Laurentide Ice Sheet dynamics across multiple glacial-interglacial cycles from Quaternary stratigraphic records in the western Hudson Bay Lowland, central Canada

Abstract

Understanding the terrestrial Quaternary stratigraphic record is necessary to reconstruct regional- to continental-scale paleo-ice sheet fluctuations and compare how these events relate to oxygen isotope proxies derived from marine sediments, sea level change, and contemporary ice sheets. Regions that contain an extensive stratigraphic record beyond the last glacial maximum are key to understanding the long-term behaviour of ice sheets and provide field-based constraints for ice-sheet reconstructions and modelling. The Hudson Bay Lowland (HBL) is one of these key regions situated in central Canada that contains a fragmented stratigraphic record of at least the last four Laurentide Ice Sheet (LIS) glaciations. The main objectives of this thesis are to 1) reconstruct the spatio-temporal evolution of the LIS from the Quaternary stratigraphic record of three relatively understudied regions of the western HBL and 2) determine the timing and climate conditions that persisted during ice-free periods across multiple glacial-interglacial cycles.

To better understand the fragmented stratigraphic record that is dominated by glacial sediments (till), this thesis developed a hybrid lithostratigraphy-allostratigraphy approach that relies on multi-parameter characterization of tills to establish sediment provenance and the ice-flow direction that deposited the sediment. This included detailed study of 70 sections, the collection of 193 stratigraphic ice-flow indicators (154 till fabrics and 39 lodged clasts) and analysis of 393 till samples. Once the till framework was established, the relative age of each nonglacial bed was then determined using the bounding surfaces of designated till units. At least 18 units have been identified this way and correlated across the western HBL. The nonglacial beds were further characterized to assess the timing of sediment deposition using radiocarbon and optical dating methods and paleo-environmental conditions that existed using pollen and foraminifera analysis. Lastly, stratigraphic frameworks presented herein did not rely on geochronology constraints to anchor correlations and the age of identified interglacial beds can be further tested to confirm the age of interglacial beds. The new stratigraphic framework for the western HBL provides important field-based constraints for LIS reconstructions. This includes evidence that there was asynchronous growth of the two major domes of the LIS during the last two glaciations, with accelerated early growth of the Quebec–Labrador Dome relative to the Keewatin Dome. In each glaciation, the Keewatin Dome becomes more active relatively later in glaciation and persists until deglaciation. During the last glaciation, ice-flowing from the Keewatin Dome likely did not occur until MIS 2 and this S-trending ice-flow transitions into late-glacial SW-trending ice-streams. During the penultimate glaciation (~MIS 6), till deposition by S- to SW-trending ice was extensive and one of the main ice-flow events across the western HBL when the Keewatin Dome was likely situated in northern mainland Nunavut. Furthermore, during deglaciation the Keewatin Dome or an ice divide was likely situated in the western HBL.

There is widespread evidence across the western HBL that sediments belonging to at least three pre-Holocene interglacial periods exists, which provides an important archive to understanding past climatic conditions in central Canada. For two of these interglacials there is evidence of marine inundation which likely occurred during Termination II (~130 ka) and Termination III (~243 ka). In the Churchill River region, the marine limits for both marine incursions are higher compared to the Tyrrell Sea (Holocene) marine limit, providing important field-based constraints for LIS modelling. Importantly, new age estimations from the uppermost intertill nonglacial sediments, combined with consideration of the paleobotanical datasets in the western HBL stratigraphic record, suggest that the region was last deglaciated during MIS 5e. This implies that the HBL, and likely Hudson Bay, remained glaciated during MIS 3.

The presence of weathered bedrock within the western HBL indicates that glacial erosion of bedrock was negligible in places during the Quaternary Period. Relict landscapes across northeastern Manitoba, such as preserved streamlined landform flowsets that are situated outside the margins of late-glacial ice streams, provide evidence of limited erosion following initial glacial advance into the area.

The results of this thesis have provided an updated Quaternary stratigraphic framework for the western HBL, a key region for understanding the long-term evolution of the LIS. The Quaternary stratigraphic record is highly fragmented, which reflects patchy erosion and deposition over multiple glacial-interglacial cycles of the region. The new stratigraphic frameworks developed provide an increased understanding of the growth, evolution and retreat of the LIS during the past two glaciations and insights into pre-Illinoian glaciations, which are essential to improving reconstruction and modelling of the ice sheet throughout the Middle and Late Pleistocene.