- 學位論文
臺灣東部海域現代沉積物之來源及傳輸
Provenance and Transportation of Modern Sediments offshore Eastern Taiwan
摘要
臺灣位於板塊交界帶與西北太平洋颱風廊道，大規模的地震與颱風強降雨事件造成的經濟損失及人員傷亡等成為臺灣常見的自然災害。前人研究中，於近岸岩心紀錄發現沉積物中保存了極端事件的災害紀錄，但因採集之岩心位置主要靠近陸坡及坡底，具有高沉積速率特徵，往往造成岩心紀錄的時間尺度不長；此外，受限於國內研究船採樣能力，東部海域深水區少有深入的研究，對於深海沉積物的來源及傳輸並不清楚。本研究利用法國研究船R/V Marion Dufresne於東部海域，即琉球弧前盆地（和平海盆、南澳海盆、東南澳海盆、波照間海盆）、耶雅瑪海脊及花東海盆所採集的10管箱型岩心，分析其統體密度、粒徑分佈及210Pb活度，藉此劃分極端事件造成的堆積層位，並利用超量210Pb訂定事件層年代。此外，透過分析臺法「渴望」航次箱型岩心和過去海研一號於近岸所採集岩心中的黏土礦物組成，藉此瞭解臺灣東部海域近岸至深海黏土礦物組成，並探討此區域現代沉積物之來源及傳輸機制。 本研究根據採樣位址之地質特性，將研究區域分成五區，分析結果顯示：（1）近岸陸棚及陸坡－臺灣東部海域近岸陸棚及陸坡沉積物之黏土礦物組成以伊利石及綠泥石為主，花蓮溪及秀姑巒溪出海口岩心膨潤石含量超過20 %，推測為陸上河川經過火成岩地層（都巒山層），將沉積物輸出至海床沉積，造成分析結果有較高含量的膨潤石。（2）南澳海盆、東南澳海盆－分析結果顯示此區岩心記錄到多層事件，且MD18-3525及MD18-3526箱型岩心粒徑中值皆觀察到粒級層的特徵，推測此區域沉積環境並不穩定。南澳海盆及東南澳海盆沉積物的黏土礦物組成以伊利石與綠泥石為主，少數層位中含有相對高量的膨潤石。（3）波照間海盆－岩心粒徑中值變化不大，但利用岩心淘選度變差劃分事件層，及210Pb活度的低值推論此區域的沉積環境不穩定。波照間海盆沉積物之黏土礦物組成以伊利石與綠泥石為主，沉積物幾乎均含有膨潤石，且在MD18-3529箱型岩心中含大量膨潤石，含量超過40 %。（4）耶雅瑪海脊－由於位處地形高區，岩心中的事件層相對較少，沉積環境相較於其他區域穩定。耶雅瑪海脊沉積物黏土礦物組成以伊利石與綠泥石為主，MD18-3532箱型岩心中有較高含量的膨潤石。（5）花東海盆－花東海盆之箱型岩心，沉積物粒徑及210Pb活度剖面具有來回跳動特徵，沉積環境受到頻繁擾動，沉積速率因此無法計算。黏土礦物組成主要以伊利石與綠泥石為主，少數層位中含有相對高量的膨潤石。 整體而言，東部海域深水區沉積物之黏土礦物組成反映出與近岸陸棚及陸坡組成相似，推論沉積物源自於臺灣東部河川所輸出；而距離臺灣較遠的波照間海盆岩心，其膨潤石比例相對其他區域較高，此區域沉積物來源與北邊鄰近八重山列島（Yaeyama Islands）有關；耶雅瑪海脊東側的MD18-3532箱型岩心含較高比例的膨潤石，推測沉積物來源與火山灰物質傳輸相關。在現代事件紀錄方面，箱型岩心皆記錄到可能受事件影響的層位，藉由超量210Pb定年法訂定事件層的年代後，對照水利署與氣象局之水文、颱風與地震資料並篩選出規模較大的事件，推測岩心中事件層同時會受到颱風與地震造成的堆積。
並列摘要
Taiwan is situated at the plate boundary and northwestern Pacific corridor of typhoons. Economic losses and casualties caused by large-scale earthquakes and heavy rainfalls have become common natural disasters in Taiwan, and the extreme events induced sediment discharges have also been recorded in sediments in the nearshore cores. However, due to the limitation of sampling capacity in the past, the cores are collected close to the slope and shelf offshore eastern Taiwan. The high sedimentation rates also limit the time scale of the cores. Only few studies off eastern Taiwan have been reported and the source and transportation sediments were not clear. In this study, 10 box cores were collected from the Ryukyu forearc basin (Hoping, Nanao, East Nanao, Hateruma Basin), Yaeyama Ridge and Huatung Basin by R/V Marion Dufresne. We analyzed bulk density, grain size and 210Pb activity to distinguish event layers. Furthermore, we analyzed the clay mineral composition of MD cores and cores collected in continental shelf and slope by R/V Ocean Research 1 to discuss the sources and transportation of sediments in this region. According to the geological characteristics of the sampling sites, we divided the study area into five regions and the results show: (1) Coastal shelf and slope region - The clay minerals composition of the coastal shelf and slope sediments in eastern Taiwan is mainly composed by illite and chlorite. The percentile of smectite in cores near the estuary of Hualien River and Hsiukuluan River exceeds 20 %, indicating that the rivers pass through the igneous rock area (Tuluanshan Formation) and transport sediments to the study area. (2) Nanao and East Nanao Basin - The results show that the cores in this area have recorded several event layers. The graded beddings were observed in MD18-3525-BC and MD18-3526-BC, indicating that the sedimentary environment was unstable. The clay minerals in this region are mainly composed of illite and chlorite, and in some layers contain high smectite. (3) Hateruma Basin - The median grain size of cores are consistent, but the event layers is characterized by poor sorting and low 210Pb activity, and inferred the sedimentary environment is unstable. The clay minerals in this region are mainly composed of illite and chlorite, and almost all sediments contain smectite. The core of MD18-3529-BC contains a large amount of smectite with a content of more than 40 %. (4) Yaeyama Ridge - Due to it is located in a high-lying area, there are relatively few event layers in the cores, and the sedimentary environment is more stable than other areas. The clay minerals in the cores collected from Yaeyama Ridge are dominated by illite and chlorite, and MD18-3532-BC has a higher content of smectite. (5) Huatung Basin - The grain size and 210Pb profiles have the characteristics of bouncing back and forth, and the sedimentary environment is frequently disturbed. It is lead to the sedimentation rate cannot be calculated. Clay minerals are mainly composed of illite and chlorite, and a few layers contain high content of smectite. In overall, the clay minerals composition in deep-sea sediments off eastern Taiwan reflects the similar composition in the nearshore shelf and slope, while the cores from the Hateruma Basin have a higher content of smectite than other areas. The source of the sediments in this area might transported from the Yaeyama Islands; the MD18-3532-BC on east side of the Yaeyama Ridge contains a higher percentage of smectite, which is related to the transportation of volcanic ash materials. In the part of modern events records, the results indicate that the cores have recorded the layers that may be affected by the event. After dating the event layers by excess 210Pb dating method, we compared the age of event layers, the hydrology data, typhoon, and earthquake data from the Water Resources Agency and the Central Weather Bureau. We suggest that the core recorded event layers caused by typhoon and earthquake at the same time.