Evaluation of luminescence-dating procedures applied to late-Holocene colluvium near St Paul's Mission, Natal, South Africa

Abstract

Colluvial deposits provide an interesting challenge for luminescence-dating techniques because of the short and varied light-exposure histories of the grains prior to deposition. Results of thermoluminescence (TL) and infrared stimulated luminescence (IRSL) measurements are presented for two stacked colluvial deposits from Natal. A radiocarbon date for bulk organic matter in the A horizon of the soil formed within the surface of the lower unit provides a means of assessing the suitability of various luminescence procedures for such material. The overlying colluvium contains grains bleached to varying extents, as shown by the range of EDs for the coarse-grain feldspar separate obtained using the single-aliquot IRSL method. For this sample all age estimates (either by IRSL or TL) are too large compared with the radiocarbon age, indicating that even the IRSL signal was not zeroed in all grains before they were buried. The underlying colluvium contains grains bleached more uniformly, resulting in a smaller range in the values of single-aliquot EDs; the agreement of the IRSL age with the radiocarbon age indicates that the IRSL signals of all grains were zeroed prior to deposition. The TL results for this sample suggest that the signals were not totally zeroed, but had experienced considerably more light at deposition than the grains from the overlying unit.