Brenda Ekwurzel’s presentation described the application of noble gases in groundwater to paleothermometry, with particular emphasis on temperature estimates at the Last Glacial Maximum (LGM).

Because the solubility of the noble gases (Ne, Ar, Kr, Xe) in water is temperature dependent, measurements of the noble gas content of groundwater can be used to estimate the temperature at the time and place of recharge. Noble gases are chemically inert, and consequently, their abundance does not change once they become incorporated into the groundwater. The heavier noble gases are most sensitive to temperature, but concentrations can also be affected by pressure differences (related to the elevation of the recharge area) and salinity. The contribution of salinity can be ignored within freshwater systems, but the influence of other non-temperature factors must be removed by measuring the full suite of noble gases. “Excess air” is the most important of these confounding factors. This factor relates to elevated noble gas concentrations caused by contributions from small air bubbles that are trapped in groundwater during water table fluctuations. Because Neon is most sensitive to excess air, it is used to correct the measurements of the other noble gases.

In contemporary hydrogeological studies, noble gas concentrations are used to determine the relative contribution of groundwater source regions. This approach is best suited for areas where potential source regions have large differences in elevation. Paleoclimatic studies exploit the temperature dependence of these gases to develop proxy temperature records that can span several tens of thousands of years. Noble gas temperature reconstructions typically have accuracies of between 0.25° to 1° C. Dating control is provided by radiometric or tracer measurements, with age-model errors on the order of ~2000 years. Because noble gas concentrations reflect mean annual temperature at the water table, a correction factor (typically + 1°+/-1° C) must be used to convert records to mean annual air temperature.

Paleothermometry from noble gases is most effective in areas with simple hydrology, where the elevation of the source area is known. Temperature estimates are more difficult to obtain from areas where the water table is less than 2 metres or greater than 30 meters below the surface, or where recharge rates are greater than several hundred millimetres per year. The technique is also not applicable to areas that were formerly ice-covered, or include karst formations.

Brenda examined the utility of noble gas temperature records by describing their contributions to the controversy over the state of the tropical oceans during the LGM. Although it is clear that the behaviour of the polar oceans was highly variable during the LGM, different records disagree regarding the degree of cooling within the tropic oceans. The CLIMAP reconstruction indicated a mean tropical SST reduction of ~ 1° C at the LGM but paleo-snowline evidence from the tropics suggests a much greater cooling. The Farrera et al. (1999) synthesis of several noble gas studies reported a cooling of ~5.8°+/-1.5°C for tropical sites at the LGM, with stronger cooling at midlatitude sites (~7.0°+/-1.8°C). However, Brenda emphasised that, before a global survey of noble gas temperature records can be completed, several issues remain outstanding, including the potential influences of (1) elevation differences between sites; (2) changes in lapse rates (due to changes in atmospheric moisture content); and (3) land cover changes.