Graduate Thesis Or Dissertation
 

Dynamics of the Laurentide Ice Sheet

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  • This dissertation concentrates on the controlling factors on the instability of the Laurentide Ice Sheet (LIS) and their effects on abrupt climate change. Northern Hemisphere climate fluctuated abruptly during the last deglaciation possibly related to variability in Atlantic meridional overturning circulation (AMOC) and reduced aerial extent of the LIS. Reductions in AMOC can be induced by changes in the hydrologic cycle which in turn may be controlled by instabilities in the LIS due to subglacial hydrology, sliding and till deformation. Sedimentological observations of the Tiskilwa Till, Illinois, and Sky Pilot Till, Manitoba determined that the LIS deposited these tills in a ≤1 m thick deforming layer as strain migrated upwards due to till accretion. These tills experienced high strain in a deforming layer thick enough contribute to ice sheet motion. Simulations of the subglacial hydrology of the James Lobe (JL) of the LIS suggest that the JL had little affect on regional groundwater flow and that subglacial aquifers were incapable of draining meltwater from the ice-till interface. However, a canal basal drainage system with canals up to 70 cm wide spaced 40 to 70 m apart could drain the ice-till interface keeping the JL coupled to its bed. To resolve the effects of LIS retreat on the hydrologic cycle, a suite of new freshwater routing proxies indicate that western Canadian Plains freshwater was routed to the St. Lawrence at the start of the Younger Dryas with an increase in base flow discharge sufficient to reduce AMOC. These proxies identified a previously unrecognized intra-Younger Dryas routing event at ~12 ka, the impact of which indicates the tight coupling of AMOC, climate and the hydrologic cycle. To determine the role of the LIS in Holocene climate change, 10Be ages from western Quebec were used to date the final disappearance of the LIS at ~6.8 ka following a period of rapid retreat and thinning that contributed to a rapid rise in sea level. The disappearance of the LIS initiated the Holocene Thermal Maximum, while the attendant reduction in freshwater runoff induced the formation of Labrador deep sea water.
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