進階搜尋


下載電子全文  
系統識別號 U0026-0906201715210700
論文名稱(中文) 湖泊自生礦物(燧石)鈾系同位素年代學及古環境意義之研究
論文名稱(英文) Studies on chronological and paleo-environmental implications of U-series disequilibria in lacustrine authigenic minerals (chert)
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 105
學期 2
出版年 106
研究生(中文) 陳素瑩
研究生(英文) Su-Yin Tan
學號 L46025014
學位類別 碩士
語文別 中文
論文頁數 72頁
口試委員 指導教授-羅尚德
口試委員-魏慶琳
口試委員-李時雨
中文關鍵字 馬加迪燧石  氣候變遷  鈾系不平衡  非洲古環境 
英文關鍵字 Magadi chert, climate change  U-series disequilibrium  paleo environmental of Africa 
學科別分類
中文摘要 燧石是許多鹼性湖泊中通過熱液活動形成的矽酸鹽為主的自生礦物。它記錄著與湖泊環境和構造活動有關的古氣候、古水文等重要訊息。本研究採自東非肯尼亞裂谷區南端馬加迪湖(Lake Magadi)的燧石為標本,分析其燧石中鈾系同位素的濃度變化,並進一步探討其鈾系定年之可行性。
燧石樣品洗淨烘乾去除表面沉積物後,利用非破壞性γ能譜法(gamma spectrometry)測量其放射性同位素活度。結果表明樣品中鈾系同位素比釷系同位素的活度高一個數量級,說明在形成燧石的過程中有U-238參與,而由外來碎屑礦物帶來的Th-232與Th-230可忽略不計。經γ能譜法初步評估後,燧石樣品經HF消化全溶、離子交換分離出铀和釷同位素並使用α譜儀(alpha spectrometry)與ICP-MS進行測量。
定年結果顯示所有燧石樣品都是在更新世形成的,LMC1與LMC3形成時間較早,是在MIS 6形成;LMC2形成時間較長,中間部分最老,形成於MIS6,往上下兩次年齡越來越小,在MIS 5停止生長。
根據本實驗結果顯示:(1)純燧石樣品幾乎不含初始Th-230,因而不需要使用等時線測年就可以獲得準確的年齡;(2)樣品中不同的部位其鈾系年齡有顯著差別。通過對樣品LMC2分層取樣發現從邊緣至中心,其年齡介於 74.5 ± 1.3Ky 和 177.8 ± 4.6Ky 之間,指示形成燧石的環境在期間內有乾濕週期變化。(3)在MIS 6形成的白色燧石含有較低的U活度,代表馬加迪湖湖水量較大,而在MIS 5形成的燧石顏色較深且U 活度高,是由於氣候變乾燥湖水蒸發濃縮造成的。(4)根據所測得之燧石年齡可對照出馬加迪湖氣候在MIS 6最濕潤,進入MIS 5變得乾燥,並有著約兩萬年乾濕週期變化。
英文摘要 SUMMARY
This study is focused on chronological and paleo-environmental implications of U-series disequilibria in lacustrine authigenic minerals (chert) in East Africa. Chert is a Pleistocene silica, precipitated from alkaline fluids around a former hot-spring. The chert samples after radiochemical purification procedures were further investigated for their naturally-occurring uranium-series radioisotope, e.g. uranium (238U and 234U), thorium (232Th, 230Th) respectively by using alpha-spectrometer and MC-ICP-MS. The major goal of this study is to: (1) assess the reliability of uranium-series dates on chert, (2) develop a method for chert U-series dating, (3) precisely date the age of chert samples focused on their respective 230Th/234U and 234U/238U measurements by MC-ICP-MS, and (4) based on the measurement uranium- and thorium- isotope ratios in the cherts, to understand the changes of paleo-environments of Lake Magadi. The purpose, methods, major results, and conclusions are summarized as follows.

INTRODUCTION
African paleo-climate and paleo-environmental changes are affected by the African monsoon and cause some wet and dry cycles during the Pleistocene. Some paleolake margins are 60m above the modern Lake Magadi level, and lacustrine authigenic minerals (chert) are deposited during these humid episodes. Lake Magadi is presently covered by a thick trona (Na2CO3·NaHCO3·2H2O) crust, which indicates that water chemistry is different from the past. The chert formation is related to the chemical change of the lake, and the chemical change of the lake is related to the environmental change. By studying of U-series disequilibria in lacustrine authigenic minerals (chert), we can understand the past changes in the environment of Lake Magadi.
The major goals of this study are to: (1) assess the reliability of uranium-series dates on chert, (2) develop a method for chert U-series dating, (3) precisely date the age of chert samples based on their respective 230Th/234U and 234U/238U measurements by MC-ICP-MS ,and (4) focused on the measurement uranium- and thorium- isotope ratios in the cherts, to understand the changes of paleo-environments of Lake Magadi.

