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