The isotopic composition of the Laurentide Ice Sheet and fossil groundwater

Abstract

Stable isotopes have been used to study large-scale changes in hydrology during the Pleistocene epoch. Many of these efforts have required an estimate of the δ18O value of runoff generated by melting ice sheets. There is no consensus on representative values. Here we examine δ18O values from fossil groundwater samples and isotope-enabled general circulation models (GCMs) to better understand the isotopic composition of the Laurentide Ice Sheet (LIS). Groundwater δ18O values ranged from -12.5 to -25.3‰ and tended to increase southward. The δ18O precipitation values predicted by GCMs follow a similar trend but increase more steeply southward. The difference in groundwater and GCM output can be explained by invoking movement of glacial ice and meltwater, along with mixing within groundwater systems. Most groundwater δ18O values are higher than an average LIS δ18O value of -25.4 ± 2.5‰ calculated based on estimated ice sheet volumes and sea level data.