A direct comparison of single grain and multi-grain aliquot luminescence dating of feldspars from colluvial deposits in KwaZulu-Natal, South Africa

Abstract. The erosional landscape of the Jojosi Dongas in KwaZulu-Natal, South Africa, expose accretionary slope deposits that preserve important geological and archaeological information. This landscape has been occupied by modern humans during the stone age for many thousands of years as evidenced by the presence of numerous stone artefacts interbedded within at least three phases of gully cut-and-fill deposits. A contextualisation of the artefacts and their role for human evolution in southern Africa, as well as developing an understanding of the environmental conditions that shaped this inhabited landscape, is only possible by establishing a robust chronological framework.

Here we use luminescence dating of feldspars to constrain the geochronological framework for the sequence of accretionary hillslope deposition at Jojosi at three sampling locations. Initial suspicion of poor bleaching led us to measure single grains of feldspar, which revealed low luminescence sensitivity of the individual grains and a variable proportion of grains in saturation. Summing the luminescence signal of individual grains and creating synthetic aliquots enabled us to study the effect of signal averaging on the luminescence sensitivity, signal saturation, and dose distributions. We then compare the results from individual grain measurements and synthetic aliquots to true multi-grain aliquots. To allow for a quantification of the results, we apply four different dose models to the distributions, including the Central Age Model, the Average Dose Model, BayLum, and a standardised growth curve (SGC) approach using an averaged L_n/T_n value interpolated onto the SGC. Doses calculated for the different samples range from ~80 Gy to ~800 Gy and contain up to 67 % saturated grains. We evaluate the performance of the different dose models over this large dose range, with samples close to the saturation level of feldspar luminescence.

On average we find good agreement between the results obtained using the different dose models, but observe that samples with a large number of saturated grains impact the consistency of the result. Overall, all dose models and data sets give consistent results below a saturated grain threshold of ~15 %, corresponding to a dose of ~120 Gy in this study.

Finally, we favoured BayLum for age calculations of the single grain and multi-grain aliquot data sets, representing the opportunity to refine the chronology by including stratigraphic information in the age calculations. We were able to establish a chronology for the three sampled sections within the Jojosi Dongas constraining erosional and depositional processes from ~100 ka to ~700 ka, and human occupation of the area in the early MIS 5 and late MIS 6.