Excess air in the noble gas groundwater paleothermometer: A new model based on diffusion in the gas phase

International audienceA key assumption for calculating paleotemperatures using noble gas concentrations in groundwater is that water equilibrates with standard air. However, if the unsaturated zone is depleted in O2, the noble gas partial pressures will be elevated, resulting in a bias of noble gas temperatures (NGTs) to low values. This oxygen depletion (OD) mechanism was used to explain low NGT values for a shallow aquifer in Michigan where new O2 saturation and CO2 measurements now confirm the OD model. Measured excess He, without an expected vertical concentration gradient in the water phase, suggests that the rate of noble gas equilibration at the base of the unsaturated zone is restricted, and that transport within the gas phase may be a rate-limiting step. A new NGT model is presented that uses the OD mechanism and that allows for partial re-equilibration of excess air via diffusion in the gas phas