柴达木盆地察尔汗古湖贝壳堤剖面碳酸盐和瓣鳃化石碳氧稳定同位素

STABLE ISOTOPES OF CARBONATE AND FOSSIL SHELLS FROM THE SHELL BAR SECTION OF THE PALEOLAKE QARHAN, QAIDAM BASIN

通过对柴达木盆地察尔汗古湖贝壳堤剖面沉积物、生物壳体碳酸盐同位素及沉积物碳酸盐含量等多项指标的分析,探讨了贝壳堤剖面记录39.6～17.1kaB.P.(未校正14C测年,下同)期间碳酸盐和贝壳化石碳氧稳定同位素的特征和相关关系,指出壳体化石及沉积物碳酸盐的碳氧同位素是湖泊水体温度和盐度的反映,其中δ18O对温度的指示意义更敏感,是湖泊和古气候演化很好的代用指标。根据这些指标重建的古气候与环境变化显示,在39.6～35.5kaB.P.期间,柴达木盆地处于温暖湿润期,周围山地降水增加,盆地内湖泊发育;自35.5kaB.P.开始,气候较前期更加湿润,是湖泊发育的最佳期;22.1kaB.P.以后,气候逐步转入较温暖的干燥期,湖泊开始萎缩、退化;末期17.1kaB.P.气候环境急剧恶化,形成石盐结晶,湖泊高湖面演化史结束。

Shell Bar, which is composed of abundant fossil shells of Corbicula fluminea MUller and Corbicula largillierti Philippi and located at the southeastern end （36°30′N, 96°12′E） of the paleolake Qarhan, is one of the sections with the most prominent features in the Qaidam Basin. Multi-method dating reveals that the high lake levels appeared between 39.6kaB. P. and 17. lkaB. P. The stable carbon and oxygen isotopes of fossil shells, bulk sedimentary carbonate and carbonate content in the lacustrine deposits in the Shell Bar section from the paleolake Qarhan in the Qaidam Basin were analyzed systematically to address the relationships between isotopes properties of shells and carbonate in bulk samples. Results show that during the period the high lake levels sustained. The stable isotopes of the shells and carbonate in bulk samples can be used as proxies of lake water temperature and salinity, while δ^18O is more sensitive to salinity of lake water, which closely relates to temperature changes. Based on these proxies, the paleoenvironment change history was reconstructed. During the period between 39. 6kaB. P. and 35.5kaB. P., the δ^13C and δ^18O values are generally high, together with high carbonate content, they imply a strong evaporation in the earlier stage of the lake development. During this stage, the climate was changed from cold-dry to warm and humid, the precipitation in the drainage, especially in mountainous regions surrounding the basin increased, and the lake began to develop with the lake level starting to rise. From 35.5kaB. P., the climate was wetter than the earlier stage. It was the period during that the lake fully developed and lake level reached the maximum. Started from 22. 1 kaB. P., the climate became relatively warm and dry, lake level decreased step by step and the salinity of lake water increased. Around 17. lkaB. P., the climate deteriorated abruptly： lake level decreased suddenly, strong evaporation resulted in the decreasing of lake level and the formation of salt layer on the top of the studied section, and the high paleolake level history ended.