湖泊沉积物的^(14)C和光释光测年——以固城湖为例

RADIOCARBON AND OPTICAL DATING OF LACUSTRINE SEDIMENTS——A CASE STUDY IN LAKE GUCHENG

富含有机质的湖泊沉积物被认为是14C测年建立古环境记录年代标尺的理想材料,光释光测年方法近年开始应用于水成沉积物的定年。应用14C和光释光两种方法对江苏固城湖湖心钻孔岩芯进行了年龄测定,结果表明全样有机质的14C年龄与石英的光释光年龄存在系统差异,后者较前者年轻约2000年。系统光释光测年研究排除了光释光年龄低估的可能性,所以,二者的差异可能是湖泊沉积物碳库效应的反映。

Organic-rich lacustrine sediments cored from Lake Gucheng, Jiangsu, China have been dated by radiocarbon and optically stimulated luminescence （OSL） methods. Five samples for radiocarbon and three for OSL were taken from the depth between 50cm and 178cm. Correlated ages are obtained from the two sets of samples but a consistent offset between them is also observed. The AMS ^14C ages of bulk sediment range from 7840 ± 40aB. P. to 8740 ± 50aB. P. while the OSL ages of fine-grained quartz range from 6.7 ± 0.6ka to 7.1 ± 0.7ka. Therefore the calibrated AMS ^14C ages of bulk sediment samples are about 2000 years older than the optical ages. The optical ages are obtained by the single aliquote regeneration （SAR） protocol with a preheat temperature of 200℃. Systematic analysis of the optical dating data, including dose recovery and preheat plateau test, rules out the possibility that the OSL age might be underestimated. The age difference is therefore attributed to the reservoir effect of the lake sediment samples. Therefore, caution should be made when bulk sediments are used for ^14C dating. More studies involving comparison among different dating methods are needed for establishing accurate chronologies of lake sequences.