中国西北干旱区湖泊表泥孢粉及其植被、气候指示意义研究

Researches on the past vegetation succession and climate change in the arid region of northwestern China provide an indispensable key to unravel the modern vegetation-climate relationship in this region. Pollen and spore fossils preserved in lake sediment have been used as the proxy of palaeovegetation and palaeoclimate frequently. Based on palynological data, many techniques have been developed to reconstruct the palaeovegetation and palaeoclimate quantitatively, such as Biomisation Technique, Transfer Function Method, Modern Analogue Technique, etc. The underlying connection between modern pollen assemblage and modern vegetation and climate is the base to extract the information of palaeovegetation and palaeoclimate from fossil pollen assemblage. In previous studies, surface soil pollen assemblages were always used as modern references for assisting interpretation of fossil pollen assemblages. However, the source area, sedimentation and taphonomy characters of pollen assemblages in lake sediment are different from those in surface soil. Hence, the pollen assemblages from surface lake sediments are suggested to be better modern references for fossil pollen assemblage from lake core/profile. Unfortunately, only rare attentions have been paid on pollen assemblage from surface lake sediment in northwestern China. In this project, we plan to investigate the pollen assemblages of surface lake sediments in different vegetation zone of northwestern China, and study their relationships with modern vegetation and climate. Subsequently, we compile the pollen data and relevant vegetation/climate information of sampling sites to establish a surface lake sediments pollen dataset. Based on the new pollen dataset, we 1) test the validation of Biomisation Technique in simulating modern vegetation; 2) establish the pollen - climate transfer functions for estimating climate parameters by using pollen data, 3) test the robustness of the new pollen dataset as training set when using Modern Analogue Technique to predict climate parameters. Then, we provide a case study on reconstructing the Holocene vegetation and climate of northwestern China adopting above mentioned quantitative techniques, and discuss the mechanism of past vegetation succession and climate evolution. These works are expected to advance the quantitative reconstructions of palaeovegetation and palaeoclimate in arid region of northwestern China at different spatiotemporal scales.

中国西北干旱区第四纪植被和气候变化研究是解读现代全球变化背景下该区域植被和气候响应特征及机制的重要参考。湖泊钻孔/剖面中的孢粉化石是最常用的古植被和古气候代用指标之一，而正确解读孢粉化石中的古植被和古气候信息必须以现代孢粉与植被、气候的关系研究为依据。湖泊表泥孢粉组合是湖泊钻孔/剖面中化石孢粉记录可靠的现代参照。然而，中国西北干旱区却极为缺乏湖泊表泥孢粉研究。本项目拟调查中国西北干旱区不同植被带湖泊表泥孢粉组合特征及其与植被和气候的关系，并建立中国西北干旱区湖泊表泥孢粉数据库，以此为基础，检验当前国际上常用的古植被（生物群区化方法）和古气候（转换函数法、现代类比法）定量重建方法在西北干旱区的适用性和可靠性，并尝试用这些方法定量重建西北干旱区全新世典型钻孔/剖面的古植被和古气候，探讨全新世西北干旱区植被演替与气候变化的机制。为下一步开展该区域不同时空尺度的古植被和古气候定量研究提供可靠参考。

孢粉现代过程研究是解读地层中孢粉化石数据的关键基础。中国干旱、半干旱地区孢粉现代过程研究急需加强，特别是湖泊表泥的孢粉组合研究亟待加强。本项目在中国干旱、半干旱地区开展了湖泊表泥和表土孢粉组合研究，对比了两种样品类型孢粉组合的异同，用统计分析方法研究了孢粉数据对植被和气候的指示意义，并检验了常用定量重建方法在该区域的可靠性和适用性。现阶段主要认识如下：1）中国西北干旱、半干旱区湖泊表泥和表土孢粉组合中都以藜科和蒿属花粉占优势，此外，麻黄属、莎草科、菊科、禾本科、云杉属、松属和桦木属都占重要地位；不过，孢粉组合特征和主成分分析结果显示两种样品的孢粉组合存在明显的差异，这些差异可能是两种样品花粉源区大小、花粉保存条件和花粉来源的不同引起的；主成分分析结果显示相对于表土，草原和草甸中湖泊表泥孢粉组合趋向于反映更湿润的条件；线性回归分析结果显示湖泊表泥孢粉组合的蒿属/藜科比值（A/C）与年均降水量有显著正相关，可用于指示该区域湿度的变化，而表土中蒿属/藜科比值与年均降水量无显著相关性，需要谨慎使用。2）半干旱区草原带现代孢粉研究结果表明草原现代孢粉组合以蒿属和藜科最多，其次是毛茛科、菊科、石竹科等，禾本科仅有极低的含量，出现了喜旱类群麻黄属、柽柳科、蒺藜科和白刺属等，乔木类群以松属和桦木属为主；冗余分析结果显示孢粉组合变化反映了三种草原环境（草甸草原、典型草原和荒漠草原）湿润度的变化；喜旱类群麻黄属、柽柳科和蒺藜科花粉相对丰度变化可用于划分不同草原类型，并且喜旱类群总体相对丰度与干燥度有高度正相关，有潜力作为代用指标指示干湿程度的变化。3）用现代孢粉数据对生物群区化法的可靠性和适用进行检验，发现目前该方法在干旱、半干旱地区识别荒漠植被的可靠性较高，而重建其它植被类型的可靠性尚需提高。这些研究成果为进一步在中国西北干旱、半干旱区开展不同时空尺度的古植被和古气候定量研究提供了可靠参考。