Abrupt change of sedimentary environment in ca. 7300 years ago recorded in the piston cores from the central Okinawa Trough floor

Kawamura Kiichiro, Ikehara Ken, Fujioka Kantaro

doi:10.5575/geosoc.113.184

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Other Title

中部沖縄トラフ底で採取されたピストンコアに記録された約7300年前の堆積環境の急激な変化
チュウブオキナワトラフテイデサイシュサレタピストンコアニキロクサレタヤク 7300ネンマエノタイセキカンキョウノキュウゲキナヘンカ

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Description

An abrupt change of the depositional system is recorded in two piston cores in the central Okinawa Trough. The cores are approximately 3 and 5 meters long, respectively, and contain the K-Ah tephra layer of the age of 7325 years B.P. The median of the grain size of the constituent quartz grains in the cores decreases suddenly upwards at the K-Ah horizon. This decrease results from the removal of siliciclastic grains coarser than ca. 20 μm in diameter from the sediments. This change of sedimentary environment could be explained in the following ways: 1) the distance between the Okinawa Trough and the Chinese continent of the sediment source has increased due to sea level rising at this time, 2) a barrier of the Kuroshio Current was formed along the edge of the continental shelf during 8000-7000 years B.P., and most of the sediment supplies from the continental shelf was swept away by the strong current such as the Kuroshio. The sediment microfabric also changed at this time from horizontal to random, as revealed by the change of the anisotropy of magnetic susceptibility. This horizontal fabric is considered to have been formed under the high sedimentation rate conditions without bioturbation. In conclusion, it is suggested that the present depositional system in the central Okinawa Trough was established at the age of ca. 7300 years B.P. and persists until now.<br>

Journal

The Journal of the Geological Society of Japan

The Journal of the Geological Society of Japan 113 (5), 184-192, 2007

The Geological Society of Japan

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