南沙群岛永暑礁泻湖岩心的高精度TIMS铀系年龄

HIGH-PRECISION TIMS(THERMAL ION MASS SPECTROMETRY)U-SERIES AGE OF THE CORE FROM THE LAGOON OF YONGSHU ATOLL,NANSHA ISLANDS

用高精度 TIMS铀系方法测定了南沙群岛永暑礁泻湖 5 .90 m长的岩心沉积物样品 ,结果表明 ,由于混合 ,表层样品年龄相近。混合层下的沉积物年龄随深度呈有规律的增加。以最小二乘法拟合 ,岩心底部与顶部的相对年龄相差 16 35 a,其误差小于 1%是比较合理的 ,据此校正混合层以下的年龄 ,而混合层和柱中个别异常的年龄值 ,改用沉积速率内插求出 ,从而得出了系统的环礁泻湖沉积的高精度 TIMS铀系地质年龄 ,其底部 5 .90 m的年龄为 (16 82± 15 ) a BP,即 317A D。

Sediment samples from the 5 90 m long core from the lagoon of Yongshu atoll,Nansha Islands are dated by highly precise TIMS(thermal ion mass spectrometry)U series method.The results show that ages of the surface samples are similar to each other due to their mixture,and those of sediments below the mixed layer increase with depth.Through LMS(least squares fitting),the value of the relative age difference between the bottom and the top of the core is 1 635 a and its error is less than 1%.It is proved reasonable.The abnormal ages of the mixture layer and core are adjusted through interpolating of the sedimentation rate.Based on these corrected ages,geological ages dated by highly precise TISM U series method are systematically given.The age at the bottom of the core is 1 682±15 a,that is,317 AD.