A Light in the Dark: Luminescence Dating Intermountain Ware Ceramics from Four Archaeological Sites in Northwestern Wyoming

The chronology of high-altitude archaeological sites in northwestern Wyoming is poorly understood due to limited reliable age constraints. While temporally diagnostic artifacts provide relative age control, fluctuations in atmospheric radiocarbon produce radiocarbon age results with multiple calibrated age-range intercepts that prove difficult to interpret. Age overestimates associated with the \u27old wood\u27 problem can be especially prominent at high-altitude sites where cold and semi-arid environments promote the presence of long-living trees and prolong the preservation of organic material. Moreover, regional droughts coupled with a pine beetle epidemic have resulted in increasing wildfire frequency and intensities that serve both as a benefit and hindrance to archaeological research and cultural resource management. While fires expose archaeological sites, they also limit contexts for radiocarbon dating through the destruction of prehistoric wooden structures and release modern charcoal on open-air sites. Needless to say, the convergence of these complications reduces confidence in radiocarbon results alone at many high elevation sites. Fortunately, the presence of prehistoric pottery, known as Intermountain Ware, at many of these sites provides an opportunity for direct dating with optically stimulated luminescence or OSL. Researchers can use OSL along with radiocarbon dating to strengthen resulting chronologies for these problematic geographical areas. In general, luminescence dating provides an age estimate of the time since quartz or feldspar grains were last exposed to light, during sediment transport, or heat, during firing, which resets the luminescence signal. In regards to prehistoric ceramics, quartz and feldspar grains become reset during prolonged exposure to temperatures in excess of 450oC, common during the firing process. Grains then begin to accumulate a luminescence signal due to exposure to radiation from surrounding sediments and the sherd itself. The luminescence emitted from the grain is proportional to the annual dose of environmental radiation and the amount of time since the vessel was originally fired. Luminescence results are interpreted to provide direct ages for site occupation as vessel use-life is encapsulated within the standard error of the technique. Single-grain OSL examines luminescence signals of individual grains, and provides a detailed look into the thermal resetting capabilities of wildfires on sherd luminescence signals. This is especially important as all sherds analyzed in this study, except one, come from post-wildfire inventories or excavations. Results demonstrate that single-grain OSL dating of quartz temper from Intermountain Ware ceramics can provide improved accuracy and precision over radiocarbon dating when sherds are not adversely affected by wildfires