Large glacial lakes modulated the return of meltwater to the ocean during deglaciation, and their drainage may have initiated global climate change. Yet few records of their drainage come from observations within their basins. Sediment cores from nine lakes along a 240-km transect from northwestern Quebec to northeastern Ontario cover a portion of former Lake Ojibway and provide a stratigraphy of the terminal phase of this large glacial lake. Magnetic susceptibility, density, grain size, X-ray fluorescence chemistry and X-ray diffraction data were used to characterize stratigraphic changes within the basin. The basal sequence consists of till and rhythmites, with ice-proximal debris flows overlain by varves. The varves thin up-section and become unrecognizable, which indicates decreased deposition rates. This fine-grained sediment forms the matrix of a clay-pebble conglomerate. The clay-pebbles are ice-rafted debris (IRD). The IRD flux was probably constant, whereas the sedimentation rate of the finer-grained matrix decreased. The end of IRD marks the cessation of icebergs in the lake and is the best indication for drainage of the glacial lake. The conglomerate is capped by laminated to massive gray silt deposited after lake drainage and marks the transition to organic-rich, post-glacial lakes. Such sequences place drainage into the broader context of deglaciation.
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Acknowledgments The authors express gratitude to NSF grant number EAR0643144 and to Sigma XI and GSA Grants for graduate research. Also, thanks to the Limnological Research Center at the University of Minnesota for help in initial core processing and to Erik Brown at the University of Minnesota, Duluth for XRF core scans. Special thanks to Joanne Ballard for discussion and laboratory help. We thank Warren Huff for XRD, discussion and editing and Tammy Gerke for help in interpreting XRF geochemistry data. Finally, we thank the reviewers for greatly improving this manuscript and Elsie and Cole Hamel for editing.
Copyright 2013 Elsevier B.V., All rights reserved.
- 8,200 cal yr BP event
- Glacial lake drainage
- Ice-rafted debris
- Lake Ojibway
- Laurentide ice sheet
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