On the δ18O, δD and D‐excess relations in meteoric precipitation and during equilibrium freezing: theoretical approach and field examples

Abstract

AbstractAnalysis of the δD and δ18O 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 δD, δ18O and deuterium excess (d) in meteoric precipitation and during equilibrium freezing of water under changing freezing rates. Traditionally, regression slope values (S D‐18O) between δD and δ18O 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 S D‐18O values of less than 7.3 can be suggestive of freezing under equilibrium conditions. This maximum freezing S D‐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 © 2011 John Wiley & Sons, Ltd.