A REVIEW OF THE GLACIAL VARVE RECORD FROM LAKE SUPERIOR: RECENT DEVELOPMENTS, UNRESOLVED QUESTIONS, AND FUTURE PROSPECTS

Recent studies of glaciolacustrine varves in Lake Superior have focused on construction of an annually resolvable stratigraphy in an effort to detail regional ice retreat and Lake Agassiz discharge, the latter of which may have been catastrophic at times. The chronology is constrained by paleomagnetic secular variation data and the varve stratigraphy. Power spectra of the thickness time series, inorganic carbonate data, and regional radiocarbon dates tied to the stratigraphy confirm that these are varves. The oldest varves begin with ice retreat, ca. 10,700 cal BP; diversion of Lake Agassiz and glacial meltwater into Lake Ojibway led to varve cessation, ca. 9,040 cal BP. Influx of Lake Agassiz 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 δ¹⁸O record from benthic ostracodes is consistently depleted in ¹⁸O. Between 9,900 and 9,400 cal BP, the presence of high δ¹⁸O values in Huron contrasts with the low values in Superior, suggesting Lake Superior overflow circumvented Lake Huron and discharged to Lake Ojibway. Northern drainage of Lake Superior ended with ice re-advance north of Superior ca. 9,400 cal BP. This interpretation of the δ¹⁸O records is contrary to the accepted post-glacial lake level history, which presumes Superior always overflowed to the south. Vanadium and arsenic concentrations may perhaps be used to fingerprint Lake Agassiz water. Comparable geochemical analyses in co-eval records from Lakes Agassiz and Huron, and replicate analyses from other sites within Lake Superior, are needed to verify the suitability of this proxy for tracing Lake Agassiz water. The continued study of varve records from Superior, Huron, Ojibway, and Agassiz can likely provide a tightly constrained and annually resolvable record of ice retreat and late-glacial paleohydrology for the entire Laurentide Ice Sheet.