Digging your own grave: OSL signatures in experimental graves

Abstract

Excavation of mock graves in sediments of aeolian and ?uvial origin were conducted to test the bleaching ef?ciency of grave digging in materials that commonly host ancient burials in Australia. Grave-size pits were dug into Pleistocene aeolian sediments at Willandra Lakes and younger ?uvial sediments on the Lachlan River, back?lled, and re-excavated. Samples for optical dating were taken from sediment in?lling the mock graves and from the adjacent, undisturbed substrate, and analysed using the single aliquot-regenerative dose (SAR) protocol applied to single quartz grains. The resulting equivalent dose (De) distributions revealed that 1% of grains had been fully zeroed in both settings, and an additional 1e6% of poorly bleached grains were apparent in the ?uvial sediments. Insuf?cient and heterogeneous bleaching of sediments during excavation and back?lling produced a decrease in the central dose of between 3 and 6 Gy, and an increase in over-dispersion values of between 5 and 10%. These differences were insuf?cient to clearly distinguish the disturbance event from the effects of bioturbation, biological mixing, or other sources of De variation. The use of the Minimum Age Model substantially over-estimated the burial age (zero years) in both depositional environments, with the degree of over-estimation increasing with the age of the host sediments. These results suggest that optically stimulated luminescence (OSL) techniques will not produce accurate ages for grave in?ll in a number of forensic and archaeological settings. Further study of the bleaching susceptibility of grains within grave in?lls, as well as the effectiveness of grave-digging as a bleaching mechanism is required. In other archaeological and geomorphological applications of OSL dating we recommend routine checks on the effective zeroing of sediments in modern equivalent situations.