Reconstruction of the Middle to Late Pleistocene glaciation history of the southern Cordilleran Ice Sheet (Fraser Lowland, British Columbia) – a geochemical approach.
Final Report Abstract
Reconstruction of ice-flow directions during glaciations is a critical component in understanding a region’s glacial history. Subject of the conducted project was to improve our understanding the glacial history of the Cordilleran Ice sheet at its southwestern sector, i.e., the western Fraser Lowland (BC, Canada), during marine isotope stage (MIS) 4 (Semiahmoo glaciation) and MIS 2 (Fraser glaciation with the early Coquitlam and later Vashon stades). Till samples from a series of sediment cores from the western Fraser Lowland (Port Moody) were investigated using geochemical (XRF), mineralogical (XRD) and granulometric (grain size) methods. The initial approach to use µXRF scanning has proven unsuitable for this application and was consequently abandoned. The analyzed sediments were deposited during MIS 4 and MIS 2. The data was used to infer sediment provenances, which can provide crucial insights into past ice-flow directions. Two provenances could be identified: The North Cascades to the east – and the southern Coast Mountains to the north of the Fraser Lowland. Eastern provenance material is enriched in MIS 4 tills, suggesting that climate conditions at some point during MIS 4 may have been favorable to promote an earlier or more rapid ice build-up in the east during that time. Summer insolation on the one hand, and atmospheric circulation patterns influenced by the extent of the of the Laurentide Ice Sheet (LIS) on the other hand, are the likely drivers of ice accumulation over the North Cascades and the southern Coast Mountains. Low summer insolation and a not fully developed LIS seem to have created climate conditions favorable for ice accumulation in more continental settings (e.g., North Cascades), tentatively during early MIS 4. Summer insolation maxima, leading to higher moisture uptake and over the North Pacific and resulting in increased precipitation especially in the more maritime southern Coast Mountains, may have fostered ice accumulation during late MIS 4, and also during the climactic stage of the Fraser glaciation (Vashon stade) during late MIS 2. High-resolution light detection and ranging (Lidar)imagery of the western Fraser Lowland was used to infer ice-flow directions during the MIS 2 Vashon stade based on geomorphological evidence. Streamlined bedforms such as drumlinoids, flutings and crag-and-tail structures reflect the ice movement during the Vashon stade. The data lead to the conclusion that during both stades of MIS 2 (the earlier Coquitlam stade and the Vashon stade), the ice covering the western Fraser Lowland originated from the northwest (Howe Sound/ Strait of Georgia) and the north (southern Coast Mountains). The interpretation of our data conflicts earlier hypotheses that ice during the stadial maximum of both stades of the Fraser glaciation came from the eastern Fraser Lowland and overrode the western Fraser Lowland in east-west direction. In addition, new radiocarbon ages from the east shore of the Strait of Georgia contribute to the ongoing discussion of the position of the southward advancing ice margin in the Salish Sea.
Publications
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Reconstruction of the Late Pleistocene glaciation history of southwestern British Columbia – a geochemical approach. Quaternary Science Reviews
Gromig, R., Ward B., Jackson, L. E., Opitz, S., Geisler, T., Friedrich, H. H. & Melles, M.
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New insights into the glacial and relative sea-level history of the western Fraser Lowland based on sediment cores from geotechnical drilling for the Evergreen Tunnel, British Columbia, Canada. Quaternary Research, 122, 40-61.
Jackson, Lionel E.; Ward, Brent C.; Hicock, Stephen R.; Gromig, Raphael; Clague, John J. & Turner, Derek G.
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Ice-Flow Dynamics During the Final Stage of the Fraser Glaciation (MIS2) in the Fraser Lowland, BC, Canada. Quaternary, 8(1), 13.
Gromig, Raphael; Franz, Kenya; Ward, Brent & Clague, John J.
