Study on the Fractionation Mechanism of Stable Isotope in Evaporating Water Body

Abstract

The interaction between evaporating water body and atmosphere and the influence of different factors on evaporation are analyzed by probing into the fractionation mechanism of stable isotope as water body is vaporized under kinetic non-equilibrium and by comparison of the kinetic fractionation model with the Rayleigh equilibrium model.;A simple description of stable isotopic fractionation in the process of evaporation is Rayleigh Model. Under Rayleigh equilibrium, the following results are obtained: a) stable isotopic ratio in residual water increases with the decrease of the residual water proportion %f% exponentially; b) the fractionation rate of stable isotopes is inversely proportional to temperature; c) the simulated Rayleigh distillation line changes with temperature. The higher the temperature, the less the slope and the constant of the distillation line. The simulated distillation line is near global Meteoric Water Line as the temperature is about 20 ℃.;Besides temperature, the relative humidity %h% and the stable isotopic ratio in atmosphere %δ% a also influence the variation of stable isotopic ratio in residual water in the process of kinetic evaporation. The simulations show that, the less the %h%, the quicker the variation rate, with %f%, of stable isotopic ratio in residual water. For great %h%, the ratio δ in residual water will not change with %f% after the evaporation running for a long time, namely isotopic composition in residual water reaches a stable state. The rate of isotope reaching stable state is dependent on the relative humidity in atmosphere mainly.;Compared with the simulated results by Rayleigh Model, the enrichment degree of stable isotopes in residual water is distinctly higher, and the fractionation rate of stable isotopes is directly proportional to temperature under kinetic evaporation, as taking the relative humidity equals to 50%. The higher the temperature, the quicker the fractionation rate, which is exactly reverse to the result simulated by Rayleigh Model. Using the kinetic model, the calculated variation rates, with %f%, of the {δ 18O} in residual water are {-28.27} for %T%=30℃, {-30.40} for% T%=20 ℃and {-32.70} for %T%=10 ℃, respectively, as taking %h%=40%. These values are comparative with the evaporation experiments carried out in Nagqu of the middle Tibetan Plateau.;Under the non-equilibrium condition, the gradient and constant values of the simulated distillation line are directly proportional to temperature and relative humidity. An observation in the Qinghai Lake demonstrates that the initial stable isotopic ratios δD={-60.6}‰and δ 18O={-10.86}‰ in the water of the Qinghai Lake, the annual mean temperature %T%=0^65 ℃ and the relative humidity %h%=55%. Inputting above data into the kinetic fractionation model and taking %β%=1^006, a good agreements between the simulated distillation line (δD=5^56 δ 18O{-1.51}) and the observed line(δD=5^55δ 18O{-0.30}) was obtained.