系統識別號 U0026-1606201419220100
論文名稱(中文) 利用珠江河口沉積物鈾釷同位素示蹤大陸風化過程及河口氧化還原環境之研究
論文名稱(英文) Tracing continental weathering and estuarine redox conditions with U/Th isotopes in Pearl River estuarine sediments
校院名稱 成功大學
系所名稱(中) 地球科學系
系所名稱(英) Department of Earth Sciences
學年度 102
學期 2
出版年 103
研究生(中文) 蔡孟涵
研究生(英文) Meng-Han Tsai
學號 L46014063
學位類別 碩士
語文別 英文
論文頁數 70頁
口試委員 指導教授-羅尚德
中文關鍵字 自生相和碎屑相  鈾系不平衡  化學風化  氧化還原環境  珠江河口 
英文關鍵字 Authigenic and Detrital phases  U-series disequilibrium  Chemical weathering  Redox environment  Pearl River Estuary 
中文摘要 河口區域是大陸風化物質進入海洋的一個重要通道,並且在此區域伴隨著許多生物地球化學反應的發生。為了瞭解大陸風化過程及沉積環境的生物地球化學過程,本研究設計了一種新的方法,以6N鹽酸溶液淋溶並分析沉積物顆粒中「自生相」及「碎屑相」的放射性核素210Pb、234U、238U、230Th、232Th、228Th和228Ra,並根據鈾系同位素不平衡現象探討所代表的大陸風化歷程及河口沉積環境隨時間的變化。
結果顯示,P11沉積物岩芯經由過剩210Pb定年方法計算所得之平均沉積速率約為3.56±0.89 公分/年,且在過剩210Pb活度分布中觀察到具有規律性周期變化現象。自生相中234U、238U和230Th活度隨深度減少有增加的趨勢,意味著珠江河口的水體及沉積環境從過去到現在更趨向於還原性環境,使得鈾釷同位素在還原性環境下因化學特性而富集,此結果亦可由(228Th/228Ra)活度比值隨深度的變化及P11 沉積物之總有機碳分布來驗證,並且進一步了解各層位間氧化還原環境之變化。碎屑相中(228Ra/232Th)比值大致上都高於1,顯示出在深部層位有228Ra的富集,推測其原因可能與硫酸鋇礦物存於沉積物中有關。碎屑相中所觀察到(234U/238U)比值接近或等於1與(230Th/234U)比值大於1,都指示著此沉積物過去主要是受到化學風化過程為主。本研究也發現210Pbex 與化學風化之間的變化具有關聯性,且周期恰巧與「聖嬰現象南方震盪」及「太陽黑子」之氣候循環事件周期相符,這些訊號可能記錄著珠江河口氣候環境變化的重要歷程。
英文摘要 Estuary is an important region where the fluxes of dissolved and particulate weathering materials in rivers transport from continent into the ocean and lots of biogeochemical reactions occurred. To understand the weathering processes and deposited environment recorded in sediments. This study employed a new method to investigate the U/Th isotope disequilibrium respectively in authigenic and detrital phases of estuarine sediments and showed that U-series disequilibrium in authigenic and/or detrital phases of estuarine sediments could provide useful information on the changes of continental weathering and/or land-sea interactions at the estuary in the present and past. A sediment core (P11) was collected from the Pearl River estuary and sediments sampled from various depths of the core were further separated into the authigenic and detrital phases using 6N HCl leaching technique which has been confirmed that do not affect the U/Th activity ratio with a set of leaching experiment. U-series radioisotopes, 210Pb (via 210Po), 234U, 238U, 228Th, 230Th, 232Th and 228Ra in authigenic and detrital phases were measured respectively with alpha-counting techniques following radiochemical purification procedures.
Results show that excess 210Pb activity exhibits frequent fluctuation with a general decreasing trend, suggesting periodic changes in 210Pb flux driven by climate change. From this decreasing trend, the average sedimentation rate of P11 core is dated to be 3.56 ± 0.89 cm/yr. In the authigenic phases, both 238U and 234U activities decrease with the depth, implying that the estuarine water and the underlying sediments become more and more reducing toward the present which supported by the (228Th/228Ra) activity ratio. (228Ra/232Th) ratios show that there is an enrichment of 228Ra and seem to correlate with the BaSO4 in detrital phase. The (234U/238U) ratio in the detrital phase is measured to be mostly near unity, reflecting that the Pearl River sediments have experienced a relatively rapid chemical weathering. The relatively lower ratios at some depths are mostly due to alpha recoil effect in association with the physical weathering of rocks. This conclusion is further supported by (230Th/234U) in the detrital phases, with (230Th/234U) > 1 indicating the preferential release of 234U due to chemical weathering. The correlation between chemical weathering and 210Pbex can also be found and is coincident with the significant climate circulation.
This study shows that by separating authigenic from detrital phases, uranium isotope disequilibrium can provide more valuable information than that based on total sediments. It will be a useful tool to understand a variety of continental/estuarine processes such as the origin of estuarine sediments, the chemical/physical weathering rate of continental rocks, and changes in estuarine biogeochemical process driven by human activity and/or natural climate change.
論文目次 摘要 I
Abstract III
Acknowledgments V
Contents VI
List of Figures IX
List of Tables XI

1. Introduction 1
1-1 Uranium-series isotopes and disequilibrium 1
1-2 Alpha-recoil effect 3
1-3 U-series disequilibrium during weathering and transported processes 4
1-4 Definition of Authigenic and Detrital phases 7
1-5 Pearl River Estuary 8
2. Material and analytical methods 9
2-1 Study area 9
2-2 Experimental analytic procedures 10
2-2-1 U-Th isotope analysis 10
2-2-2 X-ray diffraction analysis 11
2-3 Instrument and Methods 13
2-3-1 X-ray Powder Diffractometer 13
2-3-2 Alpha Spectrometer 14
2-3-3 Isotope dilution method 16
2-3-4 Leaching experiment 17
3. Results 23
3-1 210Pb activity 23
3-2 Uranium isotopes and (234U/238U) activity ratio 23
3-3 Thorium isotopes and U/Th activity ratios 24
3-4 Radium isotopes and Th/Ra activity ratios 25
4. Discussion 36
4-1 Evaluation of 6N HCl leaching method 36
4-1-1 Effects of mineral composition of rocks 36
4-1-2 Effects of grain size 42
4-2 210Pb dating of P11 sediment core 42
4-3 The geochemical/environmental indicators of U/Th isotopes in authigenic phase of sediments 45
4-4 U-Th isotope ratios in detrital phase of sediments: An indicator of rock weathering intensity 49
4-5 232Th-228Ra-228Th disequilibria in both authigenic and detrital phases of sediments: insight on the groundwater Ra transport associated with paleo-redox changes 52
4-6 Climate changes recorded by U-series radioisotopes in P11 sediment core 55
5. Conclusions 58
References 60
Appendix 69
1. Activity calculation- 228Th 69
2. Activity calculation- 228Ra 70
3. Element analysis of P11 sediment core 71
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