OPTICAL AND RADIOCARBON DATING AT JINMIUM ROCK SHELTER IN NORTHERN AUSTRALIA

The Jinmium rock shelter is located in the Kimberley region of norther n Australia. Claims for ancient rock art and an early human presence a t this site(1) were based on thermoluminescence ages of 50-75 thousand years (kyr) for quartz