Ice-vapor equilibrium fractionation factor of hydrogen and oxygen isotopes: Experimental investigations and implications for stable water isotope studies

摘要

RATIONALE The equilibrium fractionation factors govern the relative change in the isotopic composition during phase transitions of water. The commonly used results, which were published more than 40 years ago, are limited to a minimum temperature of -33°C. This limits the reliability in cold regions. With recent instrumental developments it is now possible to test the accuracy of the earlier results as well as extend the temperature range. METHODS Novel measurements were made of the ice-vapor equilibrium fractionation factor α between 0°C and -40°C, from a unique experimental setup using both a Picarro cavity ringdown spectrometer and a TC/EA IRMS system. Using both systems allows for continuous monitoring of the equilibrium state of the system as well as testing for reproducibility. RESULTS The results of the experiments show fractionation factors for δ~2H and δ~(18)O values, with a temperature dependency in accordance with theory for equilibrium fractionation. We obtain the following expressions for the temperature dependency of the fractionation coefficients: lnαδ~2H=0.2133-203. 10T+48888T2lnαδ~(18)O=0.0831-49.192T+8312.5T2 Compared with previous experimental work, a significantly larger α for δ2H is obtained while, for δ~(18)O, α is larger for temperatures below -20°C and slightly lower for temperatures above this. CONCLUSIONS Using the new values for α, a Rayleigh distillation model shows significant changes in both magnitude and shape of an annual deuterium excess signal in Greenland. This emphasizes the importance of a well-defined value of α for accurate studies of the processes in the hydrological cycle and underlines the significance of the differences between the results of this work and earlier work.