长江下游地区末次冰盛期植被变化及其气候响应

Last glacial maximum (LGM) is climatically a special geologic period in greatest contrast with that at present during the last 30 ka years. The extreme cold climate exerts a profound impact on ecological environment. At that time, in the lower reaches of the Yangtze River region in eastern China, strong incision and erosion in river valley and retreat of rivers and lakes result in sediment hiatus. Furthermore, widespread aeolian sediments such as Xiashu loess and stiff hard clays in hill and plain area greatly limit ability to reconstruct palaeoclimate. Thus, this area has long been a blind zone in terms of pollen records. We will prefer to select newly-found excellent sediments from Lake Xianghu and Lake Chaohu including fine clays deposited in lowland lakes and swamps for pollen analysis. With a valid plant-based AMS 14C chronology, zonal vegetation history will be reconstructed using pollen data in response to regional climate changes during LGM in the study area. We attempt to understand the vegetation type which is composed by new xerophylious Artemisia-dominant pollen floras and their possible southward migration and southern limit of open forest-grassland communities during the glacial episode. This project is expected to provide new clues for interpreting vegetation pattern on exposed coastal continental shelves and better understanding environmental effects on biology under extreme climatic condition, etc.

末次冰盛期（LGM）为距今约3万年以来与现代环境反差最大的一个特殊气候时期，其极端严寒气候对生态环境造成了深刻的影响。当时长江下游地区，河谷强烈下切剥蚀、河湖萎缩造成地层缺失以及丘陵平原广布的下蜀黄土、硬土层等不利埋藏环境，极大限制了孢粉重建古环境的能力，一直为孢粉记录的“盲区”。我们优选新近发现的浙江湘湖、安徽巢湖等优质地层及低洼地湖沼相沉积物的孢粉详细分析，基于可靠的植物残体AMS14C等测年手段，重建研究区植被史，旨在揭示地带性植被对冰期气候变化的区域响应。依据新发现的多个以蒿属为主的喜旱型l陆生草本优势孢粉记录，着重判别植被的性质，探讨冰期疏林草地等草原型植被分布的可能性及其南界，捕捉与其关联的夏季风尾闾区北界。本研究将为沿海大陆架植被探讨提供新线索、对深入理解极端气候生物环境效应等具有重要的学术意义。

末次冰盛期（LGM）是地球环境变迁史上是距今约7万年以来与现代环境反差最大的特殊极端冷干时段，当时严酷的气候对生态环境格局和地貌造成了深刻的影响。为弥补和揭开长江下游地区长期缺乏合适生物记录的“盲盒”，我们紧扣项目的设计和规划，优选安徽巢湖、南京林蜂桥、南通余东、江苏宜兴、浙江湘湖、浙江义乌等由北向南取得了不可多得富含有机质的低洼地湖沼相地层，通过详细的孢粉分析，建立可靠的AMS14C年代框架，由点到面综合重建了研究区在LGM时期总体上都以蒿属（Artemisia）、禾本科(Poaceae)为主的旱生地带性草原景观，指示了类似现今内蒙古—黄土高原一带北方疏林草原植被，与当今研究区亚热带常绿落叶阔叶林形成鲜明反差。由此推断LGM气候极端冷干，年均温下降达7℃以上，年降水相当于现在的1/3左右。本研究结合大陆架孢粉资料，首次以明确的年代约束材料提出我国LGM“东部大草原”观点，与先前杨怀仁教授 (1980) 提出的“冰期冷槽”学说不谋而合。同时，根据东部平原地形易受强盛冬季风入侵，提出草原南界扩展越过钱塘江—杭州湾一线至少达到浙江中部，大大低于欧洲冰期草原南界。由此推断当时一路退缩的夏季风尾闾区北界也在这一线附近，甚至更南。这项研究表明LGM东部疏林草原的形成主要受全球性冰期降温、太阳辐射减弱及海平面大幅下降控制和驱动。本研究为长期争议的沿海大陆架植被性质及我国东部低海拔有否第四纪冰川的学术探讨提供新的资料和视角。同时，对深入理解极端气候生物环境效应等具有重要的科学价值和学术意义。