5:15 PM - 7:15 PM
[SVC36-P03] Vertical Variation in Chemical Compositions of Volcanoclastic materials around Nishinoshima Island collected during Cruise KR20-E06
★Invited Papers
Keywords:Nishinoshima Island, Chemical Compositions, Tephra
Nishinoshima Island in the Ogasawara arc is a volcanic island that has exhibited intermittent volcanic activity since 2013. The intense volcanic activities have been identified during Episode 1 (2013–2015), Episode 2 (2017), Episode 3 (2018), Episode 4 (December 2019–August 2020), and Episode 5 (2021) [1–3]. Notably, in Episode 4, there was a marked shift in the eruption style. The Strombolian Eruption with lava flows continued from Episode 1 to the early phase of Episode 4 (Stage 1), but it abruptly changed to violent Strombolian Eruption in the mid to late phase of Episode 4 (Stages 2 and 3) [2]. The whole rock composition of volcanic ejecta changed from andesite in Episode 1–3 and early Episode 4 (Stage 1) to basaltic andesite in Episode 4 (Stage 2) [1, 2]. Nonetheless, the number of the Episode 4 tephra samples still remains small [1–3].
In this study, we report the vertical and regional variations in the chemical composition of tephra associated with Episode 4 eruptions to estimate the eruption period of the tephra samples and to investigate volcanic activity linked to the geochemical changes in tephra during Episode 4. We measured submarine surface sediment samples around Nishinoshima Island (depths of 945–1471 m) collected during Cruise KR20-E06 in December 2020, just after the violent Strombolian Eruption. The samples were collected using a Box Corer, which is able to strip the upper 10–20 cm of seafloor sediments, preserving the sedimentary structure. As such, the obtained sediments are considered to represent the intense tephra ejection during the mid to late Episode 4 eruption. This study performed serial analyses of XRF and ICP-MS for bulk sediments with a 1-cm interval after collecting a <1 mm fraction in size. The bulk chemical compositions suggest all the samples were deposited in the mid to late Episode 4 as anticipated. However, the results show a vertical compositional variation with a downward decrease in SiO2 content by up to 1 wt%. The vertical variation of tephra with a size of < 1 mm might represent gravitational grading as the ash falls through the water. Thus, tephra samples in this study seem to be fallout, not redeposited nor disturbed after their emplacement. The chemical compositions of tephra also present a regional variation, which could depend on the wind direction.
[1] Nagai et al. (2023) Ogasawara Research, 49, 45–69.
[2] Maeno et al. (2021) Front. Earth Sci., 9, 773819.
[3] Tamura et al., (2023) Front. Earth Sci., 11, 1137416.
In this study, we report the vertical and regional variations in the chemical composition of tephra associated with Episode 4 eruptions to estimate the eruption period of the tephra samples and to investigate volcanic activity linked to the geochemical changes in tephra during Episode 4. We measured submarine surface sediment samples around Nishinoshima Island (depths of 945–1471 m) collected during Cruise KR20-E06 in December 2020, just after the violent Strombolian Eruption. The samples were collected using a Box Corer, which is able to strip the upper 10–20 cm of seafloor sediments, preserving the sedimentary structure. As such, the obtained sediments are considered to represent the intense tephra ejection during the mid to late Episode 4 eruption. This study performed serial analyses of XRF and ICP-MS for bulk sediments with a 1-cm interval after collecting a <1 mm fraction in size. The bulk chemical compositions suggest all the samples were deposited in the mid to late Episode 4 as anticipated. However, the results show a vertical compositional variation with a downward decrease in SiO2 content by up to 1 wt%. The vertical variation of tephra with a size of < 1 mm might represent gravitational grading as the ash falls through the water. Thus, tephra samples in this study seem to be fallout, not redeposited nor disturbed after their emplacement. The chemical compositions of tephra also present a regional variation, which could depend on the wind direction.
[1] Nagai et al. (2023) Ogasawara Research, 49, 45–69.
[2] Maeno et al. (2021) Front. Earth Sci., 9, 773819.
[3] Tamura et al., (2023) Front. Earth Sci., 11, 1137416.