TEMPORAL CORRECTION AND SPECTRAL ANALYSIS OF EPHEMERAL SAND SIGNALS FROM EMBAYED LAKES EAST OF LAKE MICHIGAN

A series of embayed lakes along the eastern shore of Lake Michigan preserve records of dune mobilization as laminations of sand interbedded with organic rich silts and clays. These two distinct lithologies represent two different sources: episodic inputs of dune sand and continuous deposition of suspended load to the lakes. The period of sedimentation is constrained with a radiocarbon date at the base of each core (5665, 6820, and 9430 BP). If the sedimentation rate was constant, the age of each sand lamination could be approximated by linear interpolation between the carbon date at the bottom of the core and present day at the top of the core. However, the presence of episodic sand inputs indicates that the sedimentation rate was not constant. By assuming that the background sedimentation of fine sediments is constant and by subtracting the volume of sand from the lithological record, the age of sand laminations can be more accurately interpolated. This reconstruction gives a time series of sand input into the lakes, i.e., a temporal sand signal related to mobilization of the sand dunes between Lake Michigan and the leeward embayed lakes. Plotting the sand percent vs. rate of Lake Michigan level fluctuations over the past 4,500 years shows a correlation between high sand content and -0.4 ft/yr rate of change. This suggests that the dunes are mobile during dropping lake levels possibly driven by increased windiness and resulting evaporation.