Abstract
Varve thickness records from glacial Lake Ojibway provide a detailed picture of ice margin dynamics and paleohydrology for the Laurentide Ice Sheet during the early Holocene. The demise of Lake Ojibway occurred when an ice dam of the shrinking Laurentide Ice Sheet failed, perhaps releasing enough freshwater to impede Atlantic meridional overturning circulation and precipitate the 8200 cal BP climatic cooling event. Previously published records and new information from lacustrine sediment cores are combined to reconstruct a varve record over 2100 years long. Between varves numbered 1 to 1500, average rates of ice margin retreat increased from 140 m/y to as high as 400 m/y. Beginning prior to varve year 1800, and lasting to around year 2000, ice readvanced along a front at least 300 km wide during an episode called the Cochrane. The total distance of the advance was at least 75 km, and averaged around 200 m/y. During the advance, a series of ∼50 anomalously thick varves were deposited, the cause of which is uncertain. A widespread unconformity occurred during the end of the Cochrane advance, which was followed at around varve year 2060 by the return of varves, which thicken abruptly. Unlike previous varves, these varves include ice-rafted pellets of Lake Ojibway sediment. These varves may have resulted from rapid melting and calving, rather than a second advance, and may relate to basin re-filling following a lowstand. There are no direct dates on the Lake Ojibway varves, but a sudden increase in varve thickness at year 1528 is a regional stratigraphic marker that may have resulted from glacial sediment and meltwater (including Lake Agassiz overflow), diverted from routes to the Lake Superior basin, into the Lake Ojibway basin. Dates from the Lake Superior basin suggest that the diversion of this water occurred at around 9040 cal BP. Applying this date to varve 1528 constrains the age of the Lake Ojibway varves numbered 1 to 2100 to between 10,570 and 8470 (±200) cal BP.
Published Version
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