Vertical and Spatial Geochemical Variations of Nishinoshima Volcaniclastic Deposits, Japan, During Episode 4

Nishinoshima is a volcanic island in the Ogasawara Arc that has exhibited intermittent activity since 2013, including four major eruptive episodes: 2013–2015 (Episode 1), 2017 (Episode 2), 2018 (Episode 3), and 2019–2020 (Episode 4). Previous studies reported a change in eruption style—from a Strombolian eruption with lava flows (Episode 1 to the early phase of Episode 4) to violent Strombolian eruptions (mid to late phase of Episode 4)—accompanied by a shift in the bulk chemical composition of volcanic ash from andesite to basaltic andesite. However, the number of tephra samples from Episode 4 remains limited, and additional data are required to clarify the details about changes in chemical composition during this episode. In this study, we present the major and trace element compositions of offshore tephra collected from Episode 4. We analyzed <1 mm ash samples from the topmost sediments around Nishinoshima Island, collected from the seafloor during Cruise KR20‐E06 by R/V Kairei in December 2020. The bulk chemical compositions indicate that these samples were classified into andesite and basaltic andesite and were deposited during the mid‐ to late phase of Episode 4. Furthermore, their chemical compositions fall between those of the Nishinoshima andesites and the Episode 4 basalts reported previously. This suggests that the magma composition associated with the eruption shifted from andesite to basaltic andesite within a period of only 1.5 months. In addition, the samples show both vertical and spatial compositional variations. The former may reflect gravitational settling of ash particles through the water column and their floating behavior at the sea surface, whereas the latter may be influenced by eruption timing, local wind directions, or ocean currents.