ANALYSIS OF INTRA-DUNE VARIABILITY OF OPTICAL AGES IN LATE HOLOCENE COASTAL DUNES ALONG THE SOUTHERN SHORE OF LAKE MICHIGAN

Optical dating, or OSL dating, is rapidly becoming a vital geochronologic tool for coastal dune systems. OSL allows geomorphologists and Quaternary scientists to design a sampling strategy for establishing dune chronologies rather than building studies around chance occurrences of buried organic matter. Modern OSL experimental and analytical methods as well as a re-examination of error reporting philosophies are yielding increasingly precise ages. With these new levels of precision, age variations within a single dune can be contemplated and analyzed. Here we report the results of transverse OSL sampling transects from excavated pits parallel to the predominant wind direction across two post-Nipissing, stabilized, parabolic dunes on the Tolleston Beach within the Indiana Dunes National Lakeshore. Sampled positions included the stoss slope, the dune crest, the brink of the lee slope, the base of the lee slope, and limbs of each dune. At the crest, samples were also taken by bucket augering – a popular, but controversial sampling method. Our results indicate that both dunes consistently record a period of eolian deposition from 3700 to 3500 years ago. Although the OSL ages were statistically indistinguishable at the 2 std. err. level, the results suggest the dunes experienced continued deposition or a later stage of accumulation around 3300 to 3200 years ago. The crest samples collected by bucket augering yielded age over-estimates as compared to the crest samples collected from pit excavations, however, the ages again overlapped at the 2 std. err. level. In conclusion, this study shows that with careful field-to-lab strategies, optical dating can provide a more robust and detailed chronology of shoreline development then previously considered and has the potential to correlate eolian activity to water-level fluctuations with multi-decadal scale resolution.