摘要
珊瑚骨骼的微量元素比已廣泛地作為重建海洋環境的指標，如：Sr/Ca可指示海水表面溫度，而Ba/Ca可用以追蹤陸源的輸出量與海水湧升流強度的變化。本研究的微孔珊瑚(Porties, sp.)採自蘭嶼西北方外海，蘭嶼為一火山島，位於太平洋黑潮經過之處，又是颱風侵襲的最前哨，因此可利用蘭嶼珊瑚Ba/Ca變化量來探討其河川輸出和湧升流作用。為了取得更精確元素的比值，本研究評估不同清洗方法對珊瑚骨骼中元素比值的影響，慎選有效去除有機物的清洗方法，且將標準品與樣品的鈣濃度皆稀釋至100 ppm，以避免不同濃度的基質所產生的誤差。研究中也同時建立了高精確度珊瑚骨骼中次要(Sr/Ca和Mg/Ca)與微量元素(Ba/Ca、B/Ca、U/Ca、Pb/Ca和Mn/Ca)的分析技術，其精確度分別為±0.7%, ±0.8%, ±2.8%, ±3.0%, ±3.0%, ±5.2%和±5.7% (2)，足以用於解析珊瑚骨骼中微量元素的變化。
研究結果顯示珊瑚骨骼中Sr/Ca比值與海水表面溫度有高相關性，同時配合X-ray及螢光的條帶比對，可用以建立蘭嶼珊瑚1966-1997的年代模式。珊瑚Ba/Ca比值變化範圍為1.5-6.7 μmol/mol，且呈現週期性的變化，一年有兩個高峰值與一個低峰值。將Ba/Ca比值的變化與蘭嶼降雨量比對，可明顯發現降雨量與Ba/Ca比值具高度的正相關性；推論當降雨量增大，河流的陸源輸出量隨之增加，造成沈積物表面脫附至海水中的Ba也越高。證實珊瑚骨骼的Ba/Ca比值可忠實反映當地所發生的降雨事件或沈積物輸出。湧升流對Ba/Ca比值的影響，以現有的資料判定在受西南季風或颱風的影響之下，可能增大夏末秋初的湧升流進而產生Ba/Ca的峰值。在1966-1986年間，Ba/Ca比值在2 μmol/mol上下，然而在1986年Ba/Ca 比值急劇上升至3 μmol/mol之後，又趨於穩定，根據現有的資料，推測此一Ba/Ca比值的增高應與蘭嶼地區於1981年後設置為低核廢料放置廠所有關。此大規模的開發工程造成當地岩石裸露面積的增加，侵蝕程度及河流的輸砂量也隨之大幅增大，使得當地海水Ba/Ca比值上升。不論長時間尺度亦或短時間尺度，蘭嶼珊瑚深受本島陸源輸出物的影響，而降雨量的大小與人為開發都能使陸源輸出物增加，造成蘭嶼地區31年來Ba/Ca比值明顯改變。
關鍵字：鋇, 珊瑚, 蘭嶼

Abstract
Trace elements in coral skeletons have been widely used for reconstructing various environmental parameters in the ocean. For example, coralline Sr/Ca can be used to reconstruct the temporal changes in sea surface temperature. Furthermore, Ba/Ca ratio has also been proposed as an indicator of regional upwelling and/or sediment flux. In this research, Porites corals were collected from Lanyu Island, offshore southeastern Taiwan. The site selected often experiences abrupt climatic events, such as typhoons and heavy rainfall. Thus, the coralline Ba/Ca derived here can be used to systematically evaluate the potentials for tracing the intensity of upwelling and/or local rainfall. In order to obtain data with high precision, three different cleaning methods have been applied for testing the removing efficiency of organic matters on coral skeletons. Significant matrix effect on the determination of trace element contents can be overcome by diluting [Ca] to 100ppm in the sample solution. In addition, we developed a new analytical technique for simultaneously determining Mg/Ca, Sr/Ca, B/Ca, Ba/Ca, U/Ca, Pb/Ca and Mn/Ca ratios in the corals, and can obtain the high-precision measurements using SF-ICP-MS. The long-term precision (±2σ) is 0.7, 0.8, 2.8, 3.0, 3.0, 5.2 and 5.7%, respectively. The analytical precision is sufficient to observe the variation of trace elements in coral skeletons.
The chronology of the coral sample is based on Sr/Ca data, which is well correlated to SSTs, and also based on the growth bands using the X-ray radiograph. Ba/Ca ratio in Lanyu Island ranges from 1.53 to 6.72 μmol/mol and shows clear seasonal cycles with double peaks each year. Riverine Ba can be carried both by dissolved and particulate phases, so it is a powerful indicator for sediment flux and/or river discharge. Variations in coralline barium are correlated with the intensity of rainfall. The stronger the precipitation, the higher the river flux which carry large amounts of Ba. In the early part of the record (before 1986), the average value of Ba/Ca is ~2 μmol/mol. Following the nuclear waste dump in 1981, a significant increase of Ba/Ca can be clearly observed, possibly due to the elevated delivery of sediments after 1986. We attribute the elevated Ba during 1986-1996 to more intensive erosion due to land-use practices and the associated sediment discharge during wet periods. Whether on long or short time scale, river inputs have been identified as the major source of Ba in corals; however, precipitation and human activities can significantly affect the riverine Ba flux as well. On the basis of the presented data, it leads to conclude that the continental input is the major reason for controlling the variation of Lanyu Ba/Ca ratio over the past 31 years (1966-1997).

Key words: Ba/Ca, coral, Lanyu

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