Model of the western Laurentide Ice Sheet, North America

Abstract

The Laurentide Ice Sheet reached its maximum extent at the Last Glacial Maximum, 26 500-19 000 years before present. It is responsible for a large portion of the approximately 130 m of eustatic sea level fall since that time. During its retreat, meltwater from the Laurentide Ice Sheet caused rapid changes in sea level, and affected global climate by changing ocean circulation. However, previous estimates of the absolute volume of the Laurentide Ice Sheet through time have been limited due to deficiencies in the chronology of margin retreat and information on glacial-isostatic adjustment (GIA). In this study, I present a new numerical ice sheet model of the western portion of the Laurentide ice sheet. I constrain the model using GIA indicators, including the tilts of well dated glacial lake strandlines, tilt rates of contemporary modern lakes, uplift rates from GPS, and relative sea level indicators. I also present a new margin history based on the minimum timing of retreat. All data used in the modelling exercise are carefully assessed to ensure they are reliable.

At the Last Glacial Maximum, the ice sheet model has a broad dome that extended from the Cordillera to the area west of Great Slave Lake, Northwest Territories. The southern portion of the ice sheet is modelled to have a shallow gradient, with thickness values less than 2000 m south of 56 degrees north. This is in contrast to previous ice sheet models of the Laurentide Ice Sheet based on GIA modelling, such as ICE-5G (Peltier, 2004), that have over 5000 m of ice in this region. During deglaciation, the largest decrease in volume happened between 16,000 and 13,000 years before present, coinciding with margin retreat in Alberta and Northwest Territories. From 13 000 to 11 500 years before present, ice sheet retreat slowed, corresponding to Younger Dryas cooling. After 11 500 years before present, ice sheet retreat was more rapid, and by 6500 years before present, no ice remained in the study area. Glacial lake tilt observations support a thick elastic lithosphere, with values greater than 120 km providing the best fit to the data. A wide range of mantle viscosity values were investigated, and the calculated GIA matched observations within the range of 3-5×10 20 Pa s for the upper mantle and > 5 × 10^21 Pa s for the lower mantle for the majority of observations.