Doctor of Philosophy




Fred J. Longstaffe


This study describes contributions of glacial meltwater to Lake Superior over the last ~11,000 cal BP (calibrated years before 1950 AD). This research has been motivated by a desire to understand whether glacial meltwater discharge via the Great Lakes into the North Atlantic played a role in Holocene cooling events, such as the Younger Dryas. Rhythmites (interpreted as varves), lithological, mineralogical and grain-size variations, and radiocarbon dating were used to establish chronostratigraphic correlation among four sediment cores from Lake Superior (Duluth, Caribou and Ile Parisienne basins, Thunder Bay Trough). Glacial sediments were deposited between ~10,850 and ~8,800 cal BP. The δ18O values of ostracodes record the presence of glacial meltwater (δ18O of ~ –25 ‰) in ancient Lake Superior as the Laurentide Ice Sheet waxed and waned. Glacial meltwater was increasingly dominant between ~10,850 and ~9,250 cal BP, particularly as thick varves formed in northern portions of the Lake Superior Basin (~10,400-10,200, ~9,900 and ~9,300-9,200 cal BP). Glacial meltwater supply was reduced in the Thunder Bay Trough between ~9,250 and ~8,950 cal BP, but returned from ~8,950 to ~8,800 cal BP. Glacial meltwater flow from the Lake Superior Basin bypassed the Huron Basin several times during this period. Final termination of glacial meltwater supply occurred at ~8,800 cal BP – coincident with cessation of varve formation and inception of ancient Lakes Agassiz-Ojibway and Houghton. Primary productivity was very low and algal growth occurred under conditions of extreme nitrogen deficiency – as determined using TOC, TN and C/N ratios – until glacial meltwater supply to the Basin was ended. The postglacial sediments are non-calcareous. The diatom silica proxy record shows that water δ18O values rapidly increased after glacial meltwater termination, reaching ~ –10 ‰ during the Holocene Thermal Maximum. Water δ18O values decreased at ~3,000 cal BP in response to the Holocene Neoglacial Interval before gradually rising to Lake Superior’s modern value of –8.7 ‰. Aquatic primary productivity, inferred using TOC, TN, and δ13C and δ15N, has increased gradually since ~8,800 cal BP. Primary δD and δ18O values are not preserved by porewater; they instead reflect mixing between ancient and modern lake water.

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