柴达木盆地东北缘中中新世以来古气候变化

全球变冷、青藏高原隆升以及特提斯洋海退被认为是控制亚洲大陆新生代气候变化的三大因素，然而由于长时间尺度古气候记录的匮乏，对于三者影响的认识并不清晰。柴达木盆地是青藏高原北部最大的一个封闭盆地，发育巨厚且连续的新生代地层，详细记录了周缘山体构造隆升过程和区域气候演化历史。本文通过柴达木盆地东北缘怀头他拉剖面的中中新世-早更新世沉积物气候代用指标碳酸盐、硫酸根和氯离子含量分析，显示该研究区中中新世以来气候变化经历了15.3~13.0Ma相对湿润、13.0~6.6Ma半干旱-半湿润和6.6~1.8Ma干旱的3个演化阶段。结合生物特征和前人研究成果，认为柴达木盆地干旱化历史可能从中中新世13Ma左右开始逐步发展，而真正意义上的干旱化始于6.6Ma以来。全球变冷是控制柴达木盆地干旱气候形成和发展的主导因素，同时青藏高原晚期持续阶段性的强烈隆升起到了催化剂的作用。

Paleoclimate change since the Middle Miocene in the northeastern Qaidam Basin

As the largest temperate arid region, Central Asia has been attracting widely attention for its great impacts on global climatic change. Global cooling, Tibetan Plateau uplift and the retreat of the Para-Tethys Sea have been thought to be the key factors controlling the formation and evolution of the aridity of the Asian inland. However, most of the existed evidences on this issue are indirect, largely from the eolian records of the surrounding areas, and little from inland arid area. Due to the scarcity of long-term palaeoclimate records, which of these factors playing the major role keeps unclear. Therefore, obtaining a series of direct and convincing climate evidence is of great significance for us to understand the process and mechanism of Asian aridity. Qaidam Basin, the largest closed basin in the northern Tibetan Plateau, develops continuous Cenozoic sediments, which recorded the tectonic uplift of surrounding mountains and climatic evolution of the area. Here we present multiple geochemical climatic proxy records(CaCO3, SO42- and Cl-) from the Late Cenozoic sedimentary sequence of the Huaitoutala section (37°13'48"N, 96°43'10"E), which is located in the northeastern Qaidam Basin(Fig.1) and consists of the Shangyoushashan Fm., Shizigou Fm. and Qigequan Fm. at ca.15.3~1.8Ma(Fang et al., 2007). The analysis results of the climatic proxy records from 663 samples reveal an evident three-stage climate change since the Middle Miocene(Fig.2). From 15.3Ma to 13.0Ma(relatively humid), the CaCO3 contents(average 11.331%)showed an upward decrease, as well as the average concentrations of the soluble anions SO42- (average 1.464mg/g)and Cl- (average 12.786μg/g) remained at relatively low values. From 13.0Ma to 6.6Ma(semi-arid and semi-humid), the CaCO3 contents(average 10.443 %)decreased after a sharp increase with superimposed high-frequency fluctuations, while the concentrations of SO42-(average 1.526mg/g)and Cl-(average 17.394μg/g)stepwise increased and an obvious drop occurred at 8.1Ma and lasted to 6.6Ma. Since 6.6Ma(arid), the average contents of all the salinity ions were higher than before, among which the CaCO3 contents(average 11.908 %)displayed stable changes, the concentrations of SO42-(average 2.055mg/g)and Cl-(average 20.314μg/g)showed an evident increase, especially after 2.6Ma. With biological characteristics and previous research(Fig.3 and 4), it is concluded that the Qaidam Basin has experienced increased aridity since ca.13.0Ma, and the extreme aridity was initiated at 6.6Ma. We suggest that the Miocene global cooling exerted a significant influence on the drying of the Qaidam Basin. In addition, the episodic and persistent uplift of the NE Tibetan Plateau since the Late Micocene exerted an important influence superimposed upon this driving force.