MATERIALS AND METHODS
The Magadi chert samples were collected from a former hot-spring outcrop. In the first part of the experiment, we have to provide a preliminary analysis of the suitability of the chert for U-series dating. LMC1 and LMC2 were cut into several slices, which were removed any loose material from the surface of rock samples by watering and brushing, and dried in an oven at 60℃. A small slice of the sample was crushed into small pieces and put into a plastic jar to counter for the U-series and Th-series isotopes by ORTEC high-resolution, low-background gamma spectrometer.
In the second part of the experiment, the samples were further crushed into smaller pieces. LMC1 and LMC3 were separated based on their color (white or green), while LMC2 was cut into 8 parts basically by its growth stratigraphy. We used the HF-HNO3-HClO4 digestion method to totally dissolve the chert samples. Before we dissolved the sample, spikes were added as yield tracer. After the sample dissolved totally, U and Th were separated and purified from the sample by anion exchange methods. The naturally-occurring uranium-series radioisotope of U (238U or 235U, 234U) and Th (232Th, 230Th) are measured respectively by alpha spectrometry and MC-ICP-MS.

RESULTS AND DISCUSSION
The Magadi chert samples are massive or layered crystals, and they are dense in structure. Pure cherts incorporate a significant amount of uranium and very little of thorium into their minerals from the lake and/or ground waters during their formation. Individual chert samples with high U/Th ratios are pure enough for U-series dating and provide a constraint on the true age of the chert formation. These samples are very suitable for uranium-series dating.
The dating result shows that white portion of chert LMC1 was formed 165.4 ky b.p. and green portion is much younger than the white portion (135.8 ky b.p.). Bedded chert LMC2 was formed from 74.5 ky b.p. to 177.8 ky b.p. LMC3, the oldest chert sample was formed from 163.0 ky b.p. to 191.8 ky b.p.
Different portions of the chert deposits have different age, and they were formed in different time periods, even though they were in the same deposit. Isochron dating can also be applied to chert dating, however, it can only give an apparent age which does not agree with the true age of the sample. Therefore, a caution must be used in isochron dating if the sample does not satisfy the coeval assumption.
The cherts formed in MIS 6 contains lower uranium activity, and the cherts uranium activity formed in MIS 5 were significantly increased, which was the result of lake evaporation, indicating that the lake climate becomes more arid.

CONCLUSIONS
Both of Magadi chert samples are massive or layered crystals with very dense in structure. They have higher U and lower Th contents, so they are very suitable for uranium-series dating. Individual chert samples with high U/Th ratios are pure enough for U-series dating and provide a constraint on the true age of the chert formation.
Different portions of the chert deposits were formed at different time periods with different age, even though they were in the same deposit. The pure chert sample does not contain initial 230Th, so the Isochron dating is not necessary for the dating which just provides an apparent age.
The formation of the chert shows that Lake Magadi is in the wet condition with a higher water level during the Pleistocene. Our results show there were white cherts formed during MIS 6, and it suggests that the climate may be the most humid period in East Africa. The formation of the bedded chert was recording the alternations of a drought-wet cycle (~20ky), this indicates that the climate of East Africa is consistent with the earth axial precession.
論文目次 摘要 I
Abstract II
誌謝 VI
目錄 VII
圖目錄 X
表目錄 XII

內文章節
1. 前言 1
1.1 非洲湖泊研究意義 1
1.2 東非赤道氣候與古氣候 4
1.3 非洲鹽湖燧石形成及環境意義 7
1.4 鈾系不平衡定年法 9
1.4.1 鈾釷同位素地球化學行為 9
1.4.2 放射性鈾與釷衰變序列 10
1.4.3同位素稀釋法 11
1.4.4 衰變定年方程式 12
1.4.5 鈾系不平衡在自然界中之意義與定年假設 14
1.4.6 燧石定年條件與可靠性 16
2. 研究方法與步驟 17
2.1 東非水文地理 17
2.1.1 馬加迪湖現今地理位置與氣候 17
2.1.2 湖泊水文與湖水化學 19
2.2 樣品描述與分析方法 22
2.2.1 樣品外觀描述 22
2.2.2 樣品處理與實驗流程 24
2.3分析儀器介紹 28
2.3.1 γ能譜儀(γ spectrometry) 28
2.3.2 X光粉末繞射儀(XRD) 29
2.3.3 TOC-TN分析儀 30
2.3.4 α能譜儀(α spectrometry) 31
2.3.5 多收集器感應耦合電漿質譜儀(MC-ICP-MS) 33
3. 結果 34
3.1 鈾釷同位素活度分析結果 34
3.1.1 γ能譜儀放射性核素活度分析結果 34
3.3.2 α能譜儀與ICP-MS鈾釷同位素活度分析結果 35
3.2鈾釷同位素活度比值結果 36
3.3鈾釷同位素測年結果 37
3.4 TC、TOC與TIC測量結果 37
3.5 X光粉末繞射儀(XRD)礦物分析 38
4. 討論 49
4.1燧石中40K含量與湖水濃縮 49
4.2燧石樣品初始230Th與定年可靠性 50
4.2.1 樣品中230Th/232Th比值與初始230Th含量 50
4.2.2 等時線定年法與單點定年結果比較 51
4.2.3 ICP-MS 與α能譜儀重複定年結果比較,討論燧石樣品均勻性與測年結果重現性 55
4.3燧石樣品中鈾濃度與無機碳含量變化指示馬加迪湖湖水化學組成變化 56
4.3.1 燧石樣品中U含量變化 56
4.3.2 燧石樣品碳含量與鈾濃度關係變化 59
4.4燧石樣品鈾釷比值變化與馬加迪湖環境意義 61
4.4.1 234U/238U不平衡指示馬加迪湖氣候變化 61
4.4.2 燧石樣品238U/232Th變化與湖水氣候環境轉變 62
4.5 馬加迪層狀燧石定年結果與顏色變化週期規律 64
5. 結論 67
參考資料 70
參考文獻 Abrantes, F. (2003). A 340,000 Year Continental Climate Record from Tropical Africa – News from Opal Phytoliths from the Equatorial Atlantic. Earth and Planetary Science Letters, 209(1), 165-179.
Average Monthly Temperature and Rainfall for Kenya(n.d.). Retrieved March 30, 2017, from http://sdwebx.worldbank.org/climateportal/index.cfm
Baum,E.M., H. D., Miller, T.R., Nuclides and Isotopes Chart of the Nuclides 16th edition, Lockheed Martin, 88P., 2002.
Butzer, K. W., Isaac, G. L., Richardson, J. L., and Washbourn-Kamau, C. (1972). Radiocarbon Dating of East African Lake Levels. Science, 175(4026), 1069-1076.
Chabaux, F., J. Riotte and O. Dequincey (2003). U-Th-Ra Fractionation During Weathering and River Transport. Reviews in Mineralogy and geochemistry, 52(1), 533-576.
Cheng, H., R. Lawrence Edwards, C.-C. Shen, V. J. Polyak, Y. Asmerom, J. Woodhead, J. Hellstrom, Y. Wang, X. Kong, C. Spötl, X. Wang and E. Calvin Alexander (2013). Improvements in 230Th dating, 230Th and 234U Half-life Values, and U–Th Isotopic Measurements by Multi-Collector Inductively Coupled Plasma Mass Spectrometry. Earth and Planetary Science Letters, 371-372, 82-91.
Claude Hillaire-Marcel et al., O. C., and Joel Casanova (1986). C-14 and Th/U Dating of Pleistocene and Holocene Stromatolites from East African Paleolakes. Quaternary Research, 25, 312-329.
deMenocal, P. B. (2004). African Climate Change and Faunal Evolution during the Pliocene–Pleistocene. Earth and Planetary Science Letters, 220(1), 3-24.
Ekström, L. P., and Firestone, R. B. (2004). WWW Table of Radioactive Isotoprs. Database version 2.1.
Eugster, H. P. (1967). Hydrous Sodium Silicates from Lake Magadi, Kenya. Science, 157, 1177-1180.
Eugster, H. P. (1969). Inorganic Bedded Cherts from the Magadi Area, Kenya. Contr. Mineral. and Petrol., 22, 1-31.
Eugster, H. P. (1970). Chemistry and Origin of The Brines of Lake Magadi, Kenya. Mineralogical Society of America Spec. Pap., 3, 213-235.
Goetz, c. and C. Hillaire-Marcel (1992). U-series Disequilibria in Early Diagenetic Minerals from Lake Magadi Sediments, Kenya: Dating Potential, Geochimica et Cosmochimica Acta, 56, 1331-1341.
Guth, A. L. (2007). Evolution of the Southern Kenya Rift from Miocene to present with a focus on the Magadi area. Michigan Technological University.
Guth, A. L. (2014). Geology of the Magadi Area, Kenya, The Geological Society of America.
Hiess, J., Condon, D. J., McLean, N., and Noble, S. R. (2012). 238U/235U Systematics in Terrestrial Uranium-Bearing Minerals. Science, 335(6076), 1610-1614.
Hillaire-Marcel, C., Carro, O., and Casanova, J. (1986). 14C and Th/U Dating of Pleistocene and Holocene Stromatolites from East African Paleolakes. Quaternary Research, 25, 312-329.
Jones, B. F., Eugster, H. P., and Retting, S. L. (1977). Hydrochemistry of the Lake Magadi basin, Kenya. Geochmuca et Cosmochlmlca Acta, 41, 53-72.
Jones, B. F., Rettig, S. L., and Eugster, H. P. (1967). Silica in Alkaline Brines. Science, 158(3806), 1310-1314.
Junginger, A. (2011). East African Climate Variability on Different Time Scales : the Suguta Valley in the African-Asian Monsoon Domain Doctoral Thesis, Universität Potsdam.
Junginger, A., and Trauth, M. H. (2013). Hydrological Constraints of Paleo-Lake Suguta in the Northern Kenya Rift during the African Humid Period (15–5 ka BP). Global and Planetary Change, 111, 174-188.
Koide, M. and E. D. Goldberg (1963). Uranium-234/ Uranium-238 Ratios in Sea Water. Progress in oceanography, 3, 173-177.
Luo, S.-D., and Ku, T.-L. (1991). U Series Isochron Dating: A Generalized Method Employing Total-Sample Dissolution. Gmhimica et Cosmochimica Acta, 55, 555-564.
McCall, J. (2010). Lake Bogoria, Kenya: Hot and Warm Springs, Geysers and Holocene Stromatolites. Earth-Science Reviews, 103(1), 71-79.
Mensah, C. A. (2014). Urban Green Spaces in Africa: Nature and Challenges. International Journal of Ecosystem, 4(1), 1-11.
Moernaut, J., Verschuren, D., Charlet, F., Kristen, I., Fagot, M., and De Batist, M. (2010). The Seismic-Stratigraphic Record of Lake-Level Fluctuations in Lake Challa: Hydrological Stability and Change in Equatorial East Africa over the Last 140kyr. Earth and Planetary Science Letters, 290(1), 214-223.
O'neil, J. R., and Hay, R. L. (1973). 8O/16O Ratios in Cherts Associated with the Saline Lake Deposits of East Africa. Earth and Planetary Science Letters, 19, 257-266.
Orloff, K. G., K. Mistry, P. Charp, S. Metcalf, R. Marino, T. Shelly, E. Melaro, A. M. Donohoe and R. L. Jones (2004). Human Exposure to Uranium in Groundwater. Environmental research, 94(3), 319-326.
Pahnke, K., Zahn, R., Elderfield, H., and Schulz, M. (2003). 340,000-Year Centennial-Scale Marine Record of Southern Hemisphere Climatic Oscillation. Science, 301, 948-952.
Peterson, M. N. A., and Borch, C. C. V. D. (1965). Chert: Modern Inorganic Deposition in a Carbonate-Precipitating Locality. Science, 149, 1501-1503.
Rooney, T. P. (1969). Magadiite from Alkali Lake, Oregon. The American Mineralogist, 54, 1034-1043.
Sakaguchi, A., Yamamoto, M., Sasaki, K. and Kashiwaya, K. (2006) Uranium and Thorium Isotope Distribution in an Offshore Bottom Sediment Core of the Selenga Delta, Lake Baikal, Siberia. Journal of Paleolimnology, 35, pp. 807-818.
Taylor, S. R. (1964) Abundance of Chemical Elements in the Continental Crust-A New Table. Geochimica Et Cosmochimica Acta, 28, pp. 1273-1285.
Tierney, J. E., Russell, J. M., Sinninghe Damsté, J. S., Huang, Y., and Verschuren, D. (2011). Late Quaternary Behavior of the East African Monsoon and the Importance of the Congo Air Boundary. Quaternary Science Reviews, 30(7), 798-807.
Trauth, M. H., Larrasoaña, J. C., and Mudelsee, M. (2009). Trends, Rhythms and Events in Plio-Pleistocene African Climate. Quaternary Science Reviews, 28(5), 399-411.
Trauth, M. H., Maslin, M. A., Deino, A., and Strecker, M. R. (2005). Late Cenozoic Moisture History of East Africa. Science, 309, 2051-2053.
Urabe, A., Tateishi, M., Inouchi, Y., Matsuoka, H., Inoue, T., Dmytriev, A., and Khlystov, O. M. (2017). Lake-Level Changes during the Past 100,000 Years at Lake Baikal, southern Siberia. Quaternary Research, 62(02), 214-222.
Wood, J. and A. Guth (2013). East Africa’s Great Rift Valley: A Complex Rift System. Geology. com.
Yunwon, O., Jeong, S. Y., Jeong Kwon Suh, and Lee, J. M. (1995). Hydrothermal Syntheses of Na-Magadiite and Na-Kenyaite in the Presence of Carbonate. Bulletin of the Korean Chemical Society, 16, 737.
論文全文使用權限
  • 同意授權校內瀏覽/列印電子全文服務,於2020-05-31起公開。
  • 同意授權校外瀏覽/列印電子全文服務,於2020-05-31起公開。


  • 如您有疑問,請聯絡圖書館
    聯絡電話:(06)2757575#65773
    聯絡E-mail:etds@email.ncku.edu.tw