||Studies on chronological and paleo-environmental implications of U-series disequilibria in lacustrine authigenic minerals (chert)
||Department of Earth Sciences
Magadi chert, climate change
paleo environmental of Africa
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。
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.
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.
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.
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
1.4.4 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
2.3.1 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
3.4 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
4.2.1 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
4.3.1 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
4.4.1 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
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