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

堆積システムの急激な変化が中部沖縄トラフの3mと5m長の2本のコアに記録されていた.構成される石英粒子の中央粒径は,7325年前の年代のK-Ah層で下位から上位に向けて急激に減少した.その中央粒径の減少は,約20μmの粗粒粒子の除去による結果である.K-Ah層の年代で生じたこの変化は,1)海水準上昇で沖縄トラフと堆積物供給源である中国大陸とが離れたこと,2)トラフ内で8000〜7000年前に黒潮の堆積物供給バリアが形成され,大陸からの堆積物のほとんどが強い海流により掃きとばされたことによって生じたと推測される.さらに,微細組織がその下位での水平配列から上位でのランダム配列へ変化する.その水平配列は,高堆積速度条件下で生じた無生物擾乱からの結果であると判断できる.このように,中部沖縄トラフでは,今日のような堆積システムが約7300年前に形成されて,現在までにそれが持続していると考えられる.

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.