Changes of stable oxygen and hydrogen isotopes in summer Dali-nor Lake in Inner Mongolia of Northern China

Abstract

基于内蒙古高原寒-旱区达里诺尔湖（简称达里湖）相对集中的夏季大气降水、强烈的蒸发作用等典型区域气候环境特征，结合湖泊水体温度、盐度等理化指标变化，对夏季达里湖湖水、入湖河水、区域大气降水等样品中稳定氢、氧同位素（δD、δ¹⁸O）值变化进行了对比分析.结果显示：虽然整体上达里湖夏季水体并没有形成明显的物理性质跃层（如温跃层、盐跃层等），水体理化指标的空间分布相对稳定，但是夏季湖水中δD、δ¹⁸O值却随着水深增加逐渐偏负：δD值由表层水体（0~1 m）到底层水体（7~9 m）偏负约1.40‰，δ¹⁸O值由表层水体到底层水体偏负约0.27‰.达里湖夏季大气水线为δD=8.22δ¹⁸O+6.82，显示大气降水中δD、δ¹⁸O值受季风降水效应影响的同时受到区域蒸发作用影响.特别是7、8月大气降水同位素变化受降水效应的影响比6、9月更明显，而6、9月大气降水同位素变化受蒸发作用的影响比7、8月更明显.这也导致达里湖夏季上层水体受大气降水的稀释作用影响显著；伴随水深的增加，降水效应及蒸发作用的混合影响逐渐减弱，底层水体受区域地下水补给过程等因素的影响则可能更加明显，即达里湖夏季上层水体和底层水体中稳定同位素组成变化的影响因素存在差异.;As one proxy of water evolution, hydrologic cycle and water balance in lakes, the stable isotopes of hydrogen and oxygen (δD and δ¹⁸O) had been wildly utilized during lake researching. However, one problem is that a few works based on stable isotopes of lake water had been done in Cold-Arid regions of China. In this paper, to analyze the relationships of δD and δ¹⁸O and their feedbacks on precipitation process and vaporization process, totally 144 samples of lake water, river water and precipitation water had been collected in summer Dali-nor Lake, which locates in Cold-Arid area of Inner Mongolia, North China. Meanwhile, the typical physical and chemical proxies of lake water had been measured, such as temperature, salinity, pH value, and so on. By comparing the values of δD and δ¹⁸O among different types of sample, we concluded that:The values of classical physical and chemical proxies of lake water offset a certain degree following depth change, and are also a little different between different regions at the same water depth. But there are no clearly fluctuations appeared, such as thermocline and halocline, in summer Dali-nor Lake. As a whole, the characteristics of water mass are essentially stable in summer Dali-nor Lake. But, the values of δD and δ¹⁸O both become lighter and lighter following water depth changing. For example, the value of δD and δ¹⁸O are about -31.723‰ and -1.785‰ in surface water, and about -31.776‰ and -1.838‰ in bottom water, respectively. The meteoric water line is δD=8.22 δ¹⁸O+6.82 (R²=0.936, n=42), which shows that the change of δD and δ¹⁸O in precipitation had influenced both by rainfall process and vaporization process in Dali-nor Lake. The precipitation may have stronger influence on fractional distillation of δD and δ¹⁸O than evaporation process in July and August. Correspondingly, the evaporation process may have stronger influence on fractional distillation of δD and δ¹⁸O than precipitation process in June and September. As a result, the relationships of δD and δ¹⁸O in surface water and bottom water are different in summer Dali-nor Lake. The relationship in surface water is nearly with those in precipitation and river water, but the relationship of bottom water is close to those in groundwater. Resumptively, the value changes of δD and δ¹⁸O in surface water might be influenced mainly by evaporation process and precipitation process, and those might be influenced mainly by groundwater input in deeper water.