系統識別號 U0026-0812200913565558
論文名稱(中文) 利用矽藻中鈾同位素比值探討台灣的風化歷史
論文名稱(英文) Use of Diatomaceous Uranium Isotope Ratio in Lake Sediments to Study the Past Changes of Weathering in Taiwan
校院名稱 成功大學
系所名稱(中) 地球科學系碩博士班
系所名稱(英) Department of Earth Sciences
學年度 95
學期 2
出版年 96
研究生(中文) 蔡侑蓉
研究生(英文) Yu-Jung Tsai
學號 l4694404
學位類別 碩士
語文別 英文
論文頁數 53頁
口試委員 召集委員-扈治安
中文關鍵字 矽藻  鈾同位素  風化 
英文關鍵字 ICP-MS  chemical weathering  physical weathering  uranium isotope  diatom 
中文摘要 U-238是個具有長半衰期(4.468×109yrs)的放射性核種,它在自然界中不斷地進行自發性的放射,中間歷經一些短半衰期的子核種(ex:Th-234, Pa-234,U-234…等),最終衰變成穩定核種Pb-206;理論上在一封閉系統中,U-238及其子核種將會達到世紀平衡,但自然界中,我們可以發現到處都存在有不平衡現象,這是由於物理、化學和生物作用過程中所造成的。當U-238衰變成U-234時,會放出一個α粒子,此時由於α-recoil效應,造成U-234會存在礦物的晶格缺陷或在表面,當水流過岩石時會先將U-234 淋洗出來,造成水體的U-234/U-238比值大於1。而另一方面,鈾在一般情況下是以+4價的型態存在於岩石中,但在氧化的環境下,會形成+6價型態,並會與水溶液中的碳酸根離子形成穩定的UO2(CO3)34-,經由化學風化作用而進入到水溶液,造成鈾濃度的上升。由於鈾的同位素與濃度均和風化作用有此關聯,因此它們可以被用來作為研究地表的物理及化學風化變化的指標。矽藻中鈾同位素的比值可用來探討過去地表物理及化學風化的變化。為此,本研究建立了一套可有效由沉積物中分離出矽藻的方法,並以台灣的湖泊沉積物為例,提出湖相沉積物中矽藻紀錄的鈾同位素比值是研究過去台灣地表風化歷史有用的指標。實驗中,我們採用了重液分離法及六偏磷酸鈉懸浮法,可以將矽藻有效的從沉積物中分離出來,且將矽藻清洗乾淨後,使用0.5N NaOH溶解矽藻,看其鈾同位素比值及U/Si值,將此分離技術應用在APEC II計畫(Asian Paleo-Environment Changes II)在宜蘭雙連埤所鑽研的岩心上,發現其鈾同位素比值與碳氧同位素比值有一定的關係存在,證明鈾系應用在矽藻上的可行性,並可以補足在碳酸鹽區缺乏區域上的研究。此分析技術將可應用至鈾系定年法,古氣候學,及岩石地球化學之相關研究。
英文摘要 Uranium-238 is a radionuclide with a half-life of 4.468×109 yrs. It decays to a stable nuclide lead-206 through a series of short-lived radionuclides. Because of alpha-recoil effect, uranium-234 will exist preferentially on mineral surface or defect in mineral lattice when uranium-238 decays to uranium-234, causing preferential transport of uranium-234 to lakes or oceans when water flows through the rocks. On the other hand, uranium element exits in 4+ oxidation state in the primary igneous rocks and minerals, but uranium can be oxidized to 6+ states in oxidized environments. The 6+ oxidation state forms soluble uranyl complex ions (UO22+) transported to lakes during the weathering. This unique feature makes uranium concentration and uranium isotope ratio very useful to determine the changes of paleo-intensity of physical and chemical weatherings on the Earth surface. This work aims at using uranium isotopes in lacustrine diatomaceous microfossils as a proxy to investigate the past changes in the rate of chemical and physical weathering in Taiwan island during the past 10,000 years. Sediment cores were collected from Lake Shuang-Lien Pi in Taiwan and diatoms were separated from the sediments and cleaned to measure their U/Si and 234U/238U ratio by ICP-MS.
Measurements of 234U/238U in diatom tests, together with those of δ13C, δ18O, TC and TOC in lake sediments, provide rich information on the paleoenviromental and paleoclimatic changes in the Taiwan. Such multi-proxy approaches provide a useful calibration and validation on the proposed use of uranium isotope ratios as a proxy of paleo-weathering intensity on lands.
論文目次 摘要 I
Abstract II
Acknowledgments III
Table of Contents IV
List of Tables VI
List of Figures VII
Chapter 1 Introduction 1
1.1 Materials 2
1.2 Uranium isotope behavior during rock weathering 3
1.3 Mobility of uranium caused by chemical weathering 5
Chapter 2 Separation and purification of diatom tests from sediments 8
2.1 Sediments 10
2.2 Physical separation of diatoms from sediments. 10
2.3 Chemical cleaning of diatom tests 15
2.4 Dissolution of diatom tests 18
2.5 Analytical techniques 21
2.5.1 HR-ICP-MS analysis 21 Elemental analysis 22 Uranium and thorium isotope analysis 24
2.5.2 SEM and EDS analysis 25
Chapter 3 U-Th isotope diseqilibrium in diatomaceous earth 28
3.1 The time series of cleaning diatom tests by 3N HNO3 29
3.2 Compare the U-Th isotopes in diatom tests and clay 30
Chapter 4 U isotopes in lake sediments in Taiwan 33
4.1 Sample location 34
4.2 δ18O, δ13C, TC, OC and IC data 35
4.3 Uranium isotope data 40
Chapter 5 Conclusions 42
References 44
Appendix 50
Appendix 1: The series numbers vs. depth in SLP. 50
Appendix 2: The raw data of δ13C, δ18O, TC(total carbon), OC(organic carbon), IC(inorganic carbon) data in sediment from Lake SLP in Taiwan. 51
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