A SAULT-OUTLET-REFERENCED MID- TO LATE-HOLOCENE PALEOHYDROGRAPH FOR LAKE SUPERIOR

Even though Lake Superior is the world’s largest freshwater lake and the head of the largest surface freshwater system in the world, little is known about its recent water-level history prior to monitoring and regulation. Numerous relict shorelines, producing a ridge and swale topography, adjacent to Lake Superior were noted more than a century ago but not fully exploited for past lake-level information. Four strandplains of beach ridges were used to construct a Lake Superior paleohydrograph relative to the current outlet near Sault Ste. Marie, Ontario/Michigan. Past lake-level elevations were determined from basal foreshore contacts in about 300 vibracores of beach ridge nearshore deposits across four strandplains, and timing of ridge creation was interpreted from models of optically stimulated luminescence ages derived from 60 sediment samples collected from individual ridges at each strandplain. Glacial isostatic adjustment (GIA) was quantitatively estimated between each site by subtracting modeled elevations during Lake Superior’s four mid- to late-Holocene lake phases (Nipissing, Algoma, Sault, and Sub-Sault phases of ancestral Lake Superior). Rates of GIA derived from strandplains are similar to estimates from gauge data except for one site. Each of the four relative paleohydrographs was adjusted for GIA to produce an inferred Sault paleohydrograph, which matched a reevaluated zero-isobase strandplain record at Whitefish Point and a new sandspit record near Sault Ste. Marie. The Sault paleohydrograph shows short-term deviations from a long-term decreasing trend lasting about 4000 years followed by a relatively consistent stable trend during the last millennia. This change in trend is related to the final separation of Lake Superior from Lake Michigan/Huron at about 1 ka.