Environmental tracers can also be used for climatic reconstruction. Noble gases are particularly useful, as their atmospheric concentrations are constant, and so measured concentrations provide direct information on solubility. This is largely determined by the temperature at the water table at the time of recharge. 2H and 18O can also provide information on recharge, as isotopic ratios in rainfall are influenced by temperature, although for quantitative use, the relationship between isotope ratio and temperature needs to be independently determined at each field site. A small number of studies have also estimated past groundwater recharge rates from past changes in the chloride concentration of infiltrating water (e.g., Murphy et al., 1996).
The Great Hungarian Plain (Figure 40) consists of an upper Quaternary aquifer and a deeper Pliocene aquifer. Noble gas temperatures for these aquifer systems have been measured with Ne, Ar, Kr and Xe concentrations. Groundwater ages have been estimated using 14C, which provides the timescale for the climatic reconstruction. Samples with 14C ages less than 10,000 years have an average noble gas recharge temperature of 10.6 °C, which is close to the mean annual temperature in the recharge area under the current climate (11.4 °C). Noble gas temperatures at the glacial maxima (approximately 18,000 years ago) suggest an average mean annual temperature approximately 8.6° lower than present (Figure 41). Temperatures preceding the glacial period (28,000 – 35,000 years ago) were approximately 5° lower than present. The 18O record shows a similar pattern, revealing depleted values in samples with 14C ages exceeding 10,000 years (Stute and Schlosser, 2000).