NOBLE GASES IN GROUND WATER AS PALEOCLIMATIC INDICATORS

The solubility of the noble gases is temperature dependent. Other factors influencing solubility are the system pressure, the solute content of the water, and gravitational potentials. Most of the noble gases dissolved in ground water are from chemical equilibrium with the atmosphere. This equilibrium takes place in the recharge zone of the aquifer, typically in the soil. The final noble-gas concentrations are determined by the temperature, the elevation, the alteration of soil-gas composition by organisms, and soil-temperature gradients (which are in part a result of water-table depth and recharge rates).; If the effects of temperature can be separated from the other influences, and if the noble-gas concentrations are not altered after recharge water enters the saturated zone, variations in recharge temperature with time may be determined by measuring the noble gases in dated ground-water samples. However, analysis of available data indicates that noble-gas concentrations frequently change after recharge. This change is usually the result of reequilibration with a biogenic-gas phase produced within the aquifer, or from contact with air.; In order to extend the calculation of recharge history to samples with complex histories of gas equilibrium a general equation for two stage equilibrium was derived. The variables in this equation are the initial temperature and pressure of equilibrium, the final temperature and pressure of equilibrium, and the molar water-to-gas ratio at the second equilibration. An equation of this type is constructed for each of the gases: neon, argon, krypton and xenon. These equations are solved simultaneously for four of the variables listed above while the value of one is assumed. Graphical techniques for determining which assumptions to use are presented.; Ground-water samples were collected in glass tubes and analyzed by gas chromatograph-mass spectrometer, using double isotope dilution standardization. The analytical method is still developmental. Field sampling was undertaken in two locations, the Milk River aquifer of southern Alberta, Canada, and the Carrizo sand aquifer in southern Texas, in order to test the method. The preliminary data obtained may show correlation with known Holocene-Pleistocene climatic fluctuations, encouraging further development of the analytical technique and field research.; Finally, a method for interpreting the climatic implications of oxygen and hydrogen isotope information from ground water in conjunction with noble-gas data is given.