TY - JOUR
T1 - Fractionation of oxygen and hydrogen isotopes in evaporating water
AU - Luz, Boaz
AU - Barkan, Eugeni
AU - Yam, Ruth
AU - Shemesh, Aldo
PY - 2009/11/15
Y1 - 2009/11/15
N2 - Variations in oxygen and hydrogen isotope ratios of water and ice are powerful tools in hydrology and ice core studies. These variations are controlled by both equilibrium and kinetic isotope effects during evaporation and precipitation, and for quantitative interpretation it is necessary to understand how these processes affect the isotopic composition of water and ice. Whereas the equilibrium isotope effects are reasonably well understood, there is controversy on the magnitude of the kinetic isotope effects of both oxygen and hydrogen and the ratio between them. In order to resolve this disagreement, we performed evaporation experiments into air, argon and helium over the temperature range from 10 to 70 °C. From these measurements we derived the isotope effects for vapor diffusion in gas phase (εdiff(HD 16 O) for D/H and εdiff(H2 18 O) for 18O/16O). For air, the ratio εdiff(HD 16 O)/εdiff(H2 18 O) at 20 °C is 0.84, in very good agreement with Merlivat (1978) (0.88), but in considerable inconsistency with Cappa et al. (2003) (0.52). Our results support Merlivat's conclusion that measured εdiff(HD 16 O)/εdiff(H2 18 O) ratios are significantly different than ratios calculated from simplified kinetic theory of gas diffusion. On the other hand, our experiments with helium and argon suggest that this discrepancy is not due to isotope effects of molecular collision diameters. We also found, for the first time, that the εdiff(HD 16 O)/εdiff(H2 18 O) ratio tends to increase with cooling. This new finding may have important implications to interpretations of deuterium excess (d-excess = δD - 8δ18O) in ice core records, because as we show, the effect of temperature on d-excess is of similar magnitude to glacial interglacial variations in the cores.
AB - Variations in oxygen and hydrogen isotope ratios of water and ice are powerful tools in hydrology and ice core studies. These variations are controlled by both equilibrium and kinetic isotope effects during evaporation and precipitation, and for quantitative interpretation it is necessary to understand how these processes affect the isotopic composition of water and ice. Whereas the equilibrium isotope effects are reasonably well understood, there is controversy on the magnitude of the kinetic isotope effects of both oxygen and hydrogen and the ratio between them. In order to resolve this disagreement, we performed evaporation experiments into air, argon and helium over the temperature range from 10 to 70 °C. From these measurements we derived the isotope effects for vapor diffusion in gas phase (εdiff(HD 16 O) for D/H and εdiff(H2 18 O) for 18O/16O). For air, the ratio εdiff(HD 16 O)/εdiff(H2 18 O) at 20 °C is 0.84, in very good agreement with Merlivat (1978) (0.88), but in considerable inconsistency with Cappa et al. (2003) (0.52). Our results support Merlivat's conclusion that measured εdiff(HD 16 O)/εdiff(H2 18 O) ratios are significantly different than ratios calculated from simplified kinetic theory of gas diffusion. On the other hand, our experiments with helium and argon suggest that this discrepancy is not due to isotope effects of molecular collision diameters. We also found, for the first time, that the εdiff(HD 16 O)/εdiff(H2 18 O) ratio tends to increase with cooling. This new finding may have important implications to interpretations of deuterium excess (d-excess = δD - 8δ18O) in ice core records, because as we show, the effect of temperature on d-excess is of similar magnitude to glacial interglacial variations in the cores.
UR - http://www.scopus.com/inward/record.url?scp=70349736132&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2009.08.008
DO - 10.1016/j.gca.2009.08.008
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AN - SCOPUS:70349736132
SN - 0016-7037
VL - 73
SP - 6697
EP - 6703
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 22
ER -