臺灣東部海域的沈積物粒徑及沈積速率初探

Abstract

摘要 本文研究區域位於臺灣東部海域，除了蘭陽溪河口、花蓮溪河口與秀姑巒溪河口之外，還包括了南沖繩海槽、宜蘭陸棚、和平海盆、和平高區、新城海脊、花蓮峽谷、南縱海槽等海底地形所採集的十八根箱型岩芯，希望能藉由各個地質構造單元的岩芯分析找出沈積物的粒徑特點與傳輸方式。 依照水深及地形將岩芯分為三類，一為河口及陸棚岩芯，其二為陸坡岩芯，第三類是海底峽谷、海盆與海槽岩芯。以X光攝影結果顯示河口及陸棚岩芯常發現有旋卷構造、透鏡狀構造，也觀察到濁流層的分佈；陸坡、海底峽谷、海盆與海槽岩芯也常發現濁流沈積物的紀錄。由粒徑分析結果可得知，粒徑範圍約0-10ψ，中間粒徑範圍約2.5~7.5ψ，淘選度介於中淘選(0.8~1.4ψ)與甚劣淘選(2~2.6ψ)之間。砂質組成往北方有減少，泥質、黏土組成有增加的趨勢。表示向北傳輸的海流，如黑潮，在相似水深的地形能夠搬運泥質、黏土。臺灣東部海域中所採的河口沈積物，中間粒徑範圍約2.5~7ψ，淘選度介於中淘選與甚劣淘選之間。陸坡沈積物的中間粒徑範圍約4.5~7.5ψ，淘選度介於中淘選與劣淘選之間。海盆、海槽與峽谷沈積物的中間粒徑範圍約4~7ψ，淘選度介於中淘選與劣淘選之間。由210Pb的資料顯示，沈積速率範圍在每年0.936到0.049公分之間，其中河口與陸棚岩芯，只有花蓮東部陸棚岩芯757-14可得沈積速率每年0.486公分、陸坡區域所採的岩芯，沈積速率範圍約每年0.936到0.056公分之間、海盆、海槽與峽谷區域所採的岩芯，沈積速率範圍約每年0.642到0.049公分之間。 關鍵字：東部海域、粒徑分析、黑潮、沈積速率

Abstract In study area, 18 core samples were collected throughout the Eastern waters off Taiwan using box corers, including in the estuary of Lanyang River, Hualien River, Siouguluan River, and in southern OkinawaTrough, Ilan Shelf, Hoping Basin, Hoping Rise, Hsincheng Ridge, Hualian Valley and Southern Longitudinal Trough. This study hoped to find out the feature of grain size and the transportation of sediment through the analyses of cores collected from those physiographic units. Depending on water depth and physiographic features, these core can be divided into three categories: 1. estuaries and shelves, 2. slopes 3. valleys, basins and troughs. X-ray photographs indicated that sediment cores from estuaries and continental shelves often contain convolute, lenticular, and also find turbidite layer; in the 2nd and 3rd categories, could also discover some turbidite records. Through the result of the grain size analyze, we ascertain that particle size range between 0~10ψ,mass median diameter range between 2.5~7.5ψ,and sorting of all cores varies between moderate, poor and very poor. The value of sand is decreasing northward; the value of silt and clay is increasing northward. That means the northward currents, such as Kuroshio current could carry sediments in similar topography. The 1st category with sediments in estuaries and shelves’s mass median diameter range between 2.5~7ψ, and moderately and very poorly sorted. Sediments in slopes’s mass median diameter range between 4.5~7.5ψ, with moderately and poorly sorted. The 210Pb data shows that the sedimentation rate range between 0.936~0.049 cm per year. Category in estuaries and shelves, only Site 757-14 could count the sedimentation rate in 0.486 cm per year. Cores in slopes’s sedimentation rate range between 0.936~0.056 cm per year. The 3rd category’s sedimentation rate range between 0.632~0.049 cm per year.