Groundwater and climate dynamics derived from noble gas, 14C, and stable isotope data

Noble gas data in conjunction with stable isotopes and C-14 ages of groundwater samples from southeastern Wisconsin, USA, indicate a soil cooling of at least 6.5-7 degrees C during the last glacial period compared with modern soil temperatures. Because stable isotope and excess Ne data indicate that none of the samples contains any significant portions of glacial meltwater, samples with C-14 ages between 12 and 26 ka B.P., which is the time when the study area was ice covered, most likely infiltrated during short periods of ice retreat or represent recharge containing a significant proportion of precipitation rather than subglacial meltwater recharge. Further, all samples except for those recharged before the last glacial period show a strong correlation between noble gas temperature and delta O-18. By contrast, delta O-18 values of samples older than ca. 28 ka B.P. are too heavy with respect to their noble gas temperatures. This might be due to a stronger influence of an isotopically enriched moisture source from the Gulf of Mexico. The amount of excess air, which is closely linked to the magnitude of groundwater table fluctuations, increases shortly before and at the beginning of the last glacial period, suggesting that recharge dynamics changed considerably during that time period.