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^{1,2,3,4,5,6,7}, 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 understood^7,8,9. 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 soils^10,11,12. 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 of excess 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.