Lake Superior varves: Records of Lake Agassiz overflow and ice sheet dynamics

Lake Superior's sediments include a sequence of at least 1,600 glaciolacustrine varves that reflect the combined impacts of sediment-laden meltwater and Lake Agassiz discharge following the retreat of the Superior Lobe following the Marquette advance. Power spectra of the thickness time series, inorganic carbonate data, and regional radiocarbon dates tied to the stratigraphy confirm that these are annual rhythmites (varves).; Varve cessation coincides with the cessation of a series of thirty-six thick varves at around 9,040+/-200 cal BP, which corresponds to diversion of Lake Agassiz and glacial meltwater into Lake Ojibway. The oldest varves begin with ice retreat from western Lake Superior ca. 10,700 cal BP. Influx of Lake Agassiz water is expected ca. 10,600 cal BP; three sets of varves in western Lake Superior that abruptly thicken, then taper over the next three to nine years, may record influxes of Lake Agassiz at around 10,630, 10,600, and 10,570 cal BP.; The oxygen isotope record from benthic ostracodes is consistently depleted in 18O. Age models from cores in Lake Huron and Michigan are re-evaluated to show a strong correlation between the sequence of thick varves and the lowest delta18O values in Lake Superior and negative delta 18O anomalies in Huron and Michigan. During the 500 years prior to this period, the presence of very high delta18O values in Huron strongly contrast with much lower values in Superior, suggesting Lake Superior overflow circumvented Lake Huron, and discharged through the Pic-White Otter River Valley to Lake Ojibway. Northern drainage of Lake Superior ended with ice re-advance ca. 9,400 cal BP. This interpretation of the delta 18O record from Lake Superior necessitates that the accepted lake level history of Lake Superior is incorrect. I suggest that the northern outlet was breached during an earlier post-Minong stage, and thereafter lake levels transgressed to what was formerly described as a post-Minong II or III water plane.; Finally I present the first stratigraphically controlled trace metal data from glaciolacustrine sediments in the Great Lakes and suggest V/Cu ratios, and perhaps As concentrations, can be used to identify Lake Agassiz water.*; *This dissertation is a compound document (contains both a paper copy and a DVD as part of the dissertation).