柴达木盆地晚中新世河湖相沉积物粒度组成及其古环境意义

A GRAIN SIZE STUDY ON LATE MIOCENE HUAITOUTALA SECTION, NE QAIDAM BASIN, AND IMPLICATIONS FOR ASIAN MONSOON EVOLUTION

本文应用沉积物粒度端元分析模型对柴达木盆地怀头他拉剖面开放湖相沉积物进行分离，得到4个具有现实环境意义的端元组分，分别代表4种动力过程。端元1为河流沉积，端元2、3都为湖相沉积，端元4为三角洲沉积。这些成分含量在晚中新世的变化趋势不明显但变化幅度相对较大，表明柴达木盆地这套地层记录了沉积环境或气候的快速变化。尽管怀头他拉粒度变化趋势总体不明显，然而平均粒径在约8．3～7．0Ma期间有所降低，指示当时亚洲季风可能发生了增强，导致柴达木古湖面积扩大，到达研究站点沉积物的粒度变细。东亚和南亚地区古气候数据合成支持这一推论，由此我们推断亚洲大陆在约8．3～7．0Ma期间气候相对较湿润，是青藏高原隆升的一个直接反应；而7Ma以后气候变干可能反映了大气CO2含量下降的驱动。轨道尺度研究表明柴达木盆地气候在约8．3～7．1Ma期间发生了较大幅度的变化，支持青藏高原隆升是气候变化放大器的观点。

In this paper, we applied End-Member unmixing method developed by previous workers to try to separate different processes determining grain size distribution in the Huaitoutala section in the northeastern Qaidam Basin, northeastern Tibet. The section spans from ： 37° 13′48.5″N, 96°43′02.0″E to 37°14′37.0″N, 96°44′11.4″E. The total thickness of the section is ca. 4500m. This of the middle portion （ca.1100m in thickness） based on higher-resolution sampling, resulting 10Ma and 6Ma. section has been dated with paleomagnetic dating and the age model of the section dominated by lacustrine sediments was further refined in a solid age model constraint for the portion deposited between We reveal that the grain size data can be decomposed into 4 End-Members. End-Member 1 consists of a sand-sized main component and a silt-sized minor component. The grain size pattern is similar to fluvial sand reported before. So we attribute this End-Member to fluvial component. End-Member 2 and 3 consist of a silt-sized main component and two minor components, similar to typical lake deposits reported before. We attribute these two End-Members to lacustrine deposits with End-Member 2 indicating wetter climate than End-Member 3. It is hard to find a modern analog for End-Member 4 which exhibits a bimodal main peak distribution and some minor components. Loess is reported to exhibit similar bimodal main peak distribution pattern but the grain size is significant smaller than the End-Member 4 here. We attribute End-Member 4 to dalta depositing processes which are affected by both fluvial processes and lacustrine processes. This explains why this End-Member shows similar pattern both to End-Member 1 and End-Member 2 or 3. The relative content of these End-Members all show significant fluctuations, similar to evaporate mineral content variations reported from the other sites in the Qaidam Basin. However, separating 4 End-Members from the bulk grain size data allows a better understanding of different depositional processes through time. The mean grain size of the studied section decreased during ca. 8.3 -7.0Ma, indicating higher lake level and larger lake area in the Qaidam Basin. To further test this hypothesis, we compiled paleoenvironmental proxy records from both Eastern and Southern Asia areas. These data consistently showed that climate was wetter in Asian continent during ca. 8.3- 7.0Ma, likely associated with Asian monsoon precipitation intensification. We attribute monsoon intensification to Tibetan uplift because substantive evidence existed about northeastward growth of the Tibetan Plateau around 8Ma. Considering global sea surface temperature variations, we attribute the Qaidam drying after 7Ma to atmospheric CO2 forcing. Orbital timescale studies reveal that Qaidam climate experienced larger amplitude fluctuations during ca. 8.3 - 7.0Ma, confirming the opinion that the Tibetan uplift can increase climate system＇s sensitivity to insolation forcing.