| 摘要: | 本研究利用採自台灣河流河口以及台灣周遭深海區之沉積物，研究其黏土礦物與元素組成以及鍶釹同位素變化，探討細粒矽質沉積物由源至匯之傳輸過程。在海洋沉積物中，我們除了分析海床沉積物之黏土礦物、主要元素與微量元素以及鍶釹同位素變化之外，針對海床下50公分厚沉積物，分別採集半遠洋泥以及濁流岩，了解黏土礦物於不同沉積相的變化，進而探討其傳輸機制。
研究結果顯示半遠洋泥以及濁流岩的黏土礦物介於伊萊石+綠泥石、膨潤石等二個端成分之間。其中伊萊石+綠泥石之源岩區為台灣、膨潤石則來自呂宋島。然而，採自台灣海岸山脈河流的沉積物也有膨潤石，顯示海岸山脈也是膨潤石的源岩區之一。在琉球弧溝系統的波照間弧前盆地，含有大量的膨潤石，可能源自於琉球群島。在鄰近海岸山脈的外海獨立高區、海域的呂宋島弧高區、上部馬尼拉增積岩體以及南中國海東北部等地區，其黏土礦物組成，皆以膨潤石為主，顯示黑潮將源自呂宋島的膨潤石搬運至此堆積。然而，在琉球弧溝系統的和平海盆與南澳海盆等弧前盆地，以及台灣東南方的北呂宋海槽、台灣西南外海的高屏峽谷，以上區域含有大量的伊萊石+綠泥石，顯示這些地區常有源自台灣的濁流沉積。研究結果顯示，在同一站位岩心的半遠洋泥以及濁流岩，具有類似的黏土礦物組成，顯示兩種沉積相具有相同的源岩區。另外，在與河流相連的峽谷沉積物，其膨潤石明顯減少，顯示源自台灣的濁流沉積物，稀釋了膨潤石的含量。
在海床表層沉積物的研究中，我們結合黏土礦物指數(伊萊石結晶度與伊萊石化學指標)、主量元素指數(如化學活動性、CIX指數、K2O/(Na2O + CaO)分子比例、風化趨勢三角圖等)，以及銣、鍶、鍶-釹同位素分析結果，顯示源自台灣的沉積物歷經中等的化學風化作用。研究結果也顯示台灣鄰近深海沉積物主要源自台灣島。;Clay mineralogy, element geochemistry, and Sr and Nd isotopic study of recent deep-water seafloor sediments collected in different tectonic settings around Taiwan, as well as surrounding Taiwanese river-mouth sediments and Tainan shelf edge sediments, have been investigated to know the source-to-sink of the detrital fine-grained sediments. We determine the clay mineralogy, major and trace element geochemistry along with Sr and Nd isotopes in surface sediments around offshore Taiwan, as well as the clay mineralogy in both hemipelagites and turbidites up to the top 50 cm of the deep-sea sediment cores to infer the source and transportation of detrital fine-grained sediments.
Our findings demonstrate that the clay mineral assemblages in both hemipelagites and turbidites from various provinces gradually shift between two major end-members: illite+chlorite and smectite. They are primarily sourced from Taiwan and Luzon, respectively. Besides, the presence of more smectite in river sediments sourced from the Coastal Range indicates that smectite is an additional characteristic clay mineral for sediments derived from eastern Taiwan. The high smectite content in the Hateruma forearc basin of the Ryukyu subduction system in eastern offshore Taiwan, interpreted as derived mainly from the Ryukyu Islands. The sediment cores in a bathymetric high near offshore Coastal Range, the Luzon volcanic arc, the upper-slope perched basins of the Manila accretionary wedge, and few cores in the northeastern South China Sea off southwestern Taiwan show dominantly of smectite, indicating strong influence by Kuroshio Current pathways. Whereas, the Hoping Canyon, the Nanao Basins, the Southern Longitudinal Trough off eastern Taiwan, and the Gaoping Canyon off southwestern Taiwan consist dominantly of illite and chlorite, interpreted as derived mainly from Taiwanese rivers through the river-canyon system. Our study also indicates that the clay mineral assemblage around Taiwan is mainly controlled by supply from major provenance and current transport. In most of the cores, the relative abundance of clay minerals in turbidites and hemipelagites is very comparable. As a result, we argue that the neighboring turbidites and hemipelagites of a core share common detrital sources. Besides, less smectite is found in the river-connected canyon systems, especially for coarse fraction of turbidites, indicating that the smectite brought about by the Kuroshio Current is diluted by river-fed turbidity currents.
Clay mineral indices (illite crystallinity and illite chemistry index), as well as major element indices (chemical mobility, CIX index, K2O/(Na2O + CaO) molar ratio, and weathering trend ternary diagrams), in combination with Rb, Sr, and Sr-Nd isotopic results for surface sediments around Taiwan, indicate moderate chemical weathering in the Taiwan river sediments. Also suggest that the seafloor sediments around offshore Taiwan are highly correlated to Taiwan river sediments, indicating the high influence of Taiwan-derived sediments on the bulk fraction of offshore surface sediments around Taiwan. |
