Single-grain optical dating of grave-infill associated with human burials at Lake Mungo Australia
Recent age constraints on Australia's oldest human remains (Mungo I and III), found at Lake Mungo in western New South Wales, relied on optical dating of sands from the same stratigraphic units as those into which the remains had been inserted (42±3 ka) and those that overlay the graves (38±2 ka), giving a burial age of 40±2 ka. This indirect means of dating the burials was necessary because the original site from which the remains had been excavated had been completely eroded away. At the time of the original excavation of the Mungo III grave, blocks of sediment from the grave-infill were collected for sediment fabric analysis. These sediment blocks were impregnated with polyester resin, sectioned for analysis and the remaining resin-impregnated sediment blocks were then stored in a cupboard, where they have lain for the last 30 years. Here we report on optical dating of single grains of quartz extracted from one of these sediment blocks. Grains extracted from the centre of the block show a wide distribution of equivalent doses (0.0±0.3 to 43.7±8.3 Gy), indicating that not all of the grains have remained hidden from light (light-safe) following excavation. We show that the population of grains with the maximum equivalent dose produces an age consistent with that of the previous study, indicating that some of the grains have remained light-safe. We also use linearly modulated optically stimulated luminescence to identify light-safe grains. These yield an age of 41±4 ka, which represents a direct optical age for the grave-infill and which is consistent with the age obtained in the previous study for sands from the same stratigraphic unit as that containing the burial. The results reported here demonstrate the potential of applying optical dating to archived sediment samples that have not been stored in a light-safe environment.