17:15 〜 18:45
[SVC25-P01] Characterisation of tephra from marine sediment cores in the South Shetland Islands region, Antarctica
キーワード：tephra, Antarctica, Geochemistry, Geochronology, marine, sediment core
Marine sediment cores adjacent to Antarctica are valuable because they contain more complete records of the explosive activity of Antarctic region volcanoes than the small number of Antarctic terrestrial sediment cores or the limited tephra preserved in Antarctic ice cores. Nine marine sediment piston cores were collected from strategic new locations in the South Shetland Islands and Weddell Sea (SSI-WS) region during the Hakuhomaru KH-19-6 Leg 4 – Integrated investigation of Marine Earth Sciences in the Weddell Sea and South Pacific voyage in 2019/2020. The SSI-WS is a region of volcanological interest for several reasons including that it meets the geographic and geological location criteria needed to investigate the relationship between volcanism and glacial isostatic adjustment. Namely, it is a region of Paleocene to modern volcanism with a large ice sheet.
Volcanic glass shards were identified in two of the new marine sediment cores from SSI-WS; PC01 from a site located in the Bransfield Strait and, PC04 from a site located in the South Shetland Trough. Here we present the results from physical, geochemical and geochronological (C14 organic carbon and fossils) characterisation of the volcanic glass shards within these cores. Geochemical compositions of the volcanic glass shards was determined via electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry of individual volcanic glass shards. PC01 contains 14 layers hosting volcanic glass shards. Between ca. 6-5 ka the glass shard compositions are primarily basalt-basaltic andesite compositions. At 3150 ± 200 ka, PC01 glass shard compositions are bimodal comprising rhyolite and basalt-basaltic andesite. Trace element patterns are consistent with arc magmatic sources for all PC01 volcanic glass shards. Trace element compositions together with the major element compositions provide a geochemical fingerprint consistent with a Deception Island source for all PC01 volcanic glass shards. The volcanic history of Deception Island is characterised by the construction of a large, emergent shield volcano followed by a large caldera forming eruption at 3980 ± 125 yrs (Antoniades et al., 2018). Post-caldera formation, smaller explosive and extrusive eruptions have occurred until present with the most recent occurring in 1970 (Geyer et al., 2021). The compositions of the oldest (>5 ka) volcanic glass shards in PC01 are consistent with the compositions reported for pre-caldera collapse tephra at Deception Island and the basaltic-rhyolitic compositions of the younger (<3 500 ka) glass shards are consistent with those reported for the post caldera collapse tephra (Smellie, 2021). Rare earth element patterns indicate the presence of basaltic trachyandesite glass shards in the lowest (oldest) volcanic layers from the 3150 ± 200 ka core section that are less enriched than the mafic compositions elsewhere in the core. This suggests a separate magmatic phase associated with the beginning of post caldera explosive volcanism at Deception Island.
PC04 hosts 3 layers of tephra comprising volcanic glass shards with bimodal dacite-rhyolite and andesite-basaltic andesite compositions. Major element oxide compositions indicate a source(s) on the Antarctic Peninsula. NaO2 values of >4.0 wt% for the basaltic-andesitic volcanic glass shards suggests that Deception Island may also be the main source of the tephra in PC04.
Volcanic glass shards were identified in two of the new marine sediment cores from SSI-WS; PC01 from a site located in the Bransfield Strait and, PC04 from a site located in the South Shetland Trough. Here we present the results from physical, geochemical and geochronological (C14 organic carbon and fossils) characterisation of the volcanic glass shards within these cores. Geochemical compositions of the volcanic glass shards was determined via electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry of individual volcanic glass shards. PC01 contains 14 layers hosting volcanic glass shards. Between ca. 6-5 ka the glass shard compositions are primarily basalt-basaltic andesite compositions. At 3150 ± 200 ka, PC01 glass shard compositions are bimodal comprising rhyolite and basalt-basaltic andesite. Trace element patterns are consistent with arc magmatic sources for all PC01 volcanic glass shards. Trace element compositions together with the major element compositions provide a geochemical fingerprint consistent with a Deception Island source for all PC01 volcanic glass shards. The volcanic history of Deception Island is characterised by the construction of a large, emergent shield volcano followed by a large caldera forming eruption at 3980 ± 125 yrs (Antoniades et al., 2018). Post-caldera formation, smaller explosive and extrusive eruptions have occurred until present with the most recent occurring in 1970 (Geyer et al., 2021). The compositions of the oldest (>5 ka) volcanic glass shards in PC01 are consistent with the compositions reported for pre-caldera collapse tephra at Deception Island and the basaltic-rhyolitic compositions of the younger (<3 500 ka) glass shards are consistent with those reported for the post caldera collapse tephra (Smellie, 2021). Rare earth element patterns indicate the presence of basaltic trachyandesite glass shards in the lowest (oldest) volcanic layers from the 3150 ± 200 ka core section that are less enriched than the mafic compositions elsewhere in the core. This suggests a separate magmatic phase associated with the beginning of post caldera explosive volcanism at Deception Island.
PC04 hosts 3 layers of tephra comprising volcanic glass shards with bimodal dacite-rhyolite and andesite-basaltic andesite compositions. Major element oxide compositions indicate a source(s) on the Antarctic Peninsula. NaO2 values of >4.0 wt% for the basaltic-andesitic volcanic glass shards suggests that Deception Island may also be the main source of the tephra in PC04.