ILLUMINATING THE PAST: UTILIZING SINGLE-GRAIN OSL TO DATE PREHISTORIC INTERMOUNTAIN WARE CERAMICS FROM NORTHWESTERN WYOMING, USA

Luminescence dating has a long application history to archaeological pottery. While established luminescence methods such as thermoluminescence (TL) and infrared or optically stimulated luminescence (IRSL and OSL) are routinely used to date the fine-grain (FG) paste or coarse-grain temper of pottery, single-grain (SG) OSL is rarely utilized. This is largely due to the assumption that potsherds have homogenous luminescence signals from vessel firing and therefore, are better suited to multi-grain luminescence analyses. However, sherds may contain mixed-signal populations due to post-depositional exposure to wildfires or light-exposure in miniscule cracks. In these contexts, SG OSL dating may be the best option to identify heterogeneous grain populations within potsherds and potentially minimize the impact of eroded grain signals from contributing to the final age estimate.

We apply SG OSL to quartz sand temper in Intermountain Ware ceramics recovered from four northwestern Wyoming, USA archaeological sites and compare resulting ages to associated FG IRSL results and available radiocarbon ages. SG OSL results from the quartz temper and FG IRSL results from the ceramic paste of the same sherds produce ages that are statistically indistinguishable, although the coarse-grain quartz SG OSL ages have consistently lower standard error terms. Moreover, SG OSL results produce precision at two-sigma standard error greater than or equal to associated calibrated radiocarbon ages. In these settings, SG OSL dating of quartz temper from Intermountain Ware ceramics provides more reliable site occupation timing than radiocarbon dating, which can be conditioned by incorporation of old wood and contamination from young soil carbon. Our results highlight the importance of SG OSL dating on sherds from buried contexts when exposure to wildfires may have occurred, as ceramics recovered from the ground surface of one site after a high-intensity fire produced near-modern apparent ages, suggesting they were thermally reset during the fire. We suggest SG OSL should be applied to date similar ceramics with quartz temper to determine site age and bolster regional chronologies.