測試螢光定年法應用於土石流沉積物之可行性：以南台灣荖濃溪為例

Abstract

為了瞭解災害性土石流事件的再現性，定出土石流成因沉積物的年代成了一件重要任務。碳十四定年的材料較為稀少及難以蒐集，因此必須利用其他方法來克服困難。雖然螢光定年已被廣泛運用在不同類型的沉積物，但對於土石流環境的沉積物，不完全曬退仍然是個重要議題。又螢光定年的主要材料是常見的石英，若不完全曬退的問題可以被解決，就能變成一個有用的測定土石流年代的工具。 荖濃溪谷有很多土石流形成的扇階，為了評估螢光定年是否適合應用在這樣的沉積物，本研究於荖濃溪總共測試了三個樣本：BC01、CS01以及TY01。BC01和CS01是主流堰塞沉積物，而TY01則是土石流沉積物的基質砂。此外，這三個樣本都具有碳十四定年的結果來做為年代控制。本研究將單片再生法同時應用在單顆粒法及小片法，以找出較適合用於測量等效劑量的方法，然後將結果用來評估不完全曬退的影響。劑量率估算，則是同時採用X光螢光光譜法配合感應耦合電漿質譜儀、及伽瑪光譜法來估算。由於曬退的程度可能有所不同，所以螢光年代的計算採用不同的年代模型，並以碳十四定年結果做為控制，找出最適合土石流沉積物的螢光年代模型。 本研究結果顯示，樣本的等效劑量分布較廣。利用感應耦合電漿質譜儀加上X光螢光光譜法所計算出來的年劑量，和只使用伽瑪光譜儀的測量結果計算的年劑量並不相同。本研究認為分散的等效劑量可能代表沉積物複雜的搬運及堆積歷史，而利用不同方法所計算的年劑量分歧，可能指示不穩定的環境。螢光年代可以涵蓋碳十四年代結果，因此螢光定年確實有潛力應用在土石流事件定年。雖然土石流沉積物所表現的特性令人對螢光定年法的適用性有所懷疑，本研究認為結合單顆粒法及最小年代模型可能是個解決之道。

In order to understand the recurrence of disastrous debris flow events, dating of the debris flow derived deposits has become an important mission. However, using radiocarbon dating can only partially accomplish this mission because of the rare and nonsystematic sample findings. Therefore, it is necessary to find another method to solve above-mentioned problems. Although luminescence dating has been widely applied on several types of deposits, the issue of incomplete bleaching is still a challenge for sediments deposited in debris flow environments. If this problem can be properly solved, luminescence dating will become a useful tool since its major material is commonly distributed quartz. In the Lao-Nung River valley there are a lot of fan terraces which were formed by debris-flow events. For the purpose of assessing luminescence dating for such kind of deposits, three sampling sites were targeted: BC01, CS01, and TY01. BC01 and CS01 are both identified as dammed-up fluvial sediments, and TY01 is sand matrix within debris flow sediments. Furthermore, for all samples radiocarbon ages as independent age control are available. To test the feasible way for equivalent dose, single-aliquot regenerative-dose (SAR) protocol has been applied to both single grain and small aliquot. Then use these two methods to examine the incomplete bleaching problem. For the dose rate estimate, combing x-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS), and gamma-ray spectrometry (GRS) measurements were both carried out. Since the bleaching effect is various between different samples for such sediments, the burial ages were therefore corrected through several age models and yielded the final results close to their corresponding radiocarbon ages. The results show widely distributed equivalent doses and also a discrepancy between ICP-MS + XRF and GRS in the annual dose evaluation. In fact, the scattered equivalent dose distribution may indicate the complexity of transport and deposition histories, and the discrepancy of the annual dose may indicate an unstable environment. The luminescence dating results show good agreements with the radiocarbon dating results; hence, luminescence dating has a potential to date sediments deposited in debris flow environments. Though those characteristics related to debris flow were left questions in the traditional luminescence dating, this study gives the acceptable solution that is to combine the single-grain method plus the minimum age model (MAM) together.