臺灣西南外海重力流堆積物之可能物源

Abstract

臺灣受到地理位置影響，乾濕季節分明，降雨集中在颱風季節造成地表的沖刷強烈，雨季時會帶出大量沉積物進入海洋環境中。本研究區域位於台灣西南外海之高屏海底峽谷上段、高屏陸棚、高屏陸坡、枋寮海底峽谷等地，區域中主要的沉積物貢獻來自於高屏溪輸出的沉積物。當洪氾事件發生時，可能會在海床上造成數公分的事件沉積層，或是在不穩定的海床上啟動沉積物重力流（gravity flow），將近岸的沉積物大量且快速的傳送到深海環境中，使高屏陸坡內盆地等半遠洋環境也能發現新鮮的陸源物質。 本研究使用沉積物的粒徑資料與鉛-210剖面推斷出岩芯沉積物中的事件層位與可能的流體運動形態，觀察沉積物顆粒分佈與全岩礦物組成間之關聯性。結果顯示事件層中的粗顆粒沉積物常帶有較高比例的屬於中央山脈特徵的綠泥石成份，隨著沉積作用中發生的顆粒大小重力分異，細顆粒增加則綠泥石成份漸減。在高屏陸棚的樣本中可定出莫拉克颱風事件層與海棠颱風事件層，兩事件層中細顆粒部分的礦物組成差異甚大，顯示降雨分佈區域的不同也會在沉積物的性質上造成差異。

The distinct rainy season in Taiwan frequently induced strong erosion in central mountain range and washing out the surface sediments. The study area of this research is focus on the upper Gaoping submarine canyon, Gaoping shelf, upper Gaoping slope and Fangliao submarine canyon. Gaoping River is one of the main contributors of sediments in the area. According to the sedimentation records, the flooding events usually left turbidite layers deposit on the seafloor and may further triggered gravity flows and transport fresh terrestrial materials into semi-pelagic environment. Grain size and Pb-210 analysis results are used to distinguish event deposits in cores and discussthe correlation between mineral composition and turbidites. The coarser sediments in the event deposits are usually with higher chlorite which was transported from the Central Mountain Range. The flood events on land can directly bring out coarse sediments from the mountain to ocean. The fine-grained sediments increased during the sedimentation process by gravity redistribution and decreased the chlorites. Follow the gravitational grain size separation of sediments, the fine-grained sediments increased with decreasing chlorite contents. From the Haitang and Morakot Typhoon events, we find the differences of precipitation pattern plays an important role on the characteristic of sediments and their mineral composition.