Behavior of U and Th isotopes in diagenetically-altered fossil corals: its implication for U-series dating

• Main Authors: Bo-RenWang, 王伯仁
• Other Authors: Shangde Luo
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/57774961542072695743

碩士 === 國立成功大學 === 地球科學系碩博士班 === 98 === To understand the behavior of uranium-series isotopes in diagenetically-altered fossil corals, this study collected fossil corals from Kenting, Heng Chun Peninsula, Taiwan. After separating each coral sample into 3-5 sub-coral samples, we measured the distributions of uranium (238U, 234U) and thorium (230Th, 232Th) isotopes in each of these subsamples. Results show that the modern and unaltered corals collected from Eruanbi (鵝鑾鼻) at modern sea-level have 238U activities 2.28 ± 0.08 dpm/g in consistence with those of unaltered corals reported previously for other locations of Kenting. Corals from Lung-Pan (龍磐) at ~80 m a.s.l. (above sea-level) and She-Ding (社頂) at ~150 m a.s.l. have 238U activities mostly smaller or equal to 1 dpm/g, which are much smaller than those of modern corals at Kenting as well as the unaltered corals from various parts of world oceans. The observed decrease of 238U activities in altered corals suggests that the diagenesis of coral results in significant loss of uranium to groundwater. Consistent with the observations on diagenesis-driven leads to loss of uranium from corals, 230Th/238U ratios are found in some corals to be significantly greater than 1, suggesting that significant diagenesis occurs during the last 350 kyrs. It is interesting that 234U/238U ratios from various parts of the same coral shows variable values of ~0.98 to 1.13, a result of exchange of uranium between coral and groundwater during the diagenesis. 230Th/238U ratios in various parts of the same coral were also found to vary significantly, suggesting various degree of diagenesis in various parts of the same corals. Based on these findings, we conclude that no reliable U-series dates can be obtained from those fossil corals by assuming a closed system for corals. To evaluate the impact of diagenesis on U-series dating of corals, we proposed three possible about timing of uranium loss from the corals : (1) The uranium loss by diagenesis occurs in the early short period of coral formation, (2) the uranium loss occurs recently, or (3) U loss occurs at any time after coral formation. Case (1) is datable but requires that the measured 230Th/238U and 234U/238U ratios in all subsamples are the same with the uncertainty. Case (2) is datable only if the recent loss of U can be corrected. Case (3) is undatable.