| Summary: | The last glacial/non-glacial transition has important implications for understanding how abrupt climate changes were transmitted throughout the Northern Hemisphere. The Younger Dryas cold period, for example, is believed to be linked to a reorganization of the thermohaline circulation in the North Atlantic Ocean. The late Wisconsin deglaciation of the Fraser Lowland (3500 km²) in southwestern British Columbia and northwestern Washington was reconstructed from multiple lines of evidence. Traditional mapping and field techniques were integrated with computer-generated digital images to visualize morphologic features. New terminal positions are recognized associated with fluctuations of a piedmont ice lobe that occupied the central Fraser Lowland during the late stages of the Fraser Glaciation. The start of deglaciation is recorded by glaciomarine and glaciofluvial sediments of the Fort Langley Formation (Everson Interstade). Marine shells from stony mud range in age from c. 12,500 to 11,400 ¹⁴C yr BP. Multiple ice readvances of the Sumas Stade, after c. 11,600 ¹⁴C yr BP deposited moraines, outwash, and ice contact sediments in the Fraser Loland and adjacent valleys. Between c. 10,900 and 10,250 ¹⁴C yr BP, ice retreated an unknown distance and readvanced again, constructing moraines near the towns of Mission and Sumas. These ice fluctuations are bracketed by more than 70 radiocarbon dates, but the analytical uncertainties of c. ± 45-200 ¹⁴C yr BP indicate that more accurate dating of these rapid fluctuations will be difficult to achieve. However, constraining information provided at key localities by morphology, stratigraphy, and radiocarbon dating gives confidence in the broad evolving pattern proposed. A review of present-day glacier fluctuations on Mt. Baker (3285 m), 50 km southeast of the Fraser Lowland, indicates a linkage between alpine glaciers and Pacific Decadal Oscillation of sea surface temperatures. This link highlights the sensitivity and response of the glaciers to changes in the oceanic-atmospheric system. The emerging picture is that the glacier fluctuations in the Fraser Lowland and adjacent valleys may have responded to regional late-glacial climate oscillations similar to those of the North Atlantic and northeast Pacific, including the Allerad, Younger Dryas, Holocene transition. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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