青藏高原西北部末次冰消期以来的植被、气候变化与西南季风北界演化历史

The climate, ecosystem and environment on the Tibetan Plateau (TP) have received profound influences from the Southwest Monsoon and the Westerlies. Evolution history of these two circulation systems and their environmental impacts on the TP are very important for the understanding of influence mechanism for environment and climate changes in this area. In this project, we propose to conduct high-resolution analyses of fossil pollen assemblages covering the last twenty thousand years, with four to five lacustrine sediment cores across the transect between 32 N and 35 N in the northwest TP. And then, based on quantitive relationships between modern pollen assemblages and contemporary vegetation and climate condition on the TP, histories of vegetation and climate changes since the Last Deglacial will be quantitively reconstructed by using various methods such as biomization, discriminant analysis and the established pollen-based climate transfer functions. The responses of alpine vegetation to climate change during this time span will be revealed by comparison between paleovegetation and paleoclimate records. Vegetation distributions and migrations, tempo-spatial variations of precipitation and inferred moisture source changes will be applied to determine the major circulation system at different intervals, to recover the possible environmental impacts from the Southwest Monsoon, and monsoon boundary migrations and intensity changes, and finally to discuss the interplay between Southwest Monsoon and the Westerlies in the northwest TP. Furthermore, dynamic mechanisms for paleoenvironment change and evolution of Southwest Monsoon and the Westerlies on the TP will be discussed, based on regional comparison and remote correlation with paleoclimate records from the high and low latitudes. This study will provide scientific basis for paleoclimate modeling, and development of vegetation restoration strategy and construction of ecological security shelter zone on the TP.

西南季风与西风带两大环流系统深刻的影响着青藏高原气候与生态环境的变迁。地质历史时期青藏高原西南季风-西风演化与环境影响的研究，对于正确理解高原生态环境与气候的影响机制具有重要科学意义。项目在研究基础薄弱的高原西北部沿纬度布置4-5个湖泊钻孔（32 -35 N），开展过去2万年来高分辨率的孢粉化石分析；结合现代孢粉-植被-气候定量关系，通过生物群区化、植被判别分析和孢粉-气候转换函数方法量化植被与温度、降水变化序列，阐明该地区植被演替对气候变化的响应；以植被分布与迁移、降水格局的时空变化来分析末次冰消期以来高原西北部不同时段的主导大气环流格局，追索西南季风的可能影响、强度变化、北界迁移及其与西风带环境影响的相互作用。通过区域与全球高、低纬古气候记录的对比，探讨末次冰消期以来青藏高原气候与季风-西风环境演化的动力机制，为古气候模拟与青藏高原植被恢复、生态安全屏障建设等提供重要的科学依据。

青藏高原过去植被与气候变化的研究对于深入理解高原生态环境与气候的影响机制、预测未来气候情景下植被变化趋势和生态屏障建设等具有重要的科学意义。项目在研究基础薄弱的青藏高原西北-北部地区（32°-35°N），利用令戈错、班公错等湖泊的钻孔开展了过去约2万年来百年分辨率的孢粉分析，获得了化石孢粉序列；通过生物群区化方法构建了古植被演替定量序列，依据孢粉多样性量化了植物多样性的变化过程，基于大样本量的现代孢粉数据库与转换函数方法获得了气候要素的定量序列。项目研究获得的重要结果有：（1）证实了高原大型湖泊沉积物孢粉组合具有区域地带性植被的代表性；这对于未来湖泊科学钻探计划的点位选取、化石花粉记录的植被、气候定量解译工作具有重要的参考意义。（2）约20-13.8 ka BP青藏高原西北-北部以高寒荒漠稀疏植被为主；全新世期间高寒草原逐渐占据主导地位，中全新世适宜期高寒草原最为繁盛，但植物多样性较低；晚全新世高寒草原有所退缩，荒漠植被扩张。（3）青藏高原西北-北部植被变化对降水变化具有敏感的响应，而高原中部与东部植被变化则对温度变化具有敏感的响应。这一发现揭示了青藏高原过去植被演替的气候驱动因素具有区域差异。（4）高原西北部约12 ka BP、北部地区不晚于13.8 ka BP，西南季风已对当地植被和气候产生了影响。项目后续研究将重点开展多支钻孔的化石孢粉-气候定量重建，基于同一指标与方法对不同时间断面上季风-西风影响范围的空间格局进行分析。