On the delta O-18, delta D and D-excess Relations in Meteoric Precipitation and During Equilibrium Freezing: Theoretical Approach and Field Examples

Analysis of the delta D and delta O-18 composition of ice is commonly used to provide insight into the origin of ice bodies. However, studies have questioned the use of the co-isotope relationship to differentiate ground ice types. This study reviews the principles of fractionations affecting delta D, delta O-18 and deuterium excess (d) in meteoric precipitation and during equilibrium freezing of water under changing freezing rates. Traditionally, regression slope values (SD-18O) between delta D and delta O-18 of less than 6 have been used to suggest that ground ice was formed by freezing of liquid water but here it is shown that SD-18O values of less than 7.3 can be suggestive of freezing under equilibrium conditions. This maximum freezing SD-18O value falls within the range of many local meteoric water lines at sites in the Arctic, which can complicate proper identification of subsurface ice types. Many studies are starting to use the calculation of d to infer the origin of subsurface ice. However, d values do not provide much information on the origin of subsurface ice, as d is dependent on freezing conditions. To make proper use of d, its relation with D needs to be investigated, with no relation reflecting meteoric precipitation and a negative relation indicative of freezing. In all cases, it is recommended that stable O-H isotope measurements be supported by additional distinguishing tools (i.e. entrapped gases) when attempting to infer subsurface ice types. Copyright (c) 2011 John Wiley & Sons, Ltd.