ESR-铀系联合分析与等时线法结合在考古遗址化石测年中的应用探索

Combined ESR and U-series isochron dating of fossil tooth from Longgupo cave

在ESR考古测年中,外部剂量率通常是年代结果的一个主要误差来源,其在历史过程中不断变化,所以很难通过现今的野外测量准确测定。等时线技术可解决化石样品测年中外部剂量率的不确定性问题。本研究尝试将ESR-铀系联合化石分析结合等时线技术应用于重庆龙骨坡遗址CⅢ’6层相邻探方中采集的5颗牙齿。ESR-铀系联合分析显示全部样品均经历了铀的近期吸收过程。平均外部剂量率通过等时线方法分步迭代计算,得到样品等时线年龄为1.77±0.09 Ma。本研究表明,将ESR-铀系联合分析与等时线技术相结合可同时解决牙齿化石样品ESR测年中内部剂量率和外部剂量率的问题。

Background： In ESR and luminescence archaeological dating, the assessment of external radiation dose rate is one of the constant sources of uncertainty because of its variation in the past time and it cannot be determined accurately using the present-day measurements. Purpose： ESR isochron protocol was proposed to solve this uncertainty for the tooth samples. This protocol is applicable wherever multiple samples with different internal doses have all experienced a common external dose. The variable uranium concentration of tooth samples makes it possible to plot the equivalent dose versus the internal dose rate of each sample, and the slope of isochron line gives hence the age. For isochron dating of teeth, combined ESR/U-series dating analysis must be done together with isochron protocol. Methods： In this study, we try to use combined ESR/U-series isochron method on 5 tooth samples collected from immediate adjacent square in layer C Ili＇6 of Longgupo archaeological site, Chongqing, China. Combined ESR/U-series analysis with in situ external dose rate shows recent uranium uptake of all the samples. Results： The time-averaged external dose rate was iterative calculated by isochron protocol, and gives an isochon age of 1.77~0.09 Ma for layer C 1II＇6, which consistent with the mean US-ESR ages of 5 samples （1.64＋0.16/-0.21 Ma） in the error range. The calculated time-averaged external dose rate （-807 ~tGy/a） was basically in agreement with the in situ measured gamma dose rate value （850 p.Gy/a） in 2006, indicating the geochemical alterations may not occurred or do not affect the environmental dose rate obviously during the burial history. Conclusions： This study indicates the potential of solving both internal and external dose rate problems of ESR dating of fossil teeth by combining with U-series analysis and isochron protocol.