DERIVING AN SINGLE RECORD OF LAKE LEVEL AND ISOSTASY FOR LAKE MICHIGAN FROM FOUR RELATIVE LAKE-LEVEL CURVES

Four Late Holocene relative lake-level curves (RLLCs) from five embayments around Lake Michigan show strikingly similar timings of high and low lake-levels. This is expected because all of these sites should experience the same lake-level change at the same time. However, differential rates of long-term vertical movement caused by isostatic rebound (or tectonism) have caused these RLLCs to diverge from each. It is not, therefore, possible to determine the absolute elevations of high and low lake levels from individual relative lake-level curves.

Assuming that all differential vertical movement recorded in the four RLLCs is due to isostatic rebound, we subtracted best-fit linear rebound rates from each RLLC. The rates of rebound calculated from this study are in agreement with other published rates with the notable exception being that the southern shore of Lake Michigan experienced a change in uplift rate at about 1,400 B.P.

After subtracting rebound from the RLLCs , the resulting residual lake-level curve for each site should represent the absolute elevation of the lake at that particular time. These residual lake-level curves should be (and are) similar at all sites. We combined all these curves together and calculated a Fourier smoothing through the data to produce a single record of lake level for Lake Michigan over the past 4,700 years B.P.. Identified within this absolute lake-level curve are the Nipissing and Algoma phases of Ancestral Lake Michigan, as well as several lower magnitude and shorter duration quasi-periodic lake-level fluctuations of about 160 years (120 to 200 years).