Various methods in tephrochronology : Case studies in the Philippines, China, Korea, Japan, and the Aleutian Islands

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  • Okuno Mitsuru
    Department of Earth System Science, Faculty of Science, Fukuoka University AIG Collaborative Research Institute for International Study on Eruptive History and Informatics, Fukuoka University

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  • テフラ編年学の多様な役割 : フィリピン，中国，韓国，日本，アリューシャン列島の研究例
  • テフラヘンネンガク ノ タヨウ ナ ヤクワリ : フィリピン,チュウゴク,カンコク,ニホン,アリューシャン レットウ ノ ケンキュウレイ

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Abstract

Tephra layers provide time markers to sedimentary sequences in marine and terrestrial environments. Therefore, this feature is useful as platform for crosschecking among various dating and environmental studies. On a land surface, continuous accumulation of loam and humic soil layers allows the preservation of tephra layers. Throughout just the Holocene, for example, Adak Island in the Aleutian Islands has been covered by a sequence of soil-tephra complexes. In the Philippines, the discovery of co-ignimbrite ash-falls of Irosin ignimbrite is a first step for establishing a chronological framework. It is necessary to conduct core-drilling and stratigraphic studies in an artificial trench to obtain good samples for various analyses. In Tanegashima Island, the radiocarbon dates for Tn2 and Tn4 imply that accumulation rate is not constant between K-Tz (95ka) and AT (30ka). Precise age determination for B-Tm has been done by varve chronology and 14C wiggle-matching methods. This crosschecking process is very important for developing a dating method. Tephra layers in lacustrine play a role as time control. The data sets from varve counting and 14C dates provide accurate ages of tephra layers. Crypto-/micro-tephras contribute as time-markers. However, this method has the potential to be wrong. In fact, U-Oki has been correlated with U-2 in Ulleung Island, based on recognition of a K-Ah horizon. Radiocarbon dating in the island allows us to correlate U-4 with U-Oki.

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