Re-examination of previous correlation of distal Ks5 Tephra and proximal Oda pyroclastic flow deposits in South Kyushu, SW Japan

Two pyroclastic flow deposits named Nabekura (Nb) and Oda (Oda) Pyroclastic Flow Deposits in ascending order, in the southern Kyushu Island, SW Japan, are intercalated in the Kokubu Group distributed around the northern coastal area of the Kagoshima Bay. The purpose of this study is to reveal their differences in petrographic properties such as glass chemistry and refractive indices of volcanic glass shards, hornblende, and orthopyroxene. Nb and Oda are evidently distinguished by refractive indices of orthopyroxene and variation in the chemical composition of glass shards. We re-examined the proposed correlation of Oda with Ks5 (Boso Peninsula, Central Japan) by previous studies, comparing other two distal tephras petrographically similar to Ks5. It is concluded that Oda is not correlated to Ks5, OgA, and Hg-7

Chemical characterization of volcanic glass shards by energy dispersive X-ray spectrometry with EDAX Genesis APEX2 and JEOL JSM-6390

A chemical characterization of individual volcanic glass shard by electron-beam(E-beam) methods including SEM-EDS is effective for tephra fingerprinting. We installed a new SEM-EDS system composed of EDAX Genesis APEX2 and JEOL JSM-6390. This paper discusses reproducibility, accuracy, and precision of this system based on the analytical data sets ofthree representative Japanese widespread tephras and four provided samples by INTAVintercomparison. 710 analyses on homogeneous natural glass shards extracted from arepresentative Japanese widespread tephra (Aira-Tn: AT) under the same analytical conditionsdemonstrate reproducibility of measurements and absence of machine drift. For testinginter-laboratory reproducibility, three representative Japanese widespread tephras (AT, K-Ah, and Toya) and four natural glasses (rhyolitic Lipari obsidian, phonolitic Sheep Track tephra, basalticLaki 1783 A.D. tephra, rhyolitic Old Crow tephra) were selected. Results of comparison prove that the new system mostly enabled us to determine the major oxide concentrations comparable with other laboratories. However, in case of lower oxide concentrations less than ca. 0.3 wt% such as TiO2, MnO, and MgO, similarity and difference in concentrations are not significant because thesevalues are less than the detection limit of SEM-EDS analysis. Also, results of comparison suggest that attention should be paid in determination of glass chemistry with higher content of CaO andNa2O concentrations for basaltic and phonolitic tephras such as Laki 1783 A.D. and Sheep Track tephras

Radiocarbon ages and widespread tephra geochemistry for borehole core KA-1 from Lake Kawaguchi, Japan

Lake Kawaguchi is the second largest lake (area = 5.7 km²) in the Mount Fuji volcanic region and is located at the northern foot of Mount Fuji at 830.5 m above sea level. A borehole core of 88 m length (KA-1; 35°30.427'N, 138°43.701'E) was recovered from the western part of the lake between 1998 and 2000. The upper 15 m of the core mainly consists of silty sediments with intercalated scoria fallout deposits from Mount Fuji over the last ca. 18 kyr. The core also includes a fine-grained white pumice layer at 399.0 cm depth, which correlates with the Amagi Kawagodaira tephra (Kg), based on its geochemical composition and refractive index. We performed compound-specific radiocarbon analyses of fatty acids, as well as conventional ¹⁴C analyses of terrestrial macrofossils from core KA-1 to determine the depositional age of sediments based on ¹⁴C age of fatty acids and evaluate their potential as a dating tool for lake sediments. The dataset includes radiocarbon ages of fatty acids and plant leaves and geochemical composition of Kg from core KA-1

テフラ編年と14C年代に基づく鹿児島湾奥，新島（燃島）の海成堆積物の編年とその意義

金沢大学国際基幹教育院GS教育系Shinjima (Moeshima) Island in Kagoshima Bay, southern Kyushu, Japan is noteworthy in volcanology, paleontology, and palaeo-environmental studies, because the island emerged from the bay bottom during volcanic activity at Sakurajima in 1780 AD. As a result, Holocene and late-Pleistocene deposits of the bay including muddy deposits, a thick pyroclastic flow deposit, and prominent molluscan shell beds occur on this island. Several tephras included in the bay deposits are critical for deriving their precise chronology. The chronology of those tephras was constructed on the basis of their identification using both refractive indices and major element compositions of constituent glass shards, as well as stratigraphic features in the field. Tephras identified in sediments from younger to older are Sakurajima-Taisho (Sz-Ts)-/P1, Sakurajima-Sueyoshi(Sz-Sy)-/P11, Yonemaru, Sakurajima-Uwaba(Sz-Ub)-/P12, Sakurajima-Takatoge3 (Sz-Tk3)-/P13, Sakurajima-Satsuma (Sz-S)-/P14, and Shinjima pyroclastic flow deposit. The Shinjima pyroclastic flow deposit, which differs stratigraphically between northern and southern areas of Shinjima Island, is the same tephra in the two areas, and is estimated to be c. 13,000 cal BP in age. The pumice clasts of Sz-Sy/P11 provided a suitable environment for the habitat of a prominent shell bed (Moeshima Shell Bed) composed mainly of Neopycnodonte musashiana. Sz-Ub/P12, Sz-Tk3/P13, and associate secondary deposits of Sz-S/P14 indicate that the deposit was formed in the last 13,000 cal BP. The chronology of the deposits of Shinjima Island is based on the findings of stratigraphic positions and ages of those tephras, and 14C ages obtained in this study, and will play an important role when examining the palaeo-environmental history of Kagoshima Bay since the last deglaciation