利用珊瑚骨骼硼同位素組成評估南海過去三百年海洋酸化速率及建立巨昇華純化方法應用於硼元素分離

• Main Authors: 王子豪, Wang, Tzu-Hao
• Other Authors: 地球科學系, 游鎮烽, You, Chen-Feng
• Format: Thesis
• Language: English
• Published: 2016
• Subjects: 海洋酸化; 硼同位素; pH值代用指標; 南海; 純化技術; ocean acidification; boron isotopes; pH-proxy; coral; South China Sea; purification technique; Pacific; Ocean acidification
• Online Access: http://ir.lib.ncku.edu.tw/handle/987654321/168595; http://ir.lib.ncku.edu.tw/bitstream/987654321/168595/1/index.html

海洋經由海洋─大氣交換吸收大氣中濃度逐漸升高的二氧化碳，溶解於水中的二氧化碳使得海洋酸化，對於海洋鈣化生物的生理與海洋生態系統造成顯著的影響。受限於現今海水pH值記錄在不同空間與時間尺度上的不足，海洋酸化實際的變化量與變化速率目前仍尚未釐清。以南海與西太平洋區域而言，更是少有長時間尺度的海水pH值記錄。因此，本研究將提供來自南海北部西沙群島連續300年珊瑚骨骼中的硼同位素組成記錄，來彌補這部分紀錄的不足與評估此區域海洋酸化的情形。對於珊瑚的採集、樣品選擇到樣品保存的過程皆小心處理避免可能的汙染。藉由X光影像與鈾─釷定年的結果，珊瑚樣品根據與定年點的距離回推相對年代，以5年時間解析度進行分樣以利後續分析。從珊瑚的清洗方法、硼元素純化技術、儀器穩定度與精確度和樣品代表性與再現性的評估，每一步驟利用與已知硼同位素組成(δ11B)的實驗室標準品Alfa-B與珊瑚標準品JCp-1進行比對，評估本研究量測得出的δ11B資料的可信程度。量測珊瑚樣品JCp-1硼同位素組成的精確度約在24.3±0.4‰(2SD)，約等於0.05個海水pH單位的變化。此批珊瑚的δ11B在21.4‰至26.0‰之間變化，換算成海水pH值變化約在7.74與8.38之間，筆者依照海水pH值的變化趨勢將本研究近300年的珊瑚記錄分成三個時間間段：(1)階段一，西元1680至1855年，海洋酸化速率約 -0.00005 pH單位/年；(2) 階段二，西元1860至1877年，海洋酸化速率約 -0.0228 pH單位/年；(3) 階段三，西元1882至1960年，海洋酸化速率約 -0.0014 pH單位/年。本研究的δ11B資料與其他同時期南海與太平洋地區的珊瑚δ11B記錄在多數時間有一致的變化趨勢，指示著海洋與大氣交換的作用是全球性的現象。在西元1850至1960年之間，南海的海洋酸化速率(-0.0004~-0.0021 pH單位/年)普遍大於大堡礁珊瑚區 (-0.0003~+0.0006 pH單位/年)，其中，西沙群島的海洋酸化速率是所有比較區域裡最快的，約為-0.0021 pH單位/年。結果指出，在工業革命後，西元1850至1960年之間，海洋酸化在全球是不均勻的發生著，在當時，南海北緣偏向為大氣中二氧化碳的儲集處，而大堡礁區則偏向為一供給區，區域性的海洋─大氣交換或其他控制因子可能是造成本研究結果的可能原因，仍有待未來的研究與評估。 目前硼的微昇華純化技術受限於應用體積(~50μL)的限制，有些低濃度的樣品並不適用。本研究亦介紹新發展的硼元素純化技術─巨昇華，搭配兩種實驗室自行設計的裝置，將系統性地評估其可行性與潛在優勢。初步結果指出，兩種裝置至少能提供相較目前微昇華技術高10倍的應用體積，使得純化過程更有效率，也因此能省去原先純化後必須稀釋才能上機量測的步驟。若能在未來對於此技術作更完整的測試與評估，藉由新的純化技術，往後硼同位素組成的量測將能應用在本身濃度較低的樣品上，拓展硼同位素系統應用之範疇。 Ocean acidification (OA) caused by the uptake of elevated atmospheric CO2 can impact profoundly on the physiology of marine calcifiers and entire ocean ecosystem. Due to the spatial and temporal limitation on currently available seawater pH records, the actual variability and ocean acidification rate are poorly known. Especially, only a few centennial-scale seawater pH record studies were conducted in the South China Sea (SCS) and western Pacific Ocean region. Here, we provide a 300-year Porites sp. coralline δ11B record from Xisha Islands, northern SCS. The collection, selection and storage of coral samples were performed carefully to avoid potential contamination. Based on the X-ray photographs and U-Th dating technique, the coral samples spanning from 1680 AD to 1960 AD were sub-sampled manually in 5-year time resolution. Boron isotopic determination methodology was systematically evaluated with δ11B-known referential materials including in-house standard Alfa-B and coral standard JCp-1, i.e., cleaning protocols, instrumental stability and accuracy, purifying technique and reproducibility of coralline δ11B records. The results of above assessment suggest that our δ11B data are reliable and applicable, and the analytical uncertainties of δ11B in JCp-1 (24.3±0.4‰) were equal to an uncertainty of ~0.05 pH unit (2SD) in pH reconstruction. The δ11B values of corals ranged from 21.4‰ to 26.0‰, corresponding to seawater pH range between 7.74 and 8.38. The seawater pH variation could be divided into three stages, i.e., (1) 1680-1855 AD, at a OA rate of -0.00005 pH unit/yr; (2) 1860-1877 AD, with OA rate -0.0228 pH unit/yr; (3) 1882-1960 AD, at a OA rate of -0.0014 pH unit/yr. Our δ11B data covaried with other western Pacific coral δ11B record at most of the time, reflecting the global ocean-air interaction of increasing atmospheric CO2. During 1850 AD to 1960 AD, the OA rates in the SCS area (-0.0004~-0.0021 pH unit/yr) were higher than that in the Great Barrier Reef(GBR) region (+0.0003~+0.0006 pH unit/yr). The OA rate was the highest one in Xisha Islands at a value of -0.0021 pH unit/yr. The preliminary results suggest that ocean acidification occurred heterogeneously in the time period 1850 AD to 1960 AD soon after the Industrial Revolution. The northern SCS region tended to be a sink of atmospheric CO2, whereas the GBR region tended to be a source of CO2. Regional ocean-air interaction or other controlling factors are possible explanations of the differences mentioned above and require further assessments. Regarding to the small volume (~50μL) limitation on currently applicable micro-sublimation protocol, here we developed a novel macro-sublimation technique with two new-designed devices. The preliminary results show that a ten-fold improvement on sample volume is achievable and provided the settings of optimal performance for two devices. By applying the novel technique, additional dilutions after sublimation are not required, and the boron isotopic determination can be done on samples with lower boron concentration. Further investigations and experiments are still necessary for a systematic evaluation of the technique.