5:15 PM - 6:30 PM
[HQR04-P07] Tephrostratigraphy during the past 30,000 years in the Jaishi Oike Moor, southern Izu Peninsula, Japan.
Keywords:Moor sediment, Cryptotephra, Jaishi Oike Moor
Recently, detailed tephrostratigraphic studies using lacustrine and moor sediment records have been conducted both in the Japanese Islands and overseas, providing more widespread distributions and eruption ages of tephras and contributing to studies of volcanic eruptive history (e.g., McLean et al. 2018; Albert et al. 2018, 2019). Such studies focusing on lacustrine sediment core records in the Japanese Islands are concentrated in the Kinki and Chubu regions. There are a limited number of lakes and moors suitable for the establishment of a detailed tephrostratigraphy in the Tokai and South Kanto coastal regions, where many tephras from Quaternary volcanoes in southwest Japan are expected to be distributed due to westerlies. In this study, we focused on the Jaishi Oike Moor in the southwestern part of the Izu Peninsula in order to establish the detailed tephrostratigraphy of the Jaishi Oike Moor, including cryptotephras (Lowe and Hunt 2001). We conducted particle size analysis, particle composition analysis, tephra analysis (measurement of refractive index of volcanic glass shards and amphiboles, and major element compositions of volcanic glass shards) using a JIS core (core length is 12 m) drilled in September 2020. In particular, the content of volcanic glass shards, amphiboles, and morphological classification of volcanic glass shards were used as indicators to identify the tephric particle concentration horizons.
The sediments of the JIS core are mostly composed of silt deposited under lake to moor environments. Additionally, we also identified tephra layers and coarse-grained sediments deposited by debris flows interbedded within silty sediments. Two visible tephra layers were identified based on tephra analysis: the Amagi-Kawagodaira tephra (Kg, 3,151-3,126 cal yrs BP: Tani et al. 2013) and Aira-Tn tephra (AT, 30,078±96 cal yrs BP: Smith et al. 2013). In addition, nine cryptotephras were identified from seven layers of tephric particle concentration horizons. They are a widespread tephra of the Kikai-Akahoya tephra (K-Ah, 7,307-7,196 cal yrs BP: McLean et al. 2018), the newly discovered tephra in this site are shown from the top as follows: the Niijima-Miyatsukayama Nanbu tephra (Nj-Mts, 5.5 ka: Kobayashi et al. 2020), Niijima-Shikinejima tephra (Nj-Sk, 8 ka: Kobayashi et al. 2020), one tephra that may be correlated with Daisen Sasaganaru tephra (DSs, 29,837 ± 96 cal yrs BP or 29,830 cal yrs BP: Albert et al. 2018) or Daisen Higashidaisen tephra (DHg, 28,895 ± 72 cal yrs BP or 28,888 ± 72 cal yrs BP: Albert et al. 2018), and one uncorrelated tephra from Izu Islands.
The distribution of Nj-Mts and Nj-Sk in the Honshu Island is identified for the first time. Therefore, JIS core can provide a type tephrostratigraphy in the Izu Peninsula for studying the distribution of tephras from the Izu Islands. On the other hand, the eastern end of the distribution of DSs and DHg in the previous studies is Lake Biwa (Takemura et al. 2010). Therefore, it is likely that the distribution area of either DSs or DHg is expanded to 250 km further east from Lake Biwa by this study and the main axis of them is east-southeast direction from the source volcano. In this study, we analyzed the moor sediments continuously and identified the tephric particle concentration horizons by several indicators, and showed the detailed tephrostratigraphy during the past 30,000 years in the southern part of the Izu Peninsula.
The sediments of the JIS core are mostly composed of silt deposited under lake to moor environments. Additionally, we also identified tephra layers and coarse-grained sediments deposited by debris flows interbedded within silty sediments. Two visible tephra layers were identified based on tephra analysis: the Amagi-Kawagodaira tephra (Kg, 3,151-3,126 cal yrs BP: Tani et al. 2013) and Aira-Tn tephra (AT, 30,078±96 cal yrs BP: Smith et al. 2013). In addition, nine cryptotephras were identified from seven layers of tephric particle concentration horizons. They are a widespread tephra of the Kikai-Akahoya tephra (K-Ah, 7,307-7,196 cal yrs BP: McLean et al. 2018), the newly discovered tephra in this site are shown from the top as follows: the Niijima-Miyatsukayama Nanbu tephra (Nj-Mts, 5.5 ka: Kobayashi et al. 2020), Niijima-Shikinejima tephra (Nj-Sk, 8 ka: Kobayashi et al. 2020), one tephra that may be correlated with Daisen Sasaganaru tephra (DSs, 29,837 ± 96 cal yrs BP or 29,830 cal yrs BP: Albert et al. 2018) or Daisen Higashidaisen tephra (DHg, 28,895 ± 72 cal yrs BP or 28,888 ± 72 cal yrs BP: Albert et al. 2018), and one uncorrelated tephra from Izu Islands.
The distribution of Nj-Mts and Nj-Sk in the Honshu Island is identified for the first time. Therefore, JIS core can provide a type tephrostratigraphy in the Izu Peninsula for studying the distribution of tephras from the Izu Islands. On the other hand, the eastern end of the distribution of DSs and DHg in the previous studies is Lake Biwa (Takemura et al. 2010). Therefore, it is likely that the distribution area of either DSs or DHg is expanded to 250 km further east from Lake Biwa by this study and the main axis of them is east-southeast direction from the source volcano. In this study, we analyzed the moor sediments continuously and identified the tephric particle concentration horizons by several indicators, and showed the detailed tephrostratigraphy during the past 30,000 years in the southern part of the Izu Peninsula.