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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L19401, doi:10.1029/2008GL035018, 2008

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

Tie Sun

Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA


Chris M. Hall

Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA


Maria Clara Castro

Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA


Kyger C. Lohmann

Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA


Patrick Goblet

Centre de Géosciences, Ecole des Mines de Paris, Fontainebleau, France


Abstract

A 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 phase.

Received 13 June 2008; accepted 22 August 2008; published 4 October 2008.

Keywords: noble gas; paleoclimate; hydrology.

Index Terms: 1620 Global Change: Climate dynamics (0429, 3309); 1829 Hydrology: Groundwater hydrology; 1833 Hydrology: Hydroclimatology; 1899 Hydrology: General or miscellaneous.

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Citation: Sun, T., C. M. Hall, M. C. Castro, K. C. Lohmann, and P. Goblet (2008), Excess air in the noble gas groundwater paleothermometer: A new model based on diffusion in the gas phase, Geophys. Res. Lett., 35, L19401, doi:10.1029/2008GL035018.