RECENT ADVANCES IN OUR UNDERSTANDING OF THE NOBLE GAS THERMOMETER IN GROUNDWATER (Invited Presentation)

Noble gases are conservative tracers and their concentrations in recharge areas of groundwater systems are mostly a function of temperature, pressure, and excess air (EA). Consequently, noble gas temperatures (NGTs) have commonly been regarded as a robust indicator of past climate and used for decades in paleoclimate reconstructions. Until recently, however, limited attention had been placed on processes taking place at the surface, in the unsaturated zone and at the interface water table/soil air capable of impacting dissolved noble gases in groundwater and thus, NGTs.

In recent years, we highlighted the potential impact of systematic O₂ depletion without corresponding CO₂ build-up in the unsaturated zone on noble gas partial pressures, leading to a bias to low NGTs in recharge areas. A recent study in southern Michigan showed, however, that recharge conditions can be significantly modified by large precipitation events such as Hurricane Ike and bring O₂ depleted soil air back to standard conditions, while injecting high EA amounts into the water table. These findings suggest that the NGT proxy in sedimentary systems should be viewed as an average of recharge conditions over several years as opposed to the commonly assumed mean annual air temperature.

Atmospheric noble gas concentrations were also measured in high-altitude springs in the fractured Galapagos Islands where rainwater infiltration is expected to be rapid. These revealed the presence of a previously unknown noble gas pattern. In addition to atmospheric He excesses, this pattern displays Ne, Kr, and Xe depletion together with relative Ar enrichment. To understand the origin of this unknown pattern we began analyzing rainwater samples. Preliminary results show that all rainwater samples present atmospheric He excesses with respect to air saturated water and can be subdivided into two groups. The first group displays a pattern remarkably similar to that previously identified in the Galapagos Islands, while the second displays a mass-dependent depletion pattern with greater depletion of the heavier noble gases. It is now apparent that dissolved noble gases in rainwater are not always in equilibrium with surface conditions. Both the impact of O₂ depletion in soil air and lack of rainwater equilibration on recorded NGTs are the subject of ongoing research.