17Oexcess in evaporated desert waters and vapor from evaporation experiments

Oxygen and hydrogen isotopes are classical proxies for the investigation of climatic effects in hydrological processes. The combination of the isotopic ratios ¹⁷O/¹⁶O and ¹⁸O/¹⁶O in water allowed the determination of mass dependent processes and enabled differentiation between equilibrium and kinetic fractionation (Barkan and Luz, 2007). In analogy to d-excess, deviation in δ¹⁷O from the global average trend of meteoric water is defined as: ¹⁷O_excess = δ'17O - 0.528 × δ'18O ¹⁷O_excess depends on the impact of diffusive evaporation into air and thus reflects relative humidity conditions. The isotope ratios of water δ¹⁷O and δ¹⁸O were determined by isotope ratio gas mass spectrometry in dual inlet mode on a ThermoFinnigan MAT 253. The oxygen was extracted by water fluorination with CoF₃. Our average measurement precision for δ¹⁷O is ×0.03 ‰, for δ¹⁸O ×0.05 ‰ and for ¹⁷O_excess approximately ×7 per meg (1σ). We compared ¹⁷O_excess in natural waters from the highly arid deserts of Sistan (East Iran) and Atacama (Chile) with data obtained from evaporation experiments. In these experiments, water was evaporated into a stream of dry nitrogen and vapor collected cryogenically. The data show a systematic depletion of ¹⁷O_excess in water with increasing degree of evaporation in the residual water body. Most negative ¹⁷O_excess were determined for samples from ponds (Sistan) and salars (Atacama). These strongly evaporated samples indicate an evaporation development, following a fractionation trend (λ) of approximately 0.523. The evaporation experiment shows a λ of 0.525 and is in agreement with water data from an experiment by Barkan and Luz (2007). The difference between natural and experimental evaporation suggests either different evaporation kinetics in the natural environment, variable proportion of kinetic and equilibrium fractionation, or additional diffusive processes during ground water seepage. References: Barkan, E. and Luz, L. (2007). Diffusivity fractionations of H₂¹⁶O/H₂¹⁷O and H₂¹⁶O/H₂¹⁸O in air and their implications for isotope hydrology. Rapid Commun. Mass Spectrom., Vol. 21, pp. 2999-3005.