额济纳荒漠非饱和带记录的大气降水补给及其古环境意义

The unsaturated zone (vadose zone) in desert areas, as a part of the generalized groundwater system, is very sensitive to precipitation and groundwater circulation, as unsaturated zone solute and isotope profiles may be interpreted in terms of past changes in recharge rate and climate in cases where water movement can be approximated by piston flow. The unsaturated zone is therefore most likely a robust achievement recording the past regional hydrology and climate change during the historical period (at decades- and hundreds-years scale). The chloride and stable/radioactive isotope profiles in unsaturated zone from desert areas in northern China, where less precipitation occurs at present and is lack of surface runoff, have not been well studied to quantify soil-water flux and its relation to climate at low recharge rate. The Ejina Basin is located at the north edge of the East-Asian summer monsoon climate and also lies in the geographical latitudes of global westerly system, so it is an ideal place for studying the records of past climate information due to the less disturbance bu human activities. In this project, we plan to take the typical unsaturated profiles (such as in sandy dunes) and regional representative water/soil samples as the research targets, using the chloride mass balance method and isotopic geochemistry and chronology techniques to obtain the information about recharge rate of unsaturated zones from precipitation and hence the reflected fluctuation signals of dry and wet climate at special time periods, based on which we will further discuss the time limits and nodes of past hydrological events and climate change and evolution during that time. This item will provide new evidences for understanding groundwater recharge from precipitation and high resolutional paleoclimate reconstruction in arid regions.

荒漠土壤非饱和带（包气带）作为广义地下水系统的一部分，对大气降水和地下水循环极为敏感，是最有可能记录干旱区过去水文-气候变化的档案；在降雨稀少、地表径流缺乏的荒漠区，土壤水以接近活塞流移动的包气带上，可溶盐氯及同位素剖面记录了大气降水的多年平均补给速率及其过去的变化情况，因而可以较好的示踪历史时期（百年或千年尺度）降雨量的变化过程及其所反映的气候特征。额济纳盆地既位于东亚夏季风的北缘，又处在全球西风环流的纬度上，受人类活动的干扰小，是研究过去气候记录的理想场所。本项目拟以额济纳荒漠区（如沙丘地）的典型包气带剖面及区域代表性水、土样品为研究对象，应用可溶盐氯质量平衡法、同位素地球化学和年代学等方法，获取额济纳荒漠区特征时段内大气降水的补给速率及其所反映的气候干湿波动信息，进而探讨区域水文、气候环境演化的时间界限和节点。本项目将为干旱区地下水补给和高分辨率古气候重建研究提供新证据。

本项目以额济纳盆地荒漠区及其周边地区（阿拉善高原）典型荒漠地貌景观（戈壁、沙丘地）的土壤非饱和带（包气带地层）为研究对象，通过区域大范围地表风成沉积物样品、大气降水—河流—湖泊系统的水盐样品等的野外采集，应用可溶盐质量平衡理论（活塞流理论等）、同位素地球化学和CAA年代学等分析方法，获取了研究区数十个包气带地层沉积剖面和环境示踪剂序列；综合已有的同类地质记录，集成并建立了研究区土壤非饱和带可溶盐序列的数据库；基于“地表氯凸”、“零通量面”、“沉积物粒度分选参数”、“孔隙导水率”等判别标准，识别了包气带剖面中的“活塞流剖面”（均质、封闭、无优先流旁路流的水分运移方式），即环境示踪剖面；基于最长尺度现代降水记录的检验，建立了可用于估算包气带大气降水补给速率的环境代用指标（如氯倒数等）；基于CMB、CAA方程等估算了研究区沙丘地过去近千年以来的大气降水对区域广义地下水（包气带）的补给速率的平均值与极端值，空间分布特征/梯度，及其与现代季风雨带的关系；重建了研究区这一时间尺度上的补给历史和区域气候干湿波动过程，并基于区域对比分析了藏北高原-阿拉善高原一带（季风/非季风边缘区）在过去千年尺度上的大气降水的同步性和差异；再与区域水文记录（水量平衡分析结果）对比，并基于活塞流理论的假设条件的广泛验证，识别了地质记录估算结果相对于水文记录的不确定性及其误差所在；同时基于大尺度水盐样品分析，识别了风成沙可溶盐的大气来源及其干、湿沉降的相对贡献，矫正了CMB估算结果的误差，重新审视了古大气降水对区域地下水补给的水文意义，及其对未来全球气候变化下的启示意义，提出了干旱区水循环和古水文重建方面的新问题和研究方向。本项目发表学术论文22篇，其中包括SCI论文10篇、非SCI论文（CSCD+国际英文期刊论文）12篇，基本完成了项目任务书制定的预期目标和考核指标。