The effect of the negative pressure of capillary water on atmospheric noble gas solubility in ground water and palaeotemperature reconstruction

Abstract

Atmospheric-derived noble gases He, Ne, Ar, Kr and Xe dissolved in ground waters record the temperature at the moment of their recharge. Palaeotemperature reconstructions should include all the parameters affecting the soil air–ground water thermodynamic equilibrium. Particularly, the occurrence of excess air, that is ANG in excess compared to the equilibrium concentrations and often mass-fractionated compared to the atmospheric pattern, is likely produced by air entrapment during water infiltration in the nonsaturated zone (NSZ). Here, we show that water persisting in the NSZ has peculiar capillary properties controlled by the internal water pressure, which decreases (even down to negative pressures) with decreasing soil–air humidity. Decreasing water pressure induces mass-dependent increasing solubility of ANG from He to Xe. In addition, in nonsaturated soils where water and air coexist, air entrapment is favored allowing nonfractionated excess air. This capillary approach has been coded in an inverse-method program called Thermo_Inver, and tested on two well-characterized noble gas datasets used to constrain palaeotemperatures in Midwestern United States and Brazil during the last glacial maximum [Quat. Res. 43 (1995) 209; Science 269 (1995) 379]. Results of these simulations highlight that under suitable climatic conditions and in fine-porous media, the capillary pressure of water may be a prominent parameter controlling the addition of ANG to ground waters. Under these conditions, the recharge zone for ANG is the NZS and this water–air coexistence zone easily produces excess air.