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鈾釷同位素在矽藻殼體中富集機制之探討

Studies on the Mechanisms Controlling the Distribution of U and Th isotopes in Diatom Frustules

作者：徐意淳
畢業學校：國立成功大學
出版單位：國立成功大學
核准日期：2008-07-01
類型：Electronic Thesis or Dissertation
權限：Copyright information available at source archive--National Cheng Kung University....

中文摘要

鈾系和釷系是具有一連串長短不同衰變週期的放射性元素，而鈾同位素(238U，234U)相較於其子核種釷同位素(234Th，230Th)具有較高的溶解度，因此在岩石風化的過程中，因地下水的淋溶作用，較易使的鈾同位素被搬運至天然水體中，如河水、湖水和海洋；同時由於α-recoil效應，使水中鈾同位素的比值(234U/238U)均大於1。當生物體如矽藻、珊瑚、有孔蟲等生活在此水體中時，會吸收水中的鈾同位素，結合至它們的殼體內；而子核種釷同位素在水體中含量極微，因此導致生物體中的鈾系不平衡。而這些不平衡不僅可以用於測量生物化石的年齡，也同時可以用於研究地表的地質營力作用情況(如風化等)並爲環境變遷提供了有用的工具。本研究以探討鈾釷同位素在矽藻殼體中的生物地球化學行為及影響它們在矽藻殼體中的富集因子，並討論矽藻殼體中鈾系不平衡在絕對地質年代和地球環境變化研究中的潛在用途。
在本研究中，經過重液分離和六偏磷酸鈉懸浮後的矽藻殼體，個體本身較完整，並無破碎的情況，但是經實驗結果證實，仍需再經過化學清洗的處理步驟。以HF溶解矽藻殼體，則會造成未清洗乾淨的陸源碎屑物大量被溶解，影響實驗結果，造成誤差。能有效洗出大量陸源碎屑物而不易溶解矽藻殼體是HNO3和NH2OH‧HCl in HCl，不同濃度結果顯示差別不大。而H2O2雖在文獻中常用於去除有機物，但清洗矽藻殼體的結果顯示不論是否在酸性體系下皆不易去除陸源碎屑物且有溶解矽藻殼體的情況，飽和KMnO4則因本身背景值過高，NaF和NH2OH‧HCl in HAc則在洗釷的效果上不佳，因此並不適合作為清洗的藥劑。
實驗中先加入示蹤劑易造成示蹤劑中有U/Th分離的現象，鈾大多存在於上清液相，而釷則吸附到殘渣相，因此在計算上須作校正，而後加入示蹤劑則無此情況。
以此次研究結果可知U主要存在矽藻殼體中，含量約在24ppb~61ppb之間，我們可藉由矽藻殼體中234U/238U及230Th/232Th比值推知樣品的來源及實驗的結果。矽藻殼體中鈾同位素含量的變化可為地質年代學、古海洋學、古環境變遷提供一個有效的工具。

英文摘要

Uranium- and thorium-series have a series of radioactive elements with different half lives. Because the parent nuclides (238U, 234U) have higher solubility than the daughter radionuclides (234Th, 230Th), when ground water flows through the rocks, the uranium isotopes are preferentially transported in natural waters, such as in rivers, lakes, seas. Meanwhile -recoil effect would cause 234U/238U activity ratio in waters to be greater than 1. Organisms who inhabit natural waters, such as diatoms, corals, and foraminifera, may uptake the uranium isotopes into their tests. These organisms incorporate relatively few daughter nuclides (234Th, 230Th) due to their low solubility in natural waters, thus causing the parent-daughter radioactive disequilibrium. The disequilibrium is used not only to determine the age of biological fossils, but also provide a tool for studying the geological processes (e.g., weathering) and environmental changes. In this thesis, I studied the distribution of uranium and thorium isotopes and their biogeochemical behavior in diatom frustules. The potential use of U and Th isotopes in diatom frustules as a tool for geochronological dating and for paleo- environmental studies will be discussed.
This study shows that while pure and integral diatom frustules can be separated from sediments by heavy-liquid and sodium hexametaphosphate separation method, a chemical cleaning is needed in order to remove the contaminants from the diatom frustules. Various cleaning and dissolution methods are tested in this study. I found that diatom frustules can be dissolved with HF or NaOH solution, however the HF dissolution is not favorable as it can also easily dissolve the detrital materials that may have been coated on the diatom frustules. The use of NaOH dissolution can selectively dissolve diatom frustules rather than the ditrital minerals. The contaminants on diatom frustules can be cleaned effectively by using HNO3 and/or HCl + NH2OH HCl solutions, but not by H2O2, NaF, and/or NH2OH in HAc solutions. Saturated KMnO4 solution, combined with 6 N HCl, can also be effective in cleaning, although the cleaning solution may contain higher uranium background.
This study shows that the U and Th isotope compositions in diatom frustules are completely different from those in detrital contaminants. In pure diatom frustules contain uranium in the range of 24 ppb to 61 ppb. In contrast to those in lithogenic detritus, pure diatom frustules also contain higher ratios of 238U/232Th and 230Th/232Th and their 234U/238ratios are usually greater than 1 (e.g., close to their seawater ratio for marine diatoms). These features suggests that diatom frustules incorporate U isotopes , rather than Th isotopes, from the natural waters, providing a new tool for geochronological, paleoceanographical, and paleoenvironmental studies.

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目錄
摘要……………………………………………………………………… Ⅰ
Abstract………………………………………………………………… Ⅱ
誌謝……………………………………………………………………… Ⅳ
目錄……………………………………………………………………… Ⅴ
圖目錄…………………………………………………………………… Ⅸ
表目錄……………………………………………………………………Ⅹ
第一章：緒論
1.1. 鈾的地球化學行為…………………………………………… 1
1.2. 釷的地球化學行為…………………………………………… 3
1.3. 矽藻的基本特徵……………………………………………… 4
1.4. 有孔蟲中鈾釷同位素可行性與可能存在的問題…………… 5
1.5. 研究動機與目的……………………………………………… 5
第二章：研究背景
2.1 分離方法…………………………………………………… 7
2.2 化學清洗方法……………………………………………… 9
2.3 樣品溶解……………………………………………………… 10
2.4 鈾系定年……………………………………………………… 12
第三章：實驗方法與步驟
3.1 分離矽藻殼體………………………………………………… 14
3.2 化學清洗……………………………………………………… 15
3.2.1. 陸源物清洗……………………………………………… 15
3.2.2. 有機物清洗………………………………………… 16
3.3 矽藻殼體溶解………………………………………………… 17
3.3.1. 酸式溶解……………………………………………… 17
3.3.2. 鹼式溶解………………………………………………… 17
3.3.3. 酸式-鹼式溶解比較…………………………………… 19
3.4 清洗液的化學處理…………………………………………… 20
3.5 U/Th離子交換樹脂分離……………………………………… 20
3.6 鈾釷同位素ICP-MS分析……………………………………… 21
3.7 標準試劑與儀器校正………………………………………… 21
3.8 實驗背景值…………………………………………………… 22
3.9 數據處理……………………………………………………… 24
第四章：結果與討論
4.1. 重液分離法與六偏磷酸鈉懸浮法分離矽藻殼體…………… 29
4.2. 矽藻殼體樣品鹼式溶解與酸式溶解的比較………………… 29
4.3. 不同化學藥劑清洗釷的效果比較…………………………… 33
4.4. 不同清洗劑對測定矽藻殼體中鈾含量的影響……………… 34
4.5. 用230Th/232Th比值評估清洗效果及鈾釷在矽藻殼體樣品中的
富集型態與程度…………………………………………… 35
4.6. 矽藻殼體樣品中234U與238U的活度比…………………… 36
4.7. H2O2與HNO3清洗效果的比較……………………………… 37
4.8. 矽藻殼體樣品中鈾釷在NaOH溶解時的行為………………… 39
4.9. 現代海洋矽藻殼體中鈾釷同位素比值的測定…………… 41
4.10. NaOH相溶解時U-236和Th-229示蹤劑的行為………… 44
第五章：結論……………………………………………………… 45
參考文獻…………………………………………………………… 47