西宁盆地晚始新世-早中新世湖相沉积记录的早期C4植被扩张

The expansion of the C4 plants is one of the most significant events of the Cenozoic ecosystem evolution. It has been regarded as a consequence of atmospheric CO2 decline and climate change, thus its expansion history has great significance in understanding the variations of CO2 concentration, climate changes, and the mechanisms behind mammalian turnovers related to the grassland ecosystem evolution. The current studies are mainly focused on the expansion history of C4 plants since last 8 Ma, the history before 8 Ma, expecially before 20 Ma, however, are very sparse. In this proposal, a 560 m thick lacustrine record from the Xining Basin (Tashan section), covering the peroid between 35 Ma and 16 Ma, was selected to study the early expansion history of the C4 plants in East Asia. The leaf wax carbon isotope will be used as a proxy to reconstruct the ratio of C4 plants, and the leaf wax hydrogen isotope combined with our previous sedimentary facies analysis will be used to reconstruct the climate changes. Our objectives are 1) to reconstruct the expansion history of C4 plants in northern China during 35-16 Ma; and 2) to understand the relationships between the expansion of C4 plants and the potential driven forces, i.e., the decline of CO2 concentration during 32-25 Ma, the onset of East Asian monsoon during 25-22 Ma, and the glabal climatic changes during 35-16 Ma.

C4植物的扩张是新生代生态系统演化的重大事件，其扩张历史的研究对于认识草原生态系统的演化过程、哺乳动物群的演替机制、新生代气候和大气CO2浓度变化的生态效应及其反馈等方面具有重要意义。当前对于C4植物扩张历史及其驱动机制的研究绝大部分集中在8 Ma以来，而针对8 Ma之前、特别是超过20 Ma的研究仍十分缺乏。本项目拟选取申请人已有较好前期工作基础的西宁盆地塔山剖面35-16 Ma期间的湖相沉积为研究对象，通过来源于陆地高等植物的长链正构烷烃单体碳同位素分析，重建C4植物在该区的早期变化历史，并基于单体氢同位素分析和前期开展的沉积环境演化研究，揭示区域气候演化特征。在上述研究的基础上探讨该区C4植物的早期扩张历史与32-25 Ma期间的大气CO2浓度显著降低、25-22 Ma期间东亚季风环境的出现、晚始新世-早中新世期间的全球性重大气候事件等潜在驱动因素之间的联系。

西宁盆地晚始新世-早中新世湖相沉积是探讨东亚中纬度地区C4植被早期扩张历史的重要记录。项目主要对该盆地塔山剖面35-16 Ma期间的地层开展了较高分辨率的长链正构烷烃单体碳、氢同位素分析。取得了如下方面的主要认识：（1）西宁盆地晚始新世碳同位素极为偏正，指示C4植被在CO2浓度较高的晚始新世时期可能就已经出现，这与前人所发现的早期C4植被扩张的证据以及分子钟的预测均要早；（2）25 Ma前后碳同位素发生了急剧偏正，我们初步推测，这种变化可能与东亚季风起源导致的夏季降雨增加所引起的C4植被扩张相关。此外，我们还对少量样品开展了初步的碎屑锆石研究，结果显示，西宁盆地晚始新世-早渐新世红色泥岩样品包含大量360~560 Ma的锆石颗粒，而200-360 Ma的锆石相对很少，这些特征明显不同于新近纪风尘堆积和第四纪黄土，但与祁连山物质的碎屑锆石特征较为一致，表明其物源并非来自与亚洲季风环流相关的粉尘物质，而很可能来自近源的祁连山，因此不支持近期一些学者所提出的晚始新世亚洲季风就存在的观点。