高精確珊瑚鈾釷定年技術之限制及應用

Abstract

珊瑚骨骼中的地球化學指標已廣泛應用於古環境重建的研究。準確解釋珊瑚所紀錄的環境和氣候變化，需仰賴精確的定年技術。前人所提出最好的鈾釷定年技術之不準度為2-3年 (after Edwards et al., 1988)，而我們新發展的方法對於100年以內的珊瑚樣本，可以提供1年或更好的精確度。首先，我們精練了化學方法，在處理程序方面僅產生相當於2-3個月的誤差。非在珊瑚骨骼內之封閉系統中衰變的230Th (230Thnr)含量，即230Th初始值，是限制鈾釷定年準確性與精確性的主要參數。 230Th初始含量可藉由230Th/232Th vs. 234U/232Th等時線圖所得之30Th/232Th初始比值做進一步的估計。台灣南部南灣地區現生微孔珊瑚的230Th/232Th初始值為5.2 ± 1.1 × 10-6 (原子數比，在此之後的比值均是原子數比) ，與海水中溶解相的值 (4.0 ± 0.5 × 10-6) 一致，但高於海水中懸浮微粒物質的比値 (3.0 ± 0.7 × 10-6)。在蘇門答臘群島中的North Pagai，珊瑚230Th/232Th初始值和海水中溶解相的釷同位素比值亦相同。這些結果說明珊瑚中的230Thnr主要來自海水中的溶解相。南灣與蘇門答臘群島South Pagai 的研究顯示，區域性珊瑚骨骼中的230Th/232Th初始值在具有相同的水文環境下，沒有發現有變異性存在。North Pagai的現代標本及 475 年珊瑚化石之間的230Th/232Th初始值在時間上並沒有顯著不同。本研究將已建立好的230Th定年技術對蘇門答臘群島1935年大地震事件做了精確定年且進一步發現其250年的地震週期。古氣候的應用方面，利用西赤道太平洋的珊瑚建立一個精確的年代模式以重建其長期的氣候系統。

Abstract Coral skeletal geochemical tracers have been used as proxies to retrieve paleo-conditions. Interpretation of coral-inferred environmental and climatic records relies on accurate dating. Our newly-developed 230Th dating technique offers a precision better than 1 year for corals, younger than 100 years, superior to 2-3 years given by previous methods (after Edwards et al., 1988). To approach this objective, chemistry has been refined and only brings about an equivalent age uncertainty of 2-3 months. The most important parameter limiting precision and accuracy of coral 230Th dating is the content of non-radiogenic 230Th (230Thnr) not due to closed system decay within the coral skeletal matrix, so called the initial 230Th level. It can be evaluated with an initial 230Th/232Th ratio earned from a 230Th/232Th vs. 234U/232Th isochron plot. The initial 230Th/232Th ratio for modern Porites corals, collected from Nanwan, southern Taiwan, is 5.2 ± 1.1 × 10-6 (atomic ratio, hereafter), consistent with a value of 4.0 ± 0.5 × 10-6 in the dissolved fraction of seawater and higher than that of 3.0 ± 0.7 × 10-6 in the suspended particulate matter. The initial 230Th/232Th ratio addressed in Porites corals in North Pagai, Sumatra, also captures the dissolved seawater values. The results indicate that 230Thnr is mainly attributed to the dissolved phase of seawater. Isochron plots for modern and fossil corals from Nanwan and from island South Pagai of Sumatra display little spatial variations of initial 230Th/232Th ratio at local settings. No significant temporal variation is found between initial 230Th/232Th values inferred from modern and 475-year-old corals from island North Pagai, Sumatra. Applications of this well-established 230Th dating technique given in this study include (1) a precise age determination of an earthquake in 1935 AD and (2) a discovery of a 250-year recurrence of giant earthquakes in the Sumatran Islands, and 2 (3) a precise age model for long-term climatic reconstruction with coral tracer records in the western equatorial Pacific Ocean.