Chronostratigraphy, Site Formation, and Palaeoenvironmental Context

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Chronostratigraphy, Site Formation, and Palaeoenvironmental Context of Late Pleistocene and Holocene Occupations at Grassridge Rock Shelter (Eastern Cape, South Africa)

Christopher J.H. Ames1, 2, Luke Gliganic3, Carlos E. Cordova4, Kelsey Boyd1, Brian G. Jones1, Lisa Maher5, and B.R. Collins6, 7

1 – School of Earth, Atmospheric, and Life Sciences, University of Wollongong 2 – Department of Anthropology, University of Victoria 3 – Institute of Geology, University of Innsbruck 4 – Department of Geography, Oklahoma State University 5 – Department of Anthropology, University of California-Berkeley 6 – Department of Anthropology, University of Manitoba 7 – Department of Archaeology, University of Cape Town

Optically Stimulated Luminescence Methodological and Results Detail

Optically stimulated luminescence (OSL) dating is a method that provides an estimate of the time elapsed since luminescent minerals, such as quartz, were last exposed to sunlight (Huntley, Godfrey- Smith & Thewalt 1985; Rhodes 2011). If a quartz grain is exposed to sunlight, thereby zeroing its latent OSL signal, and is subsequently buried, charge will accumulate in the crystal lattice of the grain at a rate that is proportional to the flux of cosmic rays and ionising radiation from the surrounding environment (i.e., the dose rate). When the grain is stimulated with light in the laboratory, the stored energy is released and photons (i.e., OSL) are emitted, the amount of which is proportional to the charge that has accumulated in the grain during burial. This signal can be compared to the OSL signals measured following known laboratory-induced irradiations to estimate the equivalent dose (De) absorbed during burial. Optical ages are calculated by dividing the measured De by the corresponding dose rate.

Samples for single-grain OSL dating (n = 8) were collected from the B-C 2/3 stratigraphic sequence by hammering ~4 cm diameter opaque stainless steel tubes into the cleaned and logged section. Samples were transported to the University of Innsbruck, Austria, where quartz grains of 180-212 µm diameter were extracted from the sediment samples in the laboratory under dim red illumination using standard procedures. Sample preparation included wet sieving, hydrochloric acid wash to remove carbonates, hydrogen peroxide soak to remove organic matter, density separation using sodium polytungstate solutions of 2.62 and 2.58 g·cm-3 to isolate quartz extracts, hydrofluoric acid soak to remove the outer rind of quartz grains and any contaminant feldspar grains, a second hydrochloric acid soak, and a final dry sieve to retain grains of the target diameter (Wintle 1997; Gliganic et al. 2017). Grains were loaded into a Risø DA20 TL/OSL reader and were optically stimulated by a green (532 nm) laser light (Bøtter-Jensen, McKeever & Wintle 2003) and the ultraviolet OSL emissions were measured using an Electron Tubes Ltd 9635Q photomultiplier tube fitted with 7.5 mm of Hoya U-340 filter. Optical stimulations were performed for 2 s (at 90% laser power) at 125°C and equivalent doses were estimated by summing first 0.17 s of signal and using the final 0.3 s as background. Laboratory irradiations were given using a calibrated 90Sr/90Y beta source mounted Risø DA20 TL/OSL reader.

The OSL signal from individual quartz grains was measured to eliminate those grains with unsuitable luminescence properties and to calculate single grain De values, which enables the identification of incomplete bleaching and post-depositional sediment mixing prior to age calculation. For each sample, between 1000 and 1300 grains were measured using the single-aliquot regenerative dose

(SAR) procedure (Murray & Wintle 2000) with natural/regenerative dose and test doses of 220°C for 10 s and 200°C for 5 s, respectively. Satisfactory single-grain dose recovery experiments (Roberts et al. 1999; Murray & Wintle 2003) indicate the appropriateness of the SAR procedure for dose estimation using these preheats. Standard tests were applied to ensure the appropriateness of the SAR procedure. These include a zero-regenerative dose check for recuperation (5%), a repeat- regenerative dose for calculation of a recycling ratio to ensure correction for sensitivity changes (recycling ratio within 2σ of unity) (Murray & Wintle 2000), and an OSL-IR depletion ratio test (OSL-IR depletion ratio within 2σ of unity) (Duller 2003) to identify and eliminate contaminant feldspar grains. Statistical models used include the CAM (Galbraith et al. 1999) and the FMM (Roberts et al. 2000). To identify the age model that best fit each De distribution and the De values that best represent deposition of the sediments, a systematic approach was used (Cohen et al. 2015; Gliganic, May & Cohen 2015) that incorporates the stratigraphic position and environmental context of samples, the overdispersion of CAM and FMM results, and two statistical measures—the maximum log likelihood and the Bayes Information Criterion (Roberts et al. 2000; Jacobs et al. 2008).

Beta and gamma dose rates for sediment samples and one sample from the underlying bedrock were estimated using GM-25-5 beta counting (Bøtter-Jensen & Mejdahl 1988), thick-source alpha counting, and the conversion factors of Guérin et al. (2011). Sample GRS 9 was collected ~8 cm above bedrock and, therefore, received a gamma ray contribution from said bedrock. Consequently, the apparent gamma dose rate received by GRS9 was modelled using the measured gamma dose rates for GRS9 (0.79±0.02 Gy/ka) and GRS BR (1.01±0.03 Gy/ka) following Aitken (1985). The cosmic- ray dose rate was calculated following Prescott & Hutton (1994) and an internal alpha dose rate of 0.03±0.01 Gy/ka was assumed.

Single-grain dose recovery experiments were performed using sample GRS 6. Grains were bleached with blue LEDs for 100 s twice before being given a surrogate natural dose of 46 Gy in the Risø TL/ OSL reader. The measured/given dose ratio of 1.05 ± 0.03 with 0% overdispersion is consistent with unity at 2σ. These results indicate that the SAR procedure is appropriate for estimating known radiation doses.

Dose rate and De data are shown in Tables 3 and 4 of the manuscript. A typical decay curve and dose-response curve are shown in Figure S1.1 and De data from all samples are plotted as radial plots in Figure S1.2. Between 6% and 17% of the measured grains have OSL signals suitable for dating using the SAR procedure. The CAM best fit most samples, but two were better fit by the FMM. All ages are stratigraphically coherent and in agreement with the radiocarbon chronology.

Figure S1.1: a) typical OSL decay curve (GRS 6) and b) typical OSL dose response curve (GRS 6).

Figure S1.2: Single grain De data for all samples displayed as radial plots.

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