ORIGINS OF BASAL SEDIMENT WITHIN KETTLE LAKES IN SOUTHERN MICHIGAN AND NORTHERN INDIANA: CLIMATE CHANGE, GROUNDWATER FLUCUATIONS, OR TRASH

Finding datable material to constrain ages of deglaciation is a challenge faced by many Quaternary geologists. The basal sediment of kettle lakes may contain datable organic material, but radiocarbon ages from this sediment are generally interpreted to mark deposition thousands of years after local deglaciation – accounting for the temporal gap between local deglaciation and basal deposition. These melt-out time lags are common in reconstructions of the Laurentide Ice Sheet’s Saginaw Lobe, which is loosely constrained by a few isolated radiocarbon dates. To further constrain the Saginaw Lobe’s retreat, radiocarbon dates were collected from lakes along the Sturgis Moraine, but turned out to be ~2,000 cal years younger than earlier age assignments for the moraine and were explained by a melt-out time lag. Surprisingly, lakes along the LaGrange Moraine (~20 km further down ice) produced similar ages to those collected along the Sturgis of ~16 cal ka BP.

All cored lakes contain two lithofacies separated by a sharp contact. The lower dated facies is interbedded fine gravel and sand with high magnetic susceptibility (MS) (10–175 SI), anomalously high ratios of carbon and nitrogen (C/N), and <5% organic C. The upper facies is a massive, silty, carbon- or carbonate-rich (>10%) mud with low MS (<10 SI) and C/N values <25. Preliminary pollen analysis just above the contact indicates an unconformity between the two facies, with the upper facies being mid- to late Holocene in age (<8 ka cal BP). Possible interpretations of the lower facies include short-lived energetic flow in a glaciofluvial, fluvial, or littoral environment. The upper facies is lacustrine, with the unconformity between ~16–8 cal ka BP recording either the absence of a lake (low groundwater table) until the mid-Holocene, or erosion during a dry stage (aridity) before refilling. Similar basal radiocarbon ages and stratigraphy from lakes of geomorphically older and younger moraines of the Saginaw Lobe cannot be explained by a regional melt-out time lag of ~2,000 years. Instead, these lakes experienced different melt-out time lags. Thus, these basal ages do not provide any further constraint for the Saginaw Lobe’s retreat, but instead may record regional warming which would explain the similar radiocarbon ages collected in the region.