Palaeotemperature reconstruction from noble gases in ground water taking into account equilibration with entrapped air

Abstract

Noble-gas concentrations in ground water have been used as a proxy for past air temperatures 1-7 (Hochgestellt, Marion Hirschkorn, but the accuracy of this approach has been limited by the existence of a temperature- independent component of the noble gases in ground water, termed 'excess air', whose origin and composition is poorly understood7-9 (hochgestellt). In particular, the evidence from noble gases in a Brazilian aquifer for a cooling of more than 5°C in tropical America during the Last Glacial Maximum4 has been called into question9. Here we propose a model for dissolved gases in ground water, which describes the formation of excess air by equilibration of ground water with entrapped air in quasi-saturated soils10-12 (hochgest.).Our model predicts previously unexplained noble-gas data sets, including the concentration of atmospheric helium, and yields consistent results for the non-atmospheric helium isotopes that are used for dating ground water. Using this model ofexcess air, we re-evaluate the use of noble gases from ground water for reconstructing past temperatures. Our results corroborate the inferred cooling in Brazil during the Last Glacial Maximum4, and indicate that even larger cooling took place at mid-latitudes.