OSL dating in multi-strata Tel: Megiddo (Israel) as a case study

Megiddo, one of the most important mounds (Tel) in the Levant, was inhabited almost continuously from the 7th millennium to the 4th C. BC and archaeological remains have accumulated to a height of ca. 20 m. Megiddo features a significant number of destruction levels, some of which can be correlated to well-known historical events. Other destruction levels are less well dated, and in order to improve the chronological control, an OSL dating campaign was designed, particularly for those periods where the radiocarbon calibration curve incorporates large errors on radiocarbon dates. Twenty-six samples were collected from a range of archaeological periods, excavation areas and sediment types. In-situ gamma and cosmic dose rates were obtained either with Al2O3:C dosimeters that were buried at the site for 2 months or with a calibrated gamma scintillator. Very-fine-sand quartz was extracted and measured using conventional SAR to obtain the equivalent dose (D-e). The OSL age of many samples is older than the expected archaeological age and their D-e values are usually scattered. This suggests that sediments were continuously reused and recycled at Tel Megiddo without exposure to sunlight and very little fresh sediment was added directly from dust to the archaeological accumulation, challenging the basic requirement for OSL dating. Using combined criteria of sequential stratigraphic order of the samples and the over-dispersion of the measured D-e values helped to reject the samples that yield ages which fail to represent the age of their archeological context. Twelve of the 26 OSL ages had to be rejected, but the 14 ages which did pass the criteria agree very well with the expected archaeological ages. Thus analysis of a single sample is ineffective for determining an archeological age for a given context. Sediments from in-between building stones are more suitable than those taken from floors, streets and ash layers; samples from destruction layers should be avoided. Megiddo provides an example of the difficulties in OSL dating in a multi-period, complex archaeological site. (C) 2011 Elsevier B.V. All rights reserved.