THE LAKE SUPERIOR VARVES: RECORDS OF ICE MARGIN DYNAMICS AND EASTERN LAKE AGASSIZ OVERFLOW, CA. 10,600 TO 9,040 CAL YBP (9.5 - 8.1 14C KA)

Between 10,700 to 8,900 cal ybp (ca. 9.5-8.0 ¹⁴C ka), the Laurentide Ice Sheet (LIS) retreated from the Superior basin, creating a thick sequence of glaciolacustrine rhythmites. LIS retreat also opened eastern Lake Agassiz outlets so that the rhythmites reflect the combined impacts of sediment-laden meltwater and Lake Agassiz overflow. Time series analyses of rhythmite thickness patterns, the nature of inorganic carbonate accumulation, and independent dates on the rhythmites demonstrate that this is an annual record (varved). Because this is a varved record, inferences regarding ice sheet dynamics and Lake Agassiz discharge are potentially resolvable with annual resolution.

Anomalously thick sets of varves that could reflect massive influxes of sediment from sudden catastrophic drawdown of Lake Agassiz are practically nonexistent. An exception is an unusual sequence of thick-thin varves from Western Superior, dated at around 10,600 cal ybp (~9.4 ¹⁴C ka). Instead the varve thickness records primarily document regional ice margin dynamics. Some of the thickest varves in the record are the youngest and most distal varves. They occur over a 36-yr period at the top of the record and probably record the creation of the Nakina moraine, beginning around 9,080 cal ybp (~8.1 ¹⁴C ka). North of Isle Royale individual varves within this sequence are up to 14-cm thick, but elsewhere they average between 1 and 3-cm in thickness, however relative changes in thickness during the 36-year sequence are correlatable. A longer, but otherwise similar sequence of thick varves between 10,400-10,200 cal ybp (~9.2-9.0 ¹⁴C ka) probably reflects the creation of the Nipigon moraine. General varve cessation is associated with the abrupt diversion of Lake Agassiz and glacial meltwater from Lake Superior into Lake Ojibway at 9,040 cal ybp (~8.1 ¹⁴C ka); however, near meltwater inlets, rhythmic sedimentation persisted for another 200 years, reflecting the abandonment and drainage of stranded pro-glacial lakes Nakina and Kelvin (Lake Nipigon).

Preliminary trace metal (particularly arsenic) and ostracode oxygen isotope data hold the greatest promise for detailing an annual record of Lake Agassiz discharge to the North Atlantic Ocean.