昆仑山垭口盆地中更新世以来的植被演化与冻土环境变化研究

The changes in both the climate on the Qinghai-Tibet Plateau and in the plateau ground-surface conditions since the Middle Pleistocene have been key scientific issues in recent global changes studies. The Kunlun Pass Basin on the northern Qinghai Plateau is covered by thick and continuous Quaternary sediments, which can provide key clues for inferring the climate and permafrost history during the Quaternary. Therefore, this project plans to choose the top 100 m sediment core from one of the four long wellbores from the Kunlun Pass Basin, which contains the thickest lacustrine sediments. The reliable chronology of the sediment core will be established using multiple dating methods which include AMS radiocarbon dating, paleomagnetic, OSL dating and Be-10 dating. By combining the pollen record with results of other proxy analysis, such as those from grain size distribution, magnetic susceptibility, organic and mineral contents, a high-resolution, well-dated vegetation and climate history since the Middle Pleistocene on the northern Qinghai Plateau would be therefore convincingly reconstructed. In addition, the modern pollen assemblages from the nearby permafrost and talik zones will be compared and contrasted for finding the pollen indicators for the permafrost environment. Furthermore, by synthesizing the results of modern pollen, grain size distribution and minerals analysis, as well as the reliable chronology and the variations in the paleo-vegetation and paleo-climate and -environment, the formation and development processes of permafrost since the Middle Pleistocene in the Kunlun Pass Basin will be established. The project results will have important implications for understanding the vegetation and permafrost responses to Glacial-Interglacial climate cycles. They will further provide key and basic data and references for understanding of the regional climate changes and for predicting future and long-term climate change.

中更新世以来青藏高原气候及地表环境变化过程是全球变化研究领域的重要科学问题。然而目前仍缺少年代可靠且连续的第四纪植被和气候环境演化的完整记录。在高原东部昆仑山垭口盆地，多年冻土发育在深厚的第四纪沉积物中，保存了丰富的高保真气候环境演化信息。项目选择盆地内已获得的4个深钻孔中湖相沉积最厚、层理较清晰的一个钻孔的顶部100 m岩芯为研究对象，利用AMS C-14、古地磁、光释光及Be-10多种测年手段建立可靠的年代学框架；主要通过孢粉分析，结合粒度和磁化率等多指标对岩芯开展古植被、古气候环境研究。在此基础上，根据冻土区与融区表土孢粉组合特征及沉积物粒度分布和矿物特征，结合年代学框架，综合理解多年冻土生成发展与古植被、古气候环境演化的关系，从而探讨该区中更新世以来冻土环境的变化历史。研究结果有助于理解区域植被及冻土对全球变化的响应，为认识过去气候变化的区域差异以及预测未来气候变化提供基础数据。

青藏高原第四纪植被及冻土环境变化的研究具有重要的科学意义。本项目以青藏高原东部昆仑山垭口盆地的钻孔岩芯为研究材料，利用AMS C-14、光释光和古地磁测年方法建立年代框架；在沉积学分析基础上，以孢粉记录为主、粒度等其他代用指标为辅建立植被及气候序列，探讨植被对气候变化的响应特征以及冻土环境的变化历史。研究发现，昆仑山钻孔岩芯中缺少理想的测年材料，冻土钻孔年代无法确定。此外，沉积物没有保存到丰富的孢粉，因此孢粉分析及古植被重建无法开展。鉴于昆仑山钻孔无法完成原设计的植被对气候变化的响应以及冻土环境历史重建的研究目标，为了弥补并完成原计划的研究目标，我们结合其他项目拓展了研究区域至青藏高原东南部的星云湖及黄河源区的万隆沃尔玛湖。通过对星云湖17.6-8.5 ka期间沉积物开展孢粉研究，并综述44个我国其他地区的植被记录，结合古气候模拟结果，讨论了大区域植被对气候突变的响应机制。结果发现，云南高原的植被对新仙女木（YD）气候变冷事件的响应表现为植被垂直带的向下移动，据此推算区域在YD期间年均温下降了约1.5-2.4℃。YD时期植被的响应在我国存在一个纬度梯度：中高纬度由于降温幅度大植被响应明显，而低纬度植被响应不显著。结合古气候资料，发现中高纬度YD时期植被的变化同时受温度和降水的控制，而低纬度地区植被的变化可能受控于春季温度。万隆沃尔玛湖钻孔沉积物的含水量、粒度分布及矿物组成变化重建了区域晚更新世以来冻土演化历史：末次盛冰期之前，万湖盆地发育多年冻土；进入末次盛冰期，盆地之前的多年冻土全部冻结，并随着沉积物的堆积冻土层不断加积；进入冰消期，盆地内发育多年冻土，形成新的冻土上限；随着气候继续变暖，全新世盆地内的多年冻土厚度变小，冻土上限下降；在距今300年左右，随着气候持续变暖，盆地多年冻土上限下降，并形成热融湖塘；300年以来，气候持续变暖，冻土上限下降，冻土持续退化。本项目研究结果有助于理解在未来全球变化背景下植被及冻土的变化趋势。