16:20 〜 16:35
[MIS03-05] Implication of a black fine thin layer in the sediment cores from Lake Tazawa, Akita Prefecture, Northeast Japan
キーワード：田沢湖、秋田駒ヶ岳、XRFコアスキャナ、湖底堆積物、完新世
Lake Tazawa that is caldera lake formed by the activity of Tazawako volcano around 1.7 Ma (Kano et al., 2020) is located at eastern part of Akita Prefecture, Northeast Japan. Four sediment cores were obtained from Lake Tazawa in 2015. Based on the data determined by 14C radiocarbon dating for 26 plant fragment samples from 3 cores, those cores record environmental information from 7,000 years ago to present (Matsuoka, 2016). Akita-Komagatake volcano is also located near Lake Tazawa (12.5 km east-northeast). Akita-Komagatake volcano is a volcano composed of basaltic volcanic rocks. The tephra delivered from Akita-Komagatake volcano was mainly observed eastern part of Akita-Komagatake volcano due to the wind from west to east (Wachi et al., 1997). The tephra has not been observed in the sediment from Lake Tazawa that is present west of Akita-Komagatake volcano as of now. However, a black thin layer in the sediment cores from Lake Tazawa was identified using XRF core scanner. This research examines the implication of presence of the thin black layer.
Sediments of TZW15-3 core (276 cm long, obtained at the center of the lake) and 15-4 core (336 cm long, obtained at the southwest part of the lake) consist mainly of dark grayish siltstone. The sediments also intercalate thin sandy silt layers, diatom layers and 2 tephra layers from Towada volcano (Matsuoka, 2016). Most sandy silt layers which are consist of quartz, plagioclase and volcanic grass can be identified easily by the observation with the naked eyes. However, identification of the thin layer consisting of black fine sand was difficult by the naked eyes. Based on the determination using XRF core scanner, a peak showing high Ca intensity was observed at the position of the black thin layer in the sediment cores of TZW15-3 and 15-4. The thickness of the thin layer is 0.8 to 1.0 mm. The grain size and roundness of the black fine sand are 0.05 to 0.3 mm in diameter (fine sand) and angular, respectively. Observation under microscope shows that grains of the black fine sand consist of euhedral plagioclase showing lath-like texture and groundmass. The texture is similar to the that of basaltic rock. Ratio of intensities of Ca/K which is obtained by XRF core scanner were examined on the black thin layer and other sand silt layers. The black thin layer shows significantly higher Ca/K ratio compared to typical sandy silt layers and tephra layers. Quantitative analysis determined by ordinary XRF also shows that the silt sample containing the black thin layer has higher Ca/K ratio compared to other samples (Ishiguro, 2020). Based on the results shown above, there is a possibility that the black thin layer consists of grains having basaltic composition.
Three different basaltic rocks which are located around Lake Tazawa are thought to be a source of the black thin layer. They are the basalt in Shiritaka Formation in Neogene strata, Takabachiyama basalt (1.7 Ma) (Suto, 1987) and basalt of Akita-Komagatake volcano. The 14C radiocarbon dating data (Matsuoka, 2016) and the stratigraphy of sediment cores indicate that the black thin layer was deposited around 1870 cal BP to 1630 cal BP. The main eruption activity of Akita-Komagatake volcano was started 13,000 years ago and intermittently continue to 1971 based on the data determined by 14C radiocarbon dating (Wachi et al., 1997, Fujinawa et al., 2004, Kudo and Hoshizumi, 2006). Regarding the source of the black thin layer in the sediment cores, the possibilities of the basalt in Shiritaka Formation and Takabachiyama basalt were excluded due to older age compared with the estimated age of the black thin layer. Based on the records of eruption of Akita-Komagatake volcano (Kudo and Hoshizumi, 2006), age of deposition of the black thin layer in the sediments of Lake Tazawa is similar to the eruption record of 2.2 to 1.9 ka (2200 BP to 1900 BP), named AK-2 of Akita-Komagatake volcano.
Sediments of TZW15-3 core (276 cm long, obtained at the center of the lake) and 15-4 core (336 cm long, obtained at the southwest part of the lake) consist mainly of dark grayish siltstone. The sediments also intercalate thin sandy silt layers, diatom layers and 2 tephra layers from Towada volcano (Matsuoka, 2016). Most sandy silt layers which are consist of quartz, plagioclase and volcanic grass can be identified easily by the observation with the naked eyes. However, identification of the thin layer consisting of black fine sand was difficult by the naked eyes. Based on the determination using XRF core scanner, a peak showing high Ca intensity was observed at the position of the black thin layer in the sediment cores of TZW15-3 and 15-4. The thickness of the thin layer is 0.8 to 1.0 mm. The grain size and roundness of the black fine sand are 0.05 to 0.3 mm in diameter (fine sand) and angular, respectively. Observation under microscope shows that grains of the black fine sand consist of euhedral plagioclase showing lath-like texture and groundmass. The texture is similar to the that of basaltic rock. Ratio of intensities of Ca/K which is obtained by XRF core scanner were examined on the black thin layer and other sand silt layers. The black thin layer shows significantly higher Ca/K ratio compared to typical sandy silt layers and tephra layers. Quantitative analysis determined by ordinary XRF also shows that the silt sample containing the black thin layer has higher Ca/K ratio compared to other samples (Ishiguro, 2020). Based on the results shown above, there is a possibility that the black thin layer consists of grains having basaltic composition.
Three different basaltic rocks which are located around Lake Tazawa are thought to be a source of the black thin layer. They are the basalt in Shiritaka Formation in Neogene strata, Takabachiyama basalt (1.7 Ma) (Suto, 1987) and basalt of Akita-Komagatake volcano. The 14C radiocarbon dating data (Matsuoka, 2016) and the stratigraphy of sediment cores indicate that the black thin layer was deposited around 1870 cal BP to 1630 cal BP. The main eruption activity of Akita-Komagatake volcano was started 13,000 years ago and intermittently continue to 1971 based on the data determined by 14C radiocarbon dating (Wachi et al., 1997, Fujinawa et al., 2004, Kudo and Hoshizumi, 2006). Regarding the source of the black thin layer in the sediment cores, the possibilities of the basalt in Shiritaka Formation and Takabachiyama basalt were excluded due to older age compared with the estimated age of the black thin layer. Based on the records of eruption of Akita-Komagatake volcano (Kudo and Hoshizumi, 2006), age of deposition of the black thin layer in the sediments of Lake Tazawa is similar to the eruption record of 2.2 to 1.9 ka (2200 BP to 1900 BP), named AK-2 of Akita-Komagatake volcano.