Mechanisms of subglacial groundwater recharge as derived from noble gas, 14C, and stable isotopic data

Noble gas, stable isotope and 14C data from samples collected along groundwater flow path within a confined Paleozoic aquifer in northeastern Wisconsin, USA are used to deduce the effect of the Laurentide Ice Sheet (LIS) on the underlying groundwater and its recharge dynamics. During the last glacial maximum the investigated area was near the center of the Green Bay Lobe of the LIS. 14C ages that extend to 26 k.a. and low δ18O derived temperatures during the time that the LIS was present indicate that aquifer recharge continued when ice covered the area. δ18O values as low as −17.5‰ and δ2H values as low as −127.7‰ indicate that a significant portion of aquifer recharge was derived from glacial meltwater that maintained its glacial isotopic signature during melting and subsequent recharge. Noble gas temperatures that remain above freezing at a constant ~3 °C, unusually high excess air (ΔNe) values and noble gas fractionation patterns indicate that recharge occurred across a very dynamic water table located within the ice sheet. This englacial hydrologic system experienced recharge heads of as much as 7.8 m. Evidence for direct recharge of basal meltwater into the aquifer is not seen. To the authorsʹ knowledge this is the first time that noble gas and isotope tracers have been used to deduce the provenance of aquifer water beneath continental ice sheets.