青藏高原不同气候区表土正构烷烃氢同位素空间变化特征及其与环境因子关系研究

The hydrological cycles on the Tibetan Plateau and its sensitive feedback for climate changes are becoming research focus of climate change science. But the progress of research on paleohydrological cycles on the Tibetan Plateau is slow because of lacking a valid environmental proxy for wide application. Hydrogen isotope ratios (δD) of soil/sedimentary n-alkanes can be applied to reconstruct paleohydrological and paleoclimate change because it faithfully preserved the δD of plant source water during biosynthesis processes. However, the δD signal of plant source water is affected by a number of environmental factors. These may cause that δD of soil/sedimentary n-alkanes have different environmental significance in different climate regions. This project intends to select different climate regions on the Tibetan Plateau for research. These climate regions have large gradient difference of environmental factors such as precipitation amount, δD of precipitation, temperature, relative humidity, aridity, vegetation composition, et al. The spatial variations of δD of soil n-alkanes in these climate regions will be analyzed based on identifying the different plant sources of soil n-alkanes firstly. Then the relationship between δD of soil n-alkanes and environmental factors will be discussed, through comparing δD of soil n-alkanes with environmental proxies and using mathematical statistics method. Thus, the main control environmental factors and the response mechanism will be revealed, and then the environmental significance of δD of soil n-alkanes on the Tibetan Plateau will be understood. The work of this project can provide the theoretical basis for reconstructing paleohydrological cycles and paleoclimate changes by using δD of soil/sedimentary n-alkanes.

青藏高原水文循环过程以其对气候变化的敏感反馈，正在成为气候变化研究的热点。由于缺乏广泛适用的环境代用指标，青藏高原过去水文循环研究进展相对缓慢。表土/沉积物正构烷烃氢同位素能够可靠保存生物合成时来源水的氢同位素组成信息，具有记录区域水文循环历史和环境变化的潜能。由于从来源水至生物合成过程中受多种环境因子影响，正构烷烃氢同位素在不同气候区的气候环境意义可能并不一致。本研究拟选择降水量、降水氢同位素、温度、相对湿度、干燥度和植被类型等环境因子具有较大梯度差异的青藏高原不同气候区，在识别其表土正构烷烃主要来源基础上，分析其正构烷烃氢同位素的空间变化特征，结合区域气候、植被、土壤水分等要素，通过数理分析方法探讨与环境因子之间的关系，揭示其主要环境控制因子与响应机制，进而理解正构烷烃氢同位素所具有的环境指示意义，为该指标应用于青藏高原过去水文循环和环境变化研究提供理论依据。

明确正构烷烃及其单体同位素的环境指示意义，是利用该类指标进行古气候古环境研究的前提和关键。本项目通过采集青藏高原不同气候区的表土和湖泊沉积物并对其进行正构烷烃指标的分析，探讨了正构烷烃δD和δ13C可能的影响因素，并在明确正构烷烃环境指示意义的基础上，有效重建了青藏高原南部普莫雍错过去19 cal ka BP以来古水文过程以及高原中部当惹雍错过去6 cal ka BP以来古环境变化，并分别探讨了导致区域气候环境变化的驱动机制。本项目研究表明：（1）冰川融水补给为主的普莫雍错沉积物中长链正构烷烃单体δD（δDlong）可指示季风降水变化；中链正构烷烃单体δD（δDmid）很大程度上受控于流域内同位素负偏的冰川融水的变化；当该湖中链与长链正构烷烃分别来自水生植物和陆生高等植物时，其氢同位素差值ΔδDmid-long可以用来反映区域相对湿度的变化。基于正构烷烃δD重建的古水文变化与周边区域古气候记录对比进一步表明，15.0 cal ka BP以来普莫雍错冰川融水补给与受夏季太阳辐射控制的印度夏季风强度变化对应较好，而全新世以来湖泊降水补给变化与印度夏季风变化并无显著关系，这强调了普莫雍错流域在西南季风气候主导下区域古水文过程响应气候变化的复杂性。（2）当惹雍错沉积物水生生物来源正构烷烃单体碳同位素（δ13C-Cmid）与Paq变化分别反映当惹雍错过去6 cal ka BP以来湖泊生产力与湖泊水位总体持续下降的过程。将当惹雍错记录与高原中部以及周边区域其他沉积记录对比研究表明，青藏高原中部过去6 cal kaBP以来湖泊环境变化趋势较为一致，且在千年尺度上主要受印度夏季风的控制。更进一步的驱动机制分析表明，青藏高原中部印度夏季风显著响应于太阳辐射与赤道辐合带（ITCZ）位置的变化。研究还认为，北大西洋高纬地区气候可能通过一些间接的机制对青藏高原中部地区气候产生影响。本项目的顺利实施，不仅加深对正构烷烃δD和δ13C环境指示意义的认识，拓展了该类指标在青藏高原不同气候区的应用，同时也深化了对青藏高原过去水文过程及古环境变化的认识。