ABSTRACT: A numerical investigation of factors influencing the occurrence of millennial-scale oscillations of the southern Laurentide Ice Sheet

CGRG Bibliography of Canadian Geomorphology
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Abstract

The termini of some lobes of the southern Laurentide Ice Sheet (LIS) fluctuated more than others around the last glacial maximum. We use a two-dimensional, time-dependent flowline model to explore causes of this varied behavior. The flowline of interest stretches southwards from James Bay on a bed that is uniformly at 200 m above present sea level prior to isostatic adjustment. A transition from a ‘hard' to a ‘soft' bed is specified at 650 km. The model treats heat flow in ice and the upper 1100 m of bedrock, and accounts for contributions to the energy balance from latent heat associated with permafrost formation. For simplicity, steady basal heat flow and climate boundary conditions are adopted. In the baseline run these are (i) a geothermal flux of 0.95 HFU, and (ii) a climate parameterization that yields a total annual precipitation of 750 mm and a mean annual air temperature of 271 K at Madison. All runs have zero initial ice. We find that binge-purge-type millennial-scale oscillations of 200 km in terminus position occur under a narrow range of conditions after an initial period of ice build-up lasting ~25 kyr. Between 10 and 60% of the bed is frozen during each oscillation. It is noteworthy that these fluctuations occur under steady boundary conditions, and are independent of northern-LIS behavior. The following factors prevent oscillations from occurring: (i) increasing water content of the soft substrate by replacing sand with silty clay results in additional latent heat production on freezing; (ii) increasing or decreasing the geothermal flux by 10% sufficiently limits or enhances basal freeze-up, (iii) southward displacement (by 100 km) of the hard-soft bed transition reduces the lobes' ability to advect cold ice from upstream; (iv) increasing precipitation by 25% causes additional basal freeze-on due to enhanced vertical heat advection; and (v) decreasing precipitation by 40% favors thawed-bed conditions. These results offer new explanations for the variability of lobe behavior in the Midwest of North America.

Bibliography of Canadian Geomorphology