台灣東北外海沖繩海槽及龜山島附近之海床沉積物特徵; Seafloor sediment characteristics in the Okinawa Trough and around the Kueishantao volcanic island offshore NE Taiwan

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Title:	台灣東北外海沖繩海槽及龜山島附近之海床沉積物特徵;Seafloor sediment characteristics in the Okinawa Trough and around the Kueishantao volcanic island offshore NE Taiwan
Authors:	張筱佩;Hsiao-Pei Chang
Contributors:	地球物理研究所
Keywords:	礦物組成;宜蘭陸棚;濁流;由源至匯;210Pb定年;Ilan shelf;210Pb dating;turbidity currents;mineral compositions;source to sink.
Date:	2011-12-27
Issue Date:	2012-01-05 14:13:36 (UTC+8)
Abstract:	台灣東北外海之南沖繩海槽的沉積物主要來自台灣，蘭陽溪每年輸送大量沉積物至本區堆積。位於沖繩海槽最西側的龜山火山島也是研究區的海床沉積物來源之一，但源自龜山島之沉積物如何傳輸至鄰近海床沉積則不甚明瞭。本研究分析由海研一號(OR1-800)與海研二號(OR2-1325、1192)等三航次所採集約1公尺長之16站重力岩心(水深介於100-500公尺)及3公尺長之3站活塞岩心(水深介於900-1500公尺）。利用沉積物粒徑分析、X光攝影、磁感率量測、礦物組成分析等方法，配合海床底質剖面及海流資料，探討此區域的沉積物型態、組成、岩理、來源及沉積物傳輸路徑。經由岩心描述與礦物組成分析，得知龜山島西側與東側海域之沉積物組成顆粒以粉砂為主。龜山島西側海域的沉積物組成主要為石英及以硬頁岩、板岩為主的變質岩岩屑，少量的火成岩碎屑；龜山島東側海域沉積物仍以石英及變質岩屑為主，但具有較多安山岩碎屑，並有較多之輝石、黑雲母等主要源自火成岩的礦物。據此，推測龜山島西側海域沉積物主要源自蘭陽溪；龜山島東側海域沉積物主要來自台灣本島及龜山島。至於龜山島東西兩側的火成岩碎屑含量有所差異，可能是受到北龜山水道及南龜山水道底流作用將沉積物搬運至東側堆積，再進入南沖繩海槽。南沖繩海槽沉積物以泥質為主，主要礦物組成為石英及變質岩屑（硬頁岩、板岩、片岩）以及少量的火成岩岩屑。推測現今南沖繩海槽的沉積物除了來自蘭陽溪外，少部分來自台灣東部之中央山脈片岩區及海岸山脈島弧區。其中OR1-800-2-P站位位於斜坡下方水深1350 m處，且遠離海底峽谷，本岩心含有多層紗質濁流岩堆積，推測此濁流可能所地震引發之海底山崩所形成。沉積物放射元素（210Pb）定年分析，得知龜山島西側海域沉積速率約為0.58 cm/yr，最高可達0.71 cm/yr。 Seafloor sediment cores taken from around the Kueishantao Island and the western tip of the Okinawa Trough, offshore NE Taiwan, were analyzed for grain sizes, mineral compositions, sedimentary features and sedimentation rate. The results are used to infer major sediment sources, pathways for sediment transports in the study area as well as how sediments sourced from the Kueishantao Island are transported to the Okninawa Trough. Seafloor sediments around the Kueishantao Island are dominated by silts. Seafloor sediments west of the Kueishantao island consist predominantly of quartz, and lithic fragments of argillite and slate; whereas seafloor sediments east of the island show similar compositions but exhibit more minerals derived from volcanic rocks, such as pyroxene, hornblende and biotite. Therefore seafloor inferred that sediments west of the Kueishantao island are interpreted to derive mainly from the Taiwan mountain belt through the Lan-Yang River, and sediments east of the island are sourced from Taiwan as well as from the Kueishantao Island. Sediments in the southern tip of the Okinawa Trough, are clay rich and consist mainly of quartz, metamorphic lithic fragments (argillite, slate), and a small amount of igneous lithic fragments. This composition suggests that sediments in this trough are still sourced from Taiwan. Analyses on radio nucleus (210Pb) for seafloor sediments west of the Kueishantao island indicate a fairly high nominal sedimentation rate of 0.58 cm/yr and a maximum rate of 0.71 cm/yr.
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