Constraining time with post-IR IRSL: A geostratigraphic age-bracketing approach for a palaeontological site in southeastern Washington, USA

Legacy fossil localities can provide valuable records of faunal response to past climate change, but their utility can be limited by inaccurate chronology resulting from technical limitations at the time of sampling. The Kennewick Roadcut site (Kennewick, Washington, USA) is a Pleistocene-to-Holocene loess succession from which nearly 15,000 microvertebrate fossil specimens were recovered in the early 1980s. Yet, the site lacked a refined geochronological framework with which to date the fossil localities effectively. Here, by utilizing infrared stimulated luminescence (post-IR IRSL) dating, we constrained the depositional chronology of the site. Additionally, we applied a conservative chronostratigraphic bracketing approach to estimate the age ranges for approximately half of the fossil localities. The post-IR IRSL ages resulting from our study range from ∼14 to 144 ka, expanding the known chronology of the site beyond the original interpretations by ∼40 ka. Hence, we show that the site's temporal range extends into the Middle Pleistocene and spans Marine Isotope Stages 1–6, including two major termination events. This refined chronology provides a temporal context for integrating the curated microvertebrate assemblages into a broader palaeoecological synthesis of faunal change at this site across the past ∼144 ka. By establishing age constraints for a substantial subset of fossil-bearing localities, this study strengthens the utility of the Kennewick Roadcut record for evaluating terrestrial vertebrate responses to late Quaternary environmental change. This project's chronostratigraphic methodology provides a framework that permits other legacy sites, whose stratigraphic controls may be limited, to be revisited.