Subglacial groundwater recharge and the isotopic composition of the Laurentide and Fennoscandanavian ice sheets

Oxygen and hydrogen isotopes are commonly used to delineate the presence of Pleistocene recharge in groundwater systems. Groundwaters that have a component of subglacial recharge and glacial runoff have δ¹⁸O and δD values that are much lower than those found in modern precipitation and recharge. The actual value of the subglacial water that entered the groundwater system is not clear. Examining the lowest δ¹⁸O and δD values in various groundwater systems across North America and Europe, constraints can be placed on the isotopic composition of Pleistocene ice sheets.

The lowest δ¹⁸O and δD values are found in Quaternary aquitards and confined aquifers with narrow and reasonably shallow recharge areas. In shallow aquitards this water may be connate water or water that has travelled very short distances since the last glaciation. In confined aquifers, the lowest values are associated with near complete replacement of existing porewater by subglacial groundwater. This often occurs at significant distance from the recharge area.

The δ¹⁸O and δD values are variable in space, with lower values observed at higher latitudes. The observed values are consistent with some, but not all, outputs from isotope-enabled general circulation models of conditions at the last glacial maximum. Defining a unique ice sheet value for paleoclimate studies involving freshwater drainage may not be possible.