华北平原典型区水体蒸发氢氧同位素分馏特征

为研究华北平原衡水地区水体蒸发氢氧同位素分馏特征,采集不同盐度的深层地下淡水(TDS 为0.61g/L)和浅层地下咸水(TDS为7.97g/L),现场开展室外器皿蒸发实验,获得了当地气象条件下氢氧同位素分馏参数.实验结果显示,淡水及咸水剩余表层水δ18O与剩余水比率f呈指数关系,与瑞利分馏模拟结果一致,δD和δ18O蒸发线斜率分别为4.78和4.69.整个蒸发过程中,淡水及咸水氢氧同位素值增量ΔδD分别为Δδ18O的4.82倍和4.76倍;剩余表层水相对于初始水δD和δ18O的变化量与累积蒸发量之比,淡水分别为2.68‰/cm和0.56‰/cm,咸水分别为2.78‰/cm和0.61‰/cm;而在不同的蒸发时段,剩余表层水δD和δ18O的变化量与蒸发量无明显相关性.受水分子扩散的影响,蒸发皿中氢氧同位素分馏在垂线上分层微弱.由于水体盐度较低,在当地气候条件下进行自由蒸发时,氢氧同位素分馏的盐效应可以忽略.

Characteristics of fractionation of hydrogen and oxygen isotopes in evaporating water in the typical region of the North China Plain

To study the characteristics of fractionation of hydrogen and oxygen isotopes in evaporating water in Hengshui, a typical region of the North China Plain, two sets of outdoor water evaporation experiments with different salinities were conducted under local meteorological conditions. Water was collected from deep fresh water (TDS=0.61 g/L) and shallow saline water (TDS=7.97g/L), respectively. The experimental results showed an exponential relationship between δD and δ¹⁸O in residual water and f (the volume ratio between the residual water and the initial water), which was in agreement with Rayleigh fractionation model results. The slopes between δD and δ¹⁸O in the residual surface water were 4.78 and 4.69 for the fresh water and saline water, respectively. It was also shown that the average degrees of enrichment of the hydrogen isotope D were 4.82 and 4.76 times those of the oxygen isotope¹⁸O in the fresh water and saline water, respectively. The ratios between the δD and δ¹⁸O variations of the residual surface water and the evaporation were 2.68‰/cm and 0.56‰/cm, respectively, for the fresh water; and were 2.78‰/cm and 0.61‰/cm for the saline water during the experiment period. Nevertheless, the ratios at different evaporating times had no significant correlations with each other. Isotopic statification was rarely observed in different water layers due to the diffusion effect of water molecules. Moreover, the salt effect on the stable isotopic fractionation during the pan evaporation experiment under the typical climatic conditions was negligible, probably due to the low salt concentration in the saline water.