柴达木贝壳堤剖面元素地球化学与环境演变

ELEMENTAL GEOCHEMISTRY AND PALEOENVIRONMENT EVOLUTION OF SHELL BAR SECTION AT QARHAN IN THE QAIDAM BASIN

通过对柴达木盆地察尔汗古湖贝壳堤沉积物中酸溶与残留(酸不溶)组分中常、微量元素及其元素对的分析,并结合酸溶与残留组分中常、微量元素相关性对比,讨论了贝壳堤剖面元素地球化学指标和沉积环境之间的关系,指出酸溶组分中的常量、微量元素及其元素对可以作为湖泊和古气候演化良好的代用指标;残留组分中的元素及元素对与原岩及其风化程度紧密相关,因此,酸溶组分与残留组分中的常量和微量元素在对于环境响应模式存在一定差异,应该将二者分开研究以避免对元素所携带环境信号解译的偏差。根据元素地球化学指标重建了43.5～22.4cal.kaB.P.(39.7～17.5kaB.P.14C年代)柴达木盆地察尔汗古湖高湖面期间古气候与环境演变过程:43.5～31.7kaB.P.期间酸溶组分中元素含量总体较低,残留组分中元素含量相对较高,且湿润度值((Fe2O3+Al2O3)/(MgO+CaO),或H)处于高值段,而Ca/Mg,Fe/Mn和Rb/Sr则处于低值段,指示了温暖湿润的气候环境,古湖泊水体增加;其中37.8～31.7kaB.P.期间酸溶组分中元素含量多出现最低值,残留组分中元素含量富集,H值较高,而Ca/Mg,Fe/Mn和Rb/Sr此段均值最小,表明此时为环境的最适宜期,此期间降雨充沛,湖泊处于高湖面;在31.7～22.4kaB.P.期间酸溶组分中元素含量普遍较高,残留组分中元素含量相对富集,且Ca/Mg,Fe/Mn和Rb/Sr处于高值段波动,H值处在低值段,指示气候环境恶化,降水逐渐减少,古湖泊水体萎缩;在约22.4kaB.P.气候快速恶化,形成石盐结晶,高湖面历史结束。柴达木盆地该期高湖面及其演化过程可与腾格里沙漠和额济纳盆地-巴丹吉林沙漠高湖面记录进行对比,揭示了大范围气候变化的历史。

Based on the analytical results of the major and trace elements of the acid soluble （AS） and insoluble （AI） fractions of the Shell Bar section from the Qaidam Basin in the NE Tibetan Plateau,and the correlations between the related elements and their ratios,the depositional environment of the section was discussed. The results show that the major and trace elements and their ratios of AS are good proxies of the salinity, redox condition, and temperature of the lake, and the elements and their ratios of AI were related with source material and chemical weathering. Because of these different environmental significances, we need to distinguish the AS and AI, and avoid drawing inaccurate conclusions during the environmental reconstruction. According to these proxies, the paleoenvironment change history between 43.5kaB. P. and 22.4kaB. P. （ 39.7 ~ 17.5kaB. P. 14C ） was reconstructed. During the period between 43.5 - 31.7kaB. P., the lower values of Ca/Mg, Fe/Mn, Rb/Sr and concentrations of some elements in the AS,and the higher values of humid degree （（ Fe2O3 ＋ Al2O3 ）/（ MgO ＋ CaO）or ＂ H＂）ratio and elements contents of the AS show that the climate was warm and wet,and the paleolake level was high. Especially during 37.8- 31.7kaB. P. ,concentrations of some elements in the AS and Ca/Mg, Fe/Mn, Rb/Sr ratios are the lowest, element contents of the AI and H ratio are both high,which imply that the lake lake reached its maximum level and the organism in the lake were blooming. Between 31.7kaB. P. and 22.4kaB. P. the higher values of Ca/Mg, Fe/Mn, Rb/Sr and concentrations of some elements in the AS,the lower value of H and element contents of the AI demonstrate that the climate started to deteriorate, with the precipitation amount particularly decreased, the evaporation strongly increased, and the lake level started to decrease. The climate deteriorated abruptly around 22.4kaB. P. The lake level decreased suddenly,strong evaporation resulted in the formation of the salt layer in the top of the studied section,and the paleolake disappeared in the end. The paleolake level change history could be correlated with the records in Tengger desert and the Badain Jaran-Ejina Basin. It could also be correlated with the paleolake evolution on Tibetan Plateau and the ice core records. The widely spread high paleolake levels in arid North Western China and Tibetan Plateau implies that the climate, especially the hydrological circulation patten during the Late Pleistocene was completely different from that of today. This might give us useful clues to understand the climate situation which was crucial for human and life evolution during the period.