Limits and applications of high-precision coral 230Th dating
Date Issued
2005
Date
2005
Author(s)
Li, Kuei-Shu
DOI
en-US
Abstract
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.
Subjects
鈾釷定年技術
珊瑚
30Th初始値
等時線圖
230Th dating technique
coral
non-radiogenic 230Th
isochron plot
Type
thesis
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