An improved OSL chronology for the Still Bay layers at Blombos Cave, South Africa: further tests of single-grain dating procedures and a re-evaluation of the timing of the Still Bay industry across southern Africa
This paper presents a series of new single-grain optically stimulated luminescence (OSL) ages for the Still Bay at Blombos Cave, South Africa, and compares them to previously published OSL, thermoluminescence (TL) and electron-spin resonance (ESR) ages for this site. Details are provided about the measurement and analytical procedures, including a discussion of the characteristics of the OSL signals of individual quartz grains. This forms the basis for further investigations into the sensitivity of the equivalent dose (De) estimates to a range of different analytical approaches, including changes in the size of the test dose, the choice of signal integration interval, the subtraction of an appropriate background, and isolation of the most light-sensitive ('fast') component of quartz OSL. We also report the results of an inter-operator test of De determination using seven new samples from Blombos Cave, and demonstrate the reproducibility of results obtained for two samples that had been dated previously at another laboratory and were measured and analysed again in this study. Together, these tests validate the robustness of the Blombos Cave single-grain OSL age estimates to a variety of alternative OSL dating procedures. We have incorporated, for the first time, these ages for Blombos Cave into a data set of all single-grain OSL ages for Still Bay and Howieson's Poort sites across southern Africa, and have used a statistical model to re-evaluate the timing and duration of the Still Bay industry. We calculate the most plausible start and end dates of the Still Bay as 72.2 ka and 71.3 ka, respectively - amounting to a duration of 0.9 ka - and estimate (with conventional 95% confidence) that this industry began no earlier than 75.5 ka, ended no later than 67.8 ka and lasted no longer than 6.6 ka.