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| 題名: | 台灣河流系統中的鉬同位素：它們對季節效應、岩石類型和風化模式變化的影響;Molybdenum isotopes in the river system of Taiwan: Their implications on changing seasonal effects, rock types, and weathering patterns |
| 作者: | 艾夏兒;EKKA, SHAIL VIJETA |
| 貢獻者: | 國際研究生博士學位學程 |
| 關鍵詞: | 鉬同位素;Molybdenum isotopes |
| 日期: | 2023-07-18 |
| 上傳時間: | 2024-09-19 16:14:16 (UTC+8) |
| 出版者: | 國立中央大學 |
| 摘要: | 本研究調查鉬（Mo）之地球化學特性以及化學風化與侵蝕對鉬同位素造成的同位素分化，以具有活躍構造活動與高海拔的島嶼-台灣作為研究對象。本研究收集並測量了25個主要河流集水區的鉬濃度及其同位素比值，並對河床母岩及河床沉積物進行分析以鑑別鉬的可能來源。於雨季期間，河流中的鉬濃度變化範圍為1.94 ~ 45.09 nM，δ98/95Mo變化範圍為 -0.28‰ ~ +1.60‰，平均值為 +0.95 ± 0.31‰（1SD，n = 42）。在乾季時，鉬濃度變化範圍為2.15 ~ 58.27 nM，δ98/95Mo變化範圍為 -0.23‰ ~ +1.34‰，平均值為 +0.99 ± 0.28‰（1SD，n = 43）。臺灣河流集水區的δ98/95Mo比值顯示僅具有些微季節性的變化。河流中的δ98/95Mo比值比岩石要重。若欲解釋本研究觀察到的鉬同位素分化現象仍具有許多後續的工作，主要是因為這些河流流經不同的岩性地區。河床沉積物中的δ98/95Mo同位素組成與微量元素（如鈮、錳和鈦等）濃度呈負相關。這些微量元素富集在細顆粒的鈦鐵氧化物中，如榍石、金紅石、鈦鐵礦和鈮鐵礦。因此，河床沉積物傾向抓取輕的鉬而使得溶解態的鉬同位素比值變重。再者，臺灣河流向大洋輸出了大量的鉬，並且平均δ98/95Mo為 +1.0‰，比世界河流平均值（約 +0.8‰）重。
除此之外，本研究另一重點為利用鉬同位素作為示蹤劑來探討淡水河(受污染)與立霧溪(未受污染)的河流傳輸過程及化學風化過程。淡水河鉬同位素比值於雨季期間變化為 +0.83‰ 到 +1.50‰，而在乾季期間變化為 +0.54‰ 到 +1.25‰，若去除極端值之影響，則雨季具有較輕的鉬同位素比值。立霧溪的鉬同位素比值變化範圍為 +0.54‰到 +1.30‰，往下游比值有漸增重之趨勢。淡水河中主要的鉬來源為海水、水-岩反應之來源以及人為活動之輸入，各端源之貢獻可以透過MixSIAR混合模式來估算。在立霧溪中，較重鉬同位素比值應為黃鐵礦氧化以及碳酸鹽風化所致，而輕的鉬受到二次生成礦物相(如鐵錳氧化物)抓取而進入河床沉積物。
這些觀察到的現象，可以幫助釐清受化學風化以及人為活動影響下河川鉬同位素比值的變化，並提供現今全球海洋鉬循環更正確之制約。另一方面，本研究之結果也顯示短時間的尺度，如季節性的變化，不會對鉬同位素的循環造成顯著的變化。
總結來說，本論文之分析結果，突顯出鉬同位素應用於探討台灣河流之化學風化過程，以及鉬同位素分化的機制上，提供新的見解與研究的方向。本研究同時也顯示鉬在環境地球化學上示蹤之潛力。
;This study investigates the behavior of molybdenum (Mo) geochemistry and the mechanism that controls Mo isotopic fractionation via weathering and erosion in a tectonically active high-stand island, 25 major river catchments in Taiwan was measured for Mo concentration and its isotopic composition. For source identification, bedrock, and riverine bedload sediments were also examined. Riverine Mo concentration ranges from 1.94 to 45.09 nM, and the δ98/95Mo ranges from -0.28‰ to +1.60‰, with an average of +0.95 ± 0.31‰ (1SD, n = 42) for the wet season. In the dry season, Mo concentration ranges from 2.15 to 58.27 nM and the δ98/95Mo from -0.23‰ to +1.34‰, with an average of +0.99 ± 0.28‰ (1SD, n = 43). The δ98/95Mo composition in the Taiwan river catchment shows little seasonal variation. Riverine δ98/95Mo signals are heavier than the bedrock. The observed Mo isotopic fractionation is challenging to explain because these rivers drain across different lithologies. Moreover, the δ98/95Mo isotopic composition in riverine bedload sediments negatively correlates with trace element (e.g., Nb, Mn, and Ti) concentrations. These trace elements are enriched in fine-grain residual Fe-Ti oxides such as titanite, rutile, ilmenite, and niobite. Thus, riverine bedload sediments may act as a light δ98/95Mo sink that makes the riverine dissolved load heavier. Furthermore, Taiwanese rivers discharge a significant Mo flux and a heavier mean δ98/95Mo (+1.00‰) to the oceans than world rivers (~ +0.8‰).
Additionally, this study also focused on Mo isotopes as a source tracer, for which Danshuei (polluted) and Liwu (non-polluted) rivers in Taiwan were chosen to analyze Mo isotopes during weathering and riverine transport. The δ98/95Mo in the Danshuei river ranges from +0.83‰ to +1.50‰ in the wet season and +0.54‰ to +1.25‰ in the dry season, respectively. The wet season is lighter, while the dry season is slightly heavier, with some outliers. The δ98/95Mo ranges from +0.54‰ to +1.30‰ in the Liwu river, becoming heavier downstream. Three Mo sources are identified in the Danshuei river using the MixSIAR model: seawater intrusion, rock–water interaction, and anthropogenic inputs. The heavy δ98/95Mo signature in the Liwu river is likely driven by pyrite oxidation and carbonate weathering, while light δ98/95Mo is sequestered into secondary mineral phases (Fe-Mn oxides) in bedload sediments.
Furthermore, these findings have significant implications for weathering-controlled riverine Mo and anthropogenic Mo sources delivered to the ocean and will help better constrain the global Mo cycling in modern seawaters, while short-term seasonal changes tend to show an insignificant effect.
Overall this thesis highlights the application of Mo-isotope systematics as a promising proxy to understand the Mo isotopic fractionation mechanism during continental weathering in small mountainous rivers (SMRs) of Taiwan. In addition, it also emphasizes source tracing applications of Mo in environmental geochemistry. |
| 顯示於類別: | [地球系統科學國際研究生博士學位學程] 博碩士論文
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