柴达木盆地陆地植物单体氢同位素对湿度变化的响应

The hydrogen isotope ratio in leaf wax is emerging as a novel tool to reconstruct palaeohydrological changes. To consider the simultaneous influence of precipitation and evaporation, many studies have suggested that the D/H ratio of plant-wax alkanes can be used as a record of continental hydrological variability and as a proxy of aridity or relative moisture. Many researches predominantly carried on the humid region around the margin of low and intermediate latitude continent. However, the understanding of the process and mechanism of hydrogen isotopic fractionation in extremely arid area, which is characterized with lower precipitation and higher evaporation, is still unclear. Therefore, we plan to strength the study of the plant hydrogen isotope fractionation in Qaidam Basin which is the typical arid region in Asian inland. The research will focus on isotopic mechanism between the source water (precipitation and soil water) and typical plant (grass and shrub) in this area and include the experiments through climatic section and moisture controlling in fields and laboratory. We expect to obtain the robust relationship of terrestrial moisture and the plant wax hydrogen index and reconstruct the moisture change during the mid to late-Holocene. The work would provide new approach and insight to understand the climatic change pattern and hydrological history in the study area.

单体氢同位素作为水文变化的重要指标，目前的研究主要在中低纬大陆边缘的湿润区，而在低降水、高蒸发的内陆极端干旱区，区域性现代过程、响应机理和古环境重建研究还有待加强。作为亚洲干旱内陆区的典型代表地区，在柴达木盆地加强单体氢同位素研究，通过自然断面、野外长观和实验培养相结合的方法，研究该区域典型陆地植物(草本、灌木)与降水/源水间的同位素分馏过程，理解极端干旱化条件下氢同位素的分馏机制，获取此地区植物氢同位素与湿度变化间的定量关系，推进单体氢同位素指标对中晚全新世以来陆地环境重建工作，这将为认识亚洲内陆干旱区的水文历史和气候模式变化提供新的手段和基础认识。

项目通过调查极端干旱区柴达木盆地和黄土高原西部半干旱区的现代环境和沉积序列的调查采样，获取了研究区草本灌木植物的叶蜡氢同位素分布特征和区域干湿变化历史。确认中国西部现代灌木和降水间的氢同位素表观分馏为-100‰，而草为-140~-150‰。提出用基于烷烃比例(C31/C29)和水生生物指数Paq指标组合的烷烃植被类型指数(Pta)来解决应用叶蜡氢同位素重建古降水同位素中的植被效应，并据此选择氢同位素分馏系数进行定量重建。通过近千来中国不同区域的氢同位素序列的验证表明，重建古降水同位素和现代降水有很好的相关性，说明该方法的有效性。利用叶蜡氢同位素和有机链长指标重建了末次冰消期以来青藏高原东北部湖深和湿度变化历史。千年尺度上，末次冰消期以来青海湖出现过5次平衡状态，湖水从15.6ka之前的最浅状态波动上升上升降，湖泊最深时间出现在5.9ka，之后缓慢下降。而叶蜡氢同位素则显示出和东部典型季风湿润区夏季风强度变化完全一致的关系。认为全新世青藏高原东北部湿度变化同时受到大趋势蒸发偏弱以及短周期上季风降水增加的叠加效果控制。在轨道尺度上，黄土高原250ka以来低分辨黄土氢同位素记录反映了植被变化和降水变化的综合信息，表现出强烈的100ka、40ka周期以及较弱的20ka干湿变化周期，夏季风增强带来的降水增多是季风边缘区环境湿润的主要因素。为了应对湖泊烯酮的种属变化，提出了一种基于C37:2丰度和温度的对数关系的新湖泊烯酮指标U37ks。这一发现为湖泊温度定量重建提供了新途径，也为评估局地冷凝温度对叶蜡氢同位素分馏的影响提供了基础。