南沖繩海槽MD012403岩芯碎屑沉積物：源區與古海洋變遷

Abstract

國際古海洋全球變遷計畫(IMAGES)於2001年在西太平洋鑽取了一系列的深洋岩芯，期望可解析晚第四紀以來千年尺度的古海洋環境變化。沖繩海槽位處黑潮主流流經的地理樞紐，本研究利用取自南沖繩海槽MD012403岩芯的碎屑沉積物的粒徑分佈、黏土礦物及主要元素的組成，探討三萬年來黑潮流徑的變遷，以及沉積物供應來源變化。MD012403岩芯位於沖繩海槽的西南端 (25° 03.90' N，123° 16.94' E)，沉積物以陸源碎屑物為主，海源生物殼體次之；相對於其他深洋岩芯而言，本岩芯擁有較高的沉積速率(每千年50-500 cm)。 MD012403岩芯細粒沉積物的粒徑中值在20-32.6 ka時為7.3-7.8 Φ，較10.1-20 ka為細；10.1 ka以後，沉積物有變粗並伴隨著淘選度變差的趨勢，這樣的粒徑變化與可淘選粉砂(sortable silt)含量的增高指示著黑潮在此時進入了沖繩海槽。 台灣北部沉積物黏土礦物的Chlorite / Kaolinite值(>5)遠遠高於東海陸棚(<2)，本研究中岩芯沉積物在0-12 ka的Chlorite / Kaolinite值(2-6)高於12-21 ka的Chlorite / Kaolinite值(0.5)，推論12-21 ka東海陸棚為南沖繩海槽沉積物主要供應來源。 台灣沉積物與東海陸棚物質在主要元素K2O/TiO2比值上也有明顯差距，台灣沉積物的K2O/TiO2會高於東海陸棚的值，本研究中K2O/TiO2值(4.5)在12-21 ka遠遠低於0-12 ka及21-32.6 ka時的6-6.5，和黏土礦物變化趨勢一致，在12-21 ka時，東海陸棚物質輸入遠大於台灣東北部的貢獻。

High sedimentation rates (50-500cm/kyr) in the South Okinawa Trough (SOT) provide high resolution records for reconstructing millennial or centennial paleo-environment changes. This research analyzes the grain size distribution , clay mineral assemblage and major elements of the IMAGE Core MD012403 retrieved from the Southern Okinawa Trough. The medians of grain size in MD012403 varied from 7.3 Φ to 7.8 Φ during 32.6 ka-20 ka and showed a coarsening trend during 21 ka-10.1 ka. After 10.1 ka, the grain sizes increased to 6.6-7.1 Φ were more stable in association with poor sorting. High content of sortable silt also occurred at this period, indicating the intensity of Kuroshio current was enhanced. The average chlorite / kaolinite ratio of detrital sediments from Taiwan (5.0) is much higher than that from the East China Sea Continental (ECS) Shelf (<2.0). The ratio of chlorite / kaolinite in Core MD012403 shows high values (2-6) during 0 ka-12 ka and maintains low (0.5) between 12 ka-21 ka. This suggests that the ECS may be the major sedimentary provenance during 12 ka-21 ka. Concentrations of MnO、CaO、MgO、ΣFe2O3 in sediment increase with depth but Na2O displays an opposite trend. Aoki and Oinuma(1974) documented that the sediments come from East China Shelf have a significantly low K/Ti ratio. In this study, the ratios of K/Ti drop down during 12 ka-21 ka indicating sources of sediments were mainly from ECS shelf. This interpretation is consistent with evidence also supports the inference derived from clay mineral assemblage.