Paper No. 81-24
Presentation Time: 9:00 AM-5:30 PM
CHARACTERISTICS OF BASAL TRASH LAYERS WITHIN KETTLE LAKES IN SOUTHERN MICHIGAN AND NORTH-EAST INDIANA
One challenging aspect of deglacial reconstructions is that datable material is often few and far between. A primary source for radiocarbon dates related to the last glacial maximum (LGM) is organic material found within kettle lakes, but this source has its drawbacks. Terrestrial plant material taken from basal, inorganic sediment beneath lacustrine sediment only provide minimum ages for deglaciation and are impacted by a time-lag for ice blocks to melt. This basal sediment is described as a “trash” layer and is interpreted to have been deposited during down wasting as kettle blocks melted. Despite its significance regarding deglacial ages, the trash layer concept is largely untested. Coring of kettles associated with ice marginal positions (moraines) of the Laurentide Ice Sheet’s Saginaw Lobe has consistently uncovered sandy, basal sediment often containing terrestrial plant material. Radiocarbon ages from trash layers within different lakes along the Sturgis Moraine vary by hundreds of years, ranging from 13.3 – 13.8 ka 14C BP. Sediment cores taken from these lakes reveal relationships between radiocarbon ages, basal stratigraphy and environmental proxies. Lakes with younger ages (and presumably longer time-lags) exhibit a well-sorted, fine-to-medium sand trash layer followed by more organic sediment composed of > 20% C and < 6% CaCO3. Alternatively, older lakes (shorter time-lags) contain trash layers below inorganic mud with higher CaCO3 contents (up to 30%) and < 7% C. Additional kettle lakes from the Sturgis Moraine and Shipshewana Moraine (to the south) are being cored and analyses will include loss-on-ignition, magnetic susceptibility, particle size, CT scanning and C/N ratios. These analyses should further highlight relationships between radiocarbon ages and kettle lake stratigraphy. A more congruent interpretation of radiocarbon ages in deglacial environments would allow for further understanding of ice sheet dynamics and timelines, assisting the development of deglacial reconstructions.