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.

Luminescence dating of hillslope deposits—A review

“…These studies provided chronologies for the deposits, but also showed that the light exposure history of individual colluvial sediments can be dramatically different. Using a single-aliquot IR-OSL protocol on coarse-grain feldspar separates, a wide range of D e values was obtained for the upper part of a colluvial sequence, whereas a much smaller range in D e s was found in the lower part (Wintle et al, 1995b). Comparison with radiocarbon age constraints showed that IR-OSL ages for the upper colluvium overestimated the burial age, whereas in the underlying colluvium-where grains were bleached more uniformly-the obtained IR-OSL ages were correct.…”

Exploring the method of optical dating and comparison of optical and 14C ages of Late Weichselian coversands in the southern Netherlands

“…The presence of a residual dose in some of the grains owing to incomplete bleaching in the past was thought not to be very likely. In such a case, one would expect a higher scatter for the natural and the lower dose points and a lower scatter for the higher dose points because of a reduction of the relative size of the residual dose when laboratory dose is added (Wintle et al, 1995). Nevertheless, a bright grain with a large residual dose (the grain might perhaps not have been bleached at all) could be present in an aliquot, and this might dominate the natural luminescence signal.…”

Sedimentology, palaeoecology and geochronology of Marine Isotope Stage 5 deposits on the Shetland Islands, Scotland

“…This technique may suffer from changes in luminescence sensitivity, but other studies applying the method to samples that have been exposed to daylight for only a short time at deposition, and which probably retain a large proportion of their TL signal, suggest that it may give reliable results (Wintle et al, 1993).…”