Dating the middle to late woodland transition in the Illinois Valley: Radiocarbon and thermoluminescence dates from the Baehr-Gust site

Midcontinental Journal of Archaeology, Spring 2003 by Holt, Julie Zimmermann, Feathers, James K

Methods

Radiocarbon Dating

Thirteen pit features-approximately one-third of the pit features NYU teams excavated at Baehr-Gust-were selected for radiocarbon dating. These 13 pits were chosen because they showed no clear evidence of disturbance other than truncation by plowing. The radiocarbon samples consisted of charred plant remains recovered from flotation samples. Because the provenience on the flotation samples was relatively broad for the purposes of radiocarbon dating, typically to entire levels within a feature, bulk samples rather than AMS samples were submitted. Dating bulk samples (i.e., maximizing sample size) produced dates that would ideally minimize the effects of any contaminating charcoal that might have been present in the samples.

Because radiocarbon dating is destructive, the plant remains to be dated were first submitted to David Asch for taxonomic identification. Taxonomic identification also ensured taxonomic uniformity and the absence of contaminants. Finally, the samples were submitted to the Illinois State Geological Survey for dating.

A date from the mounds was also desired to determine if they were built during the White Hall occupation, as suggested by the first hypothesis, or during the Havana occupation, as suggested by the second hypothesis. Because all of the NYU excavations took place off of the mounds, artifacts from the Snyder excavation now curated at the American Museum of Natural History (AMNH) in New York were examined to see if any were suitable for radiocarbon dating. Of these artifacts, the material best suited for radiometric dating was a set of bone awls (Walton 1962:Plate 14). One of these bone awls was borrowed from the AMNH and submitted to Stafford Research Laboratories, Inc., for AMS dating of bone collagen.

For comparison between the radiocarbon and TL dates (see the Discussion below), it was necessary to calibrate the radiocarbon ages to calendar years. Calibration was made using the program CALIB ((C) 2000), which is written and distributed by the University of Washington's Quaternary Isotope Lab (http://depts.washington.edu/qil/calib/). Dates were calibrated to one sigma.

TL Dating

TL dating was used to determine whether White Hall and Havana-Hopewell pottery were contemporaneous in some of these features or whether they occurred together due to mixing. TL is well suited for addressing contemporaneity because it directly dates ceramics to the time of their last heating, which usually corresponds to when they were made or used. TL is based on the continual accumulation of stored energy as a function of natural radioactivity. This energy is released as luminescence upon heating, so the intensity of the current TL signal in a ceramic is proportional to the time since it was last heated. Age is determined by calibrating the current TL signal to absorbed radiation dose-a quantity called equivalent dose-and dividing that quantity by the average dose rate. The latter consists of both internal radioactivity (alpha and beta radiation) and external radioactivity (mainly gamma radiation).