Excess Air in Groundwater as a Potential Indicator of past Environmental Changes

Dissolved noble gases in groundwater are used to reconstruct paleotemperature, but also yield information about "excess air", a component of dissolved gases in excess of solubility equilibrium, derived from dissolution of trapped air in the ground. A good characterization of the excess air component is necessary not only to obtain reliable noble gas temperatures, but also to investigate the potential of excess air as a proxy for past environmental conditions. Two excess air related quantities can be derived from groundwater noble gas data sets: The initial air/water ratio and the pressure exerted on the entrapped air. Under recharge conditions typical for many aquifers, the excess of dissolved gases, expressed by the relative Ne excess Ne, is mainly determined by the hydrostatic pressure on the entrapped air. Thus, we suggest that Ne is essentially a measure of the amplitude of water table fluctuations in the recharge area. Comparing data sets from three aquifers in temperate, humid latitudes and three aquifers in tropical, semi-arid regions, we find that Ne is generally higher in the tropical aquifers, possibly related to larger water table fluctuations in these aquifers characterized by deep unsaturated zones. Whereas Ne shows little temporal variation in the mid-latitude aquifers, there is a strong signal of higher Ne in the paleowaters of the tropical aquifers as compared to water recharged under modern climate conditions. This finding may indicate a higher variability of recharge in the past at the studied tropical sites.