台灣海峽北部現代沈積物之沈積速率與傳輸途徑

Abstract

本論文研究利用海研一、二號於台灣海峽十個航次所採之重力岩心62支與箱型岩心20支沈積物樣本，分析落塵放射核種210Pb及137Cs於岩心中垂直分布，計算沈積速率與核種儲量。此外，從各航次所採230個表層沈積物樣本所測得之7Be，及部分航次之表層沈積物粒徑分布，配合連續變頻聲納淺層沈積物或底質形貌資料，探討台灣海峽從年際、年代紀至百年尺度之沈積通量、沈積物來源及傳輸途徑。 根據210Pb及 137Cs於沈積物岩心中剖面所計算出台灣海峽百年尺度之沈積速率大多介於0.1-2.4 g/cm2/yr之間，平均沈積速率約為0.6 g/cm2/yr。沈積速率與核種儲量高值區有二：一為近濁水溪河口之東彰雲脊頂，另一為海峽西北端之大陸沿岸。依據210Pbex活度剖面多呈非單一指數遞減趨勢，可知台灣海峽沉積環境不單純，隨時間變化既快且鉅。海峽中線東側地帶在過去五十年間之沈積速率有增快現象，反映台灣西部山地土壤之侵蝕加劇，可能與山坡地過度的開發及暴雨之發生頻率增高有關。 粒徑分析、7Be活度分佈與聲納探測資料顯示，海峽中的粗砂主要分佈在台灣近岸與觀音凹陷，其來源為台灣西部河川與較老的殘餘沈積物。細砂則主要來自彰雲沙脊，受到潮流作用，往北傳輸，與海峽中央地帶的泥質沈積物混合。泥質沈積物主要由台灣西部河川所供應，入海後先於河口地區暫時堆積，之後繼續受潮流與海流作用，輾轉搬運至海峽中央地帶。西北部之泥質沈積物，依淺層沈積物構造、沈積速率與核種儲量分佈，判定其來源為中國大陸河川輸出，被沿岸流攜帶，往南傳送。 綜言之，在現代水文（潮流與海流）與氣候（颱風或暴雨）條件作用下，除海峽西北部有來自中國大陸沿岸的泥質沈積物分佈，基本上台灣海峽之淺層(一米內)沈積物大多為台灣陸源的現代(百年內)沈積物與海峽內原已存在之較老沈積物混合組成。

Sixty-two gravity cores and 20 box cores collected from 10 cruises onboard R/V Ocean Researcher I and II were analyzed for 210Pb and 137Cs to estimate sedimentation rates and nuclide inventories. In addition, surficial sediments from many of these cruises were analyzed for 7Be and grain size distribution. The results are integrated with information derived from Chirp sonar survey about the structure and distribution trend of shallow sediment strata to elucidate the sources, pathways and budget of sediment in the Taiwan Strait from seasonal to annual, decadal and centennial timescales. Based on profiles of 210Pb and 137Cs, the calculated sedimentation rates largely fall in the range from 0.1 to 2.4 g cm-2 yr-1, averaging 0.6 g cm-2 yr-1. The highest sedimentation rates as well as nuclide inventories are found at the top of the eastern Yun-Chan Ridge near the Choshui River’s estuary and in the northwestern corner of the Taiwan Strait. In much of the Taiwan Strait, sedimentation rates are highly variable with time, as indicated by non-steady state 210Pb profiles. In the eastern part of the strait, sedimentation rates appear to have increased in the last few decades, probably reflecting intensified soil erosion in Taiwan in wake of accelerated exploitation of land and more frequent occurrence of intense precipitation in the region. 7Be and grain size distribution in surface sediments, coupled with Chirp sonar survey of shallow sediment strata, indicate coarse sand is distributed mainly along part of the west coast of Taiwan and the Kuanyin depression with swift along-shore or bottom flows. The distribution of fine sand in the Taiwan Strait is controlled mainly by tidal currents which transport such particles from the Yun-Chang Ridge northward toward the middle of the Taiwan Strait where fine sand from the south merges into mud derived from western Taiwan Rivers. As for the mud in the northwest of the Strait, it is derived from Chinese rivers and transported alongshore toward the south. In summary, shallow (<1 m) sediments in the Taiwan Strait are composed of relict sand and modern (<100 years) sediment derived mainly from Taiwanese rivers. Dispersal and deposition of such sediments are controlled by modern hydrodynamics in the strait.