兰州地区黄土揭示的西部黄土高原第四纪气候环境演变研究

Important paleoclimatic progresses have been made on the temporal sequence recorded by the Chinese loess during the Quaternary period. However, researches on the spatial paleoclimate changes in the Chinese Loess Plateau (CLP) are generally weak. Currently, the 13Corg records in the different areas show that there is great divergence of views on the time, feature and driving factors of C4 plant expansion on the tectonic scale in Quaternary period, which needs to be verified urgently by the new and reliable records in adjacent regions. The special location and higher sedimentary rate of western CLP leads to more sensitiveness of the paleoclimatic proxies in response to the climatic and environmental changes. This has important value to further understand the problem of C4 plant expansion, the process and trend of climatic evolution, etc. in Quaternary period. On the base of past relevant research of the loess sequence in Xijin village in Lanzhou region, this project plans to connect the record of the Xijin loess sequence with that of the absent Quaternary loess from 2.6 to 2.2 Ma ago, after the high-resolution chronology and records of climatic change will be obtained using the drilling core. The whole Quaternary stratigraphic, chronologic and paleoclimatic change sequences will be established in Lanzhou region. The records of paleovegetation, paleoprecipitation and climatic environment changes will be extracted on the tectonic scale during the Quaternary period in the area, by the compound-specific isotopes 13C and D of the n-alkanes and 13Corg index, etc. Then the history of the C4 vegetation expansion and climatic evolution in Quaternary period will be rebuilt in Lanzhou region of the western CLP. The reason and controlling factors of the temporal and spatial differences on C4 vegetation expansion in CLP region will be revealed on the tectonic scale through regional comparison and the driving mechanism of the paleoclimatic change will be further understood in Quaternary in the region.

中国第四纪黄土记录时间序列研究成果斐然，而空间研究相对滞后。目前黄土高原不同区域有机碳同位素记录研究，对第四纪构造尺度C4植被扩张时间、特征及驱动因素等的认识分歧较大，急需临近地区新的可靠记录来检验。西部黄土高原区位特殊，黄土沉积速率高、且能敏感响应气候环境信号变化，对深入理解C4植被扩张问题和第四纪环境演变过程及趋势等具有重要价值。本项目拟在前期兰州地区西津第四纪黄土相关研究基础上，通过钻取该区2.6-2.2 Ma间缺少的黄土地层，以获取高分辨率年代和气候变化记录实现与西津黄土对接，建立该区完整第四纪地层、年代和气候环境变化序列。并通过正构烷烃单体碳、氢同位素和有机碳同位素等指标，提取第四纪构造尺度古植被、降水和气候环境记录，重建西部黄土高原地区第四纪C4植被扩张和气候环境演变历史，再通过区域对比研究，揭示黄土高原地区C4植被扩张时空差异成因及控制因素，更深入理解第四纪气候变化驱动机制。

中国第四纪黄土记录经过长期研究，在时间序列方面已取得丰硕成果，但空间研究相对滞后，黄土高原不同地区第四纪构造尺度C4植被扩张时间、特征及驱动因素等存在一定差异，本项目主要内容是通过钻取2.2 Ma之前兰州第四纪黄土缺少的岩芯地层，基于多种测试分析方法，与西津黄土地层对接建立完整的第四纪沉积、磁性地层及气候变化序列，并使用正构烷烃单体碳、氢同位素和有机碳同位素指标，提取第四纪构造尺度C3/C4植被相对丰度和降水记录变化信息，重建兰州地区第四纪C4植被扩张和气候演变历史，综合对比理解黄土高原地区C4植被扩张时空差异原因、控制因素与机制。主要获得以下认识和成果：（1）通过钻探工程获得了兰州地区2.8 -2.2 Ma的沉积地层岩芯，采用古地磁、粒度和磁化率的实验分析，基于黄土-古土壤地层的周期性气候旋回规律，对接建立了兰州地区2.8 -0 Ma完整的第四纪黄土-古土壤序列磁性磁层、沉积地层和气候环境记录序列。（2）磁化率和粒度敏感记录了2.80-2.45Ma冰期-间冰期旋回剧烈的气候环境演化过程，而在其后的2.45 -1.52 Ma之间则表现出长期低幅波动。（3）通过总有机碳和正构烷烃单体碳同位素记录提取，重建了兰州第四纪植被C3/C4演替变化过程，发现从2.0 Ma以来兰州地区C3/C4混合植被类型中C4植被的相对比例是逐渐降低的，直到末次冰期完全演变为纯C3植被类型，之后的全新世随温度上升，C4又重新出现在兰州的植被群落结构中，这主要受到了全球长期降温过程的调控。（4）兰州地区在整个第四纪C4植被经历2.35-1.24 Ma和0.40-0.13 Ma两次明显的扩张时期，各项指标的对比表明，2.35-1.24 Ma时期的C4植被扩张可能温度有一定贡献，但降水是首要的驱动因素；0.40-0.13 Ma间的C4植被扩张发生在冷干的冰期，干旱是直接驱动C4植被冰期扩张的主要原因。