We analyze both new and previously published paleomagnetic records of secular variation (PSV) from Lake Superior sediment cores and compare these records to correlated rhythmite (varve) thickness records to determine post-glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin. The results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years, particularly during the mid-Holocene. We attribute offshore, mid-Holocene sedimentation changes to alterations in whole basin circulation, perhaps precipitated by a greater dominance of the Gulf of Mexico air mass during the summer season. Nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000cal. BP. After 2000cal. BP, sedimentation rates subsided to earlier rates. The increases between 4500 and 2000cal. BP are probably due to lake level fall after the Nipissing II highstand. The older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake, except near the Lake Nipigon inlets. Multiple sediment cores reveal 36 anomalously thick varves, previously ascribed to the formation of the Nakina moraine, which were deposited just prior to varve cessation in the open lake. The PSV records support the observation that the cessation of these thick varves is a temporally correlative event, occurring at 9035±170cal. BP (calibrated years before 1950, ca 7950-8250 14C BP). This date would correlate to the eastern diversion of Lake Agassiz and glacial meltwater into Lake Ojibway.
Bibliographical noteFunding Information:
Support from the Weinert Foundation and the University of Minnesota Duluth enabled the recovery of the sediment cores. We gratefully acknowledge Captain M. King and crew aboard the R/V Blue Heron for their help with core recovery. This paper was improved by reviews from J. Ridge and J. Teller. We also acknowledge L. Valdez and the staff at the UMD Visualization and Digital Imaging Lab for their aid with the varve thickness measurements.
Copyright 2008 Elsevier B.V., All rights reserved.
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