocene.

Journal Articles

Characteristics in trace elements compositions of tephras (B-Tm and To-a) for identification tools

Nara, Fumiko*; Yokoyama, Tatsunori; Yamasaki, Shinichi*; Minami, Masayo*; Asahara, Yoshihiro*; Watanabe, Takahiro; Yamada, Kazuyoshi*; Tsuchiya, Noriyoshi*; Yasuda, Yoshinori*

Geochemical Journal, 55(3), p.117 - 133, 2021/00

 Times Cited Count:5 Percentile:61.61(Geochemistry & Geophysics)

The absolute date of the Millennium Eruption (ME) of Changbaishan Volcano is widely recognized as AD 946. The Baegdosan-Tomakomai (B-Tm) tephra dispersed during the ME is a robust-age key bed. In order to identify the tephra, refractive index and major-element compositions of volcanic glass shards are conventionally used. However, trace-element analysis has been rarely carried out, especially for rare-earth elements (REEs) and for tephra layer bulk sediments. Here we present the datasets of major- and trace-element compositions datasets for the glass shards and bulk sediments of the B-Tm and Towada caldera eruptions (To-a) tephra deposits from the Lake Ogawara sediment core, Tohoku region, northern Japan. The depth profiles of the major and trace elements show the significant peaks for the K$$_{2}$$O and some trace elements (Zn, Rb, Zr, Nb, Sn, Y, La, Ce, Nd, Th, and U) at the B-Tm tephra layer in the Lake Ogawara sediment core, but no peaks of these elements at the To-a tephra layer. High concentrations of the trace elements in the B-Tm tephra layer were observed in individual glass shards as well as in the bulk sediment. These concentrations are highlighted by the elemental abundance pattern normalized by the crustal abundance. The elemental pattern in individual glass shards from other Japanese tephras showed significant differences from those of the B-Tm tephra, especially in REEs compositions. The trace-element compositions of the glass shards and bulk sediment show strong advantages for distinguishing the B-Tm tephra from other Japanese tephras.

Journal Articles

Geochemical characteristics of paleotsunami deposits from the Shizuoka plain on the Pacific coast of middle Japan

Watanabe, Takahiro; Tsuchiya, Noriyoshi*; Kitamura, Akihisa*; Yamasaki, Shinichi*; Nara, Fumiko*

Geochemical Journal, 55(6), p.325 - 340, 2021/00

 Times Cited Count:1 Percentile:15.35(Geochemistry & Geophysics)

Geochemical signatures of natural samples could be an effective proxy to identify paleo tsunami deposits. However, less geochemical approach has been applied in the Pacific coast of middle Japan. Therefore, geochemical characteristics of the paleo tsunami deposits were studied using geological samples from the Shizuoka plain in the Pacific coast of middle Japan, and this paper shows the first geochemical dataset of paleo tsunami deposits from the area. In this study, typical geochemical biplots and ternary diagrams were applied to discriminate the paleo tsunami deposits from other layers. Additionally, vertical and horizontal distributions of titanium normalized values and statistical methods of geochemical data could provide the information for characteristic of the deposits to detect prehistorical tsunami inundation.

Journal Articles

Quantitative and semi-quantitative analyses using a portable energy dispersive X-ray fluorescence spectrometer; Geochemical applications in fault rocks, lake sediments, and event deposits

Watanabe, Takahiro; Ishii, Chikako; Ishizaka, Chika; Niwa, Masakazu; Shimada, Koji; Sawai, Yuki*; Tsuchiya, Noriyoshi*; Matsunaka, Tetsuya*; Ochiai, Shinya*; Nara, Fumiko*

Journal of Mineralogical and Petrological Sciences, 116(3), p.140 - 158, 2021/00

 Times Cited Count:3 Percentile:42.89(Mineralogy)

A portable energy dispersive X-ray fluorescence spectrometer (potable XRF) can be an effective tool for detecting chemical elements in various materials, such as geological, and environmental samples. In this study, working curves were confirmed using reference materials, such as igneous rocks and other geochemical standards, distributed by national and international organizations. Subsequently, quantification and semi-quantification analyses were performed by the portable XRF for inorganic elements in (A) fault rocks, (B) lake sediments from the middle Japan, and (C) soils with paleotsunami deposits from the Pacific coast of northeast Japan. Twenty-four elements (Mg-U) in these geological samples were measured by potable XRF using our working curves. Measured values by the portable XRF of the samples were good agreement with the reported values in almost cases.

Journal Articles

A Geochemical approach for identifying marine incursions; Implications for tsunami geology on the Pacific coast of northeast Japan

Watanabe, Takahiro; Tsuchiya, Noriyoshi*; Yamasaki, Shinichi*; Sawai, Yuki*; Hosoda, Norihiro*; Nara, Fumiko*; Nakamura, Toshio*; Komai, Takeshi*

Applied Geochemistry, 118, p.104644_1 - 104644_11, 2020/07

 Times Cited Count:9 Percentile:64.18(Geochemistry & Geophysics)

Possible tsunami inundation areas can be predicted from the distribution of paleo tsunami deposits, which are mainly composed of marine-derived sands and muds on land. Such exotic sandy and muddy layers have been identified by multiple approaches. However, there still remain uncertainties and other useful proxies need to be investigated for the identification of tsunami deposits. Here we show the geochemical signatures of modern tsunami deposits from the Pacific coast of Northeast Japan (2011 Tohoku tsunami), as well as those paleo tsunami deposits, which were taken from the Tohoku District (Jogan, ca. 1080 calibrated ages before present [cal BP] and Yayoi, ca. 2000 cal BP). A geochemical ternary diagram (Seawater-Rock-(As+Metals)) enables the weathering trend of tsunami deposits over ca. 2000 years in the Sendai Plain in the Tohoku to be shown. In the paleo tsunami layers from the Tohoku, the Na/Ti atomic ratios markedly increased to 23.4, and the average values were 19.3$$pm$$3.0 (Jogan and Yayoi tsunami deposits), which were clearly higher than those of other layers (soil deposits, 10.3$$pm$$3.5 on average). These results show that the Na/Ti ratio is a useful indicator of marine incursions in our case. Our rapid and simple method using the Na/Ti ratio can be easily applied to the Tohoku, and it may contribute to the detection of unrecorded muddy tsunami deposits.

Journal Articles

Preliminary results of $$^{10}$$Be of the rock samples from the lake terrace of the Lake Pumayum Co in Tibetan Plateau

Nara, Fumiko*; Watanabe, Takahiro; Kokubu, Yoko; Horiuchi, Kazuho*

JAEA-Conf 2018-002, p.124 - 127, 2019/02

In-situ cosmogenic radionuclides have been used to understand earth surface processes, such as the erosion rate and exposure age dating of rocks. This is a preliminary report on in-situ $$^{10}$$Be values measurements by Accelerator Mass Spectrometry (AMS) for rock samples from the shore of the Lake Pumoyum Co in Tibetan plateau. The $$^{10}$$Be concentrations ranged from 5.5-7.5$$times$$10$$^{6}$$ atom/g similar to those of the surface sediments of Lake Pumoyum Co. The calculated erosion rates were 5.2-7.6 mm/kyr, values close to those previously reported by Lal et al. (2003).

Oral presentation

The Millennium scale monsoon cycles recorded in a sediment core from alpine Tibetan lake

Watanabe, Takahiro; Nara, Fumiko*; Matsunaka, Tetsuya*; Minoura, Koji*; Kakegawa, Takeshi*; Yamasaki, Shinichi*; Tsuchiya, Noriyoshi*; Nakamura, Toshio*; Junbo, W.*; Liping, Z.*

no journal, , 

Understanding of past environmental and geochemical changes is significant issues to estimate global climate variation. Therefore, these changes have been studied by continuous terrestrial sediments. The high-time resolution past environmental changes, especially monsoon fluctuations, have been reported in previous studies using geochemical signatures in stalagmites from Asian caves. However, continuous climate records in Tibetan plateau have not yet been cleared, which a key area to understand mechanism of monsoon fluctuations. In this study, mean grain size and geochemical proxies from lake sediment cores from south Tibetan plateau were used for estimation of past monsoon changes during Holocene. Spectral analysis of the mean grain size in the Tibetan lake indicated 1000-1500 years climate cycles, which might be caused by the changes in solar activities. These monsoon fluctuations based on the mean grain size were also supported by geochemical proxies in this study.

Oral presentation

Rb/Sr ratio in Lake Baikal sediment core; The New geochemical proxy for East Asian winter monsoon strength during cool climate period

Nara, Fumiko*; Yamasaki, Shinichi*; Watanabe, Takahiro; Tsuchiya, Noriyoshi*; Miyahara, Hiroko*; Kato, Takenori*; Minoura, Koji*; Kakegawa, Takeshi*

no journal, , 

High-time resolution measurements of the major and trace inorganic elements, such as Rb and Sr, from the Lake Baikal sediment core was carried out to estimate the weathering response and the material provenance in the lake watershed. The fluctuations of the Rb/Sr ratio and mean grain size (MGS) during the Holocene were corresponding with each other. The similar profiles between the Rb/Sr ratio and the MGS from loess sediment in China have been observed in previous study. These results imply that the Rb/Sr ratio can be used as the proxy to estimate the East Asian winter monsoon intensity.

Oral presentation

Radiocarbon dating of lake sediments and tsunami deposits by AMS

Watanabe, Takahiro; Nara, Fumiko*

no journal, , 

Geochronological study of lacustrine and land sediments has been reported based on radiocarbon measurements by accelerator mass spectrometry. In particular, the studies on past climate changes and tsunami geology using the sediments have been proceeded (Watanabe et al., 2009; 2010; 2012; Nara et al., 2014). In this presentation, standard methods for radiocarbon dating of the lacustrine and land sediments are shown. These methods are applied to dating of tsunami deposits for disaster science in this study. We tried to make an age model of tsunami deposits using these methods.

Oral presentation

Rapid hydrological change during the late last glacial period in the south Siberia

Nara, Fumiko*; Watanabe, Takahiro; Kokubu, Yoko; Nakamura, Toshio*

no journal, , 

Lake Baikal sediment cores record the paleoclimatic and biological changes in the continental area corresponding with the global and local climate changes. In this study, we show the rapid changes of the sedimentation rate in Lake Baikal sediment core (VER99G12) at around 19 cal ka BP, which were recovered from the $$^{14}$$C analysis of total organic carbon. The sampling site of VER99G12 is located in front of the Selenga River, which provides the largest inflow into the lake. Therefore, the sediment core in this study was mainly composed of the deposition from the Selenga River watershed. High-time resolution $$^{14}$$C data sets from the sediment core revealed the rapid increase in sedimentation rate corresponding with the global climate warming (MWP-19Ka). The change of grain size and the K/Ti atomic ratios in the core also responded at the period as well. These results suggest the increase in the precipitation in the continental area at around 19000 years ago.

Oral presentation

Estimation of the age with the brackish lake environmental changes using continuous sediment core samples from Lake Ogawara

Nara, Fumiko*; Matsunaka, Tetsuya*; Yamasaki, Shinichi*; Tsuchiya, Noriyoshi*; Watanabe, Takahiro; Yamada, Kazuyoshi*; Yasuda, Yoshinori*

no journal, , 

Lake Ogawara, which is the brackish lake in Aomori prefecture, northern Japan, has been formed by the sea level change. Although the lake formation period is roughly estimated at the late Jomon period, the precise formation period is still unclear. The Lake Ogawara formation is directly connected with the sea level change, therefore, determination of the precise formation period of the Lake Ogawara is very important to understand the sea level history during the Jomon period. In this study, we constructed the precise age model based on the radiocarbon (C-14) for the plant residues and total organic carbon (TOC), Cs-137, Pb-210 and tephra identify in the Lake Ogawara sediment core. Two visible tephras (the Baegdosan-Tomakomai: B-Tm and Towada carudera eruption: To-a) were observed in the sediment core. Since the absolute age of the Millennium Eruption (ME) of Changbaishan volcano is widely accepted as AD 946, the B-Tm tephra in the sediment core can be used as the precice chronological key bed layer. Age profile of the trace element (Br), which was derived from the sea water, in the core showed that the Lake Ogawara started to change to the brackish lake at around 2000 cal BP.

Oral presentation

Chemical analysis of tsunami deposits; High spatial resolution measurement using scanning X-ray analytical microscope

Watanabe, Takahiro; Nara, Fumiko*; Ueki, Tadamasa*; Tsuchiya, Noriyoshi*

no journal, , 

The identification of past tsunami deposits is important task for the risk assessment of major subduction zone earthquakes. Geological investigation now requires more reliable and precise identification of tsunami deposits and reconstructions of inundation limits than before. A tsunami deposit is preliminarily identified as an exotic layer within deposits that settled out in calm environments. The exotic deposit is then characterized using multiple criteria such as the lateral extent of the layer, changes in grain size, sedimentological features, and micropaleontology. Among the criteria, geochemical analysis is used to estimate the source of the sediments. Here, we tested inorganic geochemical analyses of the paleo tsunami deposits using XGT.

Oral presentation

Trace element distributions of Lake Ogawara sediment core: Implication for the past sea level change at the late Holocene

Nara, Fumiko*; Yamasaki, Shinichi*; Watanabe, Takahiro; Tsuchiya, Noriyoshi*; Yamada, Kazuyoshi*; Yasuda, Yoshinori*

no journal, , 

Lake Ogawara is located northernmost Honshu island Japan, Tohoku region alongside Pacific Ocean. In this study, the trace elements (Br and U) in the Lake Ogawara sediment core were analyzed using XRF to reconstruct the past sea level and salinity changes during the late Holocene. The depth profiles of the trace elements (Br and U) showed the gradual decrease from 160 cm depth to the top of the core (corresponding to ca. 2,200 cal BP). These results showed that the sea level was decreased at around 2,200 cal BP.

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