環境變遷對於南海東北部沉積物中生物矽含量時序變化之影響

The Effect of Environmental Change on Biogenic Silica Temporal Variation in Sediments of the Northeastern South China Sea

蔡旭濱(Hsu-Pin Tsai)

指導教授 ： 林曉武

國立臺灣大學/理學院/海洋研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.11059

摘要

南海東北部的海洋沉積物中不僅含有矽質殼體及碳酸鈣殼體之海源物質沉積，更深受周圍陸地（台灣、中國大陸、菲律賓等）河川所傳輸之陸源物質所影響。海源物質生產與沉積主要受控於表層水溫變化、營養鹽多寡、陸源物質輸入稀釋及碳酸鈣補償深度，因氣候變遷所改變之海水溫度、海面高度及海表初級生產力，都會影響到海源物質的沉積。陸源物質之傳輸亦受到風化強度、岩石組成、降雨多寡所影響，以台灣河川為例受到高年降雨量、陸地高度等因素，年輸出懸浮顆粒可高達近400萬噸之巨，對於鄰近海域沉積物沉降量變化有巨大之影響。雖然近年來對鄰近海域陸源物質傳輸與沉降有相當多之研究，但對於海源顆粒，尤其是生物矽顆粒之沉降與年代受氣候變遷之研究卻為數不多。
海源沉積物主要組成有生物矽、有機碳及碳酸鈣等。生物矽（Biogenic Silica, BSi）是指利用化學分析方法所測定的無定形矽含量，也稱為生物性蛋白石或簡稱為蛋白石（Opal）。沉積物當中的生物矽主要來源是矽藻、放射蟲、矽鞭毛藻和海綿骨針等矽質生物死亡之後其骨骼沉降堆積在海床上。在古海洋學研究中，生物矽沉積紀錄可以被用以指示古生產力的變化，而這些變化與古海水營養鹽狀況及古氣候狀況的變化有著密切相關，因此生物矽沉積紀錄可指出大尺度的古海洋或古氣候（如冰期、間冰期）的變化。
研究結果發現南海東北部沉積物中，測站AG（北緯20.05度，東經118.10度）的生物矽含量介於3.68% ~ 9.20%之間，平均值5.64 ±1.92%；測站2914（北緯22.03度，東經119.85度）的沉積物生物矽含量介於1.63%~6.40%，平均值3.57 ±1.47%。從空間上的尺度來看，距離台灣較遠的測站AG其生物源的沉積量比距離台灣較近的測站2914高出許多；從時間上的尺度來看，兩測站生物矽含量皆以全新世時期的含量最低，末次冰期的含量最高，而轉換期次之。將此生物矽含量與有機碳含量做比較，發現此兩者有著很好的相關性，尤其是在冰期時R2值甚至高達0.70，而在全新世時期的相關性則偏低。
綜合上述結果顯示南海東北部靠近台灣端的海底沉積物受到台灣輸出大量懸浮顆粒所稀釋，造成其生物矽含量明顯較低，而在轉換期與末次冰期時南海東北部沉積物中有機碳貢獻量主要由矽質殼體生物所提供，且生物矽含量較全新世時期高。

關鍵字

南海 ； 古氣候 ； 末次冰盛期 ； 生物矽 ； 化學萃取

並列摘要

Biogenic silica, as well as carbonaceous particles, are the most important types of biogenic sediments. For the South China Sea area where huge amount of terrigenous particles from small rivers of the surrounding islands and large rivers play a very important role in transporting land derived materials to the ocean, however, the exact amount of terrigenous materials from land to the ocean may subject to climate change and varied in times. In addition, sea temperature fluctuation may also affect ocean productivity in the area. There is little study on the variation(s) of sediment deposition with respect to the temporal variation, in particular, and area subjecting to climatic changes.
Biogenic sediments mainly compose of biogenic silica, organic carbon and calcium carbonate. Biogenic Silica (BSi) is the amorphous silicon content which is usually determined by wet chemical extraction. It is also known as or referred to as biogenic opal (Opal). Main sources of the biogenic silica in sediment come from diatoms, radiolarians, silicoflagellates and sponge spicules and other siliceous organisms, with their skeletons accumulated on the seabed. Biogenic silica content variation in sediments usually indicate a change(s) of oceanic paleoproductivity. Changes of paleoproductivity were closely related to the status of paleoseawater nutrient and the change of paleoclimatic condition. Hence, sedimentary record of biogenic silica could serve as an indicator of large scale changes of the ancient oceans or paleoclimatic (e.g. glacial or interglacial) change.
The result of this study showed that biogenic silica content in sediments in the northeastern South China Sea at sampling site AG (ssAG; Latitude: 20.05° N, Longitude: 118.10° E) range between 3.68% and 9.20%, with an average of 5.64 ± 1.92%, and at site 2914 (ss2914; Latitude: 22.03° N, Longitude: 119.85 ° E) range between 1.63% and 6.40%, with an average of 3.57 ± 1.47%. From the perspective of spatial scales, ssAG is further away from Taiwan compare to ss2914. ssAG has much higher level of biological source of sediment. The amount of biogenic silica in both ssAG and ss2914 are relatively low for Holocene and high during the last glacial. In between, the amount of biogenic silica correlate very well with the amount of organic carbon, especially in glacial period where R2 reach up to 0.70. This correlation is much weaker during Holocene.
In conclusion, large amount of suspended particles from Taiwan do play an important role in diluting biogenic sediments. During the last glacial and the transition period, siliceous shell was a major source of sediment organic carbons in the northeastern South China Sea’s sediments. The amount of biogenic silica in last glacial is relatively higher than in Holocene.

並列關鍵字

South China Sea ； paleo-climate change ； Last Glacial Maximum ； biogenic silica ； chemical extraction

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