A Comparative Study of Lake Sediments from Early and Late Stage Deglaciation of the Laurentide Ice Sheet

Two different lake cores within the context of the last glacial retreat were examined across a geographic distance of 1200 kilometers to compare differences in associated sediment sources and mineralogies. Records from these two lakes, Rabbit Lake located in Otter Tail Co., Minnesota (N 46.13326, W 96.2071) and Lillabelle Lake in Ontario, Canada (N 49.1021, W 81.0263), were investigated in this study. Sediments were characterized by XRD and SEM-EDAX as well as digital and optical microscopy. Despite the influence of the Laurentide Ice Sheet, both sections have distinctly different mineralogies reflecting changes in source areas, depositional processes and climate. Differences were also seen in the overall core stratigraphy. Rabbit Lake has a clay unit with pockets of aeolian sand and gypsum dispersed throughout, which grades upward into banded, mottled clay that contains carbonate and is capped with a dark homogeneous gyttja. A radiocarbon date taken from the middle of the core yielded an age of 11,800 B.P. indicating that conditions before and during the Younger Dryas should be represented in the core. Lillabelle Lake has a basal varve package that grades up-section into a disturbed varve section with abundant clasts and poorly defined bedding consisting of lenses and laminae of silt and clay. Differences between the sections are marked by an abundance of evaporate minerals such as gypsum and polyhalite as well as expanding clays in the Rabbit lake sediments and the presence of more mafic minerals in the Lillabelle core. Dolomite, quartz, illite, chlorite and calcite are common between both lakes. Grain size data show sand to fine clay-sized fractions d10 (0.5 µm) to d90 (2.2µm) with a mean size of approximately 1.25µm. The presence of clay-sized quartz in the Lillabelle core reflecting a significant aeolian influence is noteworthy.