A high-resolution chronology for the archaeological deposits at Pinnacle Point 5-6, Western Cape Province, South Africa

Pinnacle Point Site 5-6 (PP5-6) is a key archaeological and paleoenvironmental site located on the edge of the Palaeo-Agulhas Plain along the southern Cape coast of South Africa. Construction of high-resolution chronologies for archaeological sites beyond the range of radiocarbon dating is challenging. Geochronological methods such as optical dating are hampered by the availability of applicable materials that are directly associated with the events of interest. Optical dating relies on assumptions made about time-dependent changes and is made up of a series of measurements each with its own random and systematic uncertainties that together make up the age estimates. In this study, we explicitly took on the challenge to systematically produce a high-resolution chronology for PP5-6 made up of 197 individual age estimates of which 169 were input into a Bayesian age model. PP5-6 is ideal because of its fine-scale stratigraphy and use of modern excavation techniques and detailed recording of stratigraphy and plotted finds. Excavations and dating took place concurrently over almost two decades to inform the dating strategy, contextualise sample choice and data analysis, and to bring the scales of analysis of different proxies closer together. Here we present the optical dating process, including sensitivity tests of our instruments, data analysis procedures and modelling approach. We then construct a final timeline for comparisons with other proxy data and interpretation of the sedimentary sequence and occupation of PP5-6 over an interval of similar to 60,000 years from similar to 110,000 to similar to 50,000 years ago. We show how closely linked sediment deposition is to changes in global climate and sea-level, identify a few Pleistocene and Holocene erosional events that modified the site post-depositionally and place a variety of interconnected causes and effects coincident with different types of occupation on this timeline. This approach opens up opportunities to reduce the resolution of chronologies closer to the human timescales required to improve our understanding of changes through time and to make more direct comparisons between other sites and proxies that contain similarly highly resolved archives of human occupation and change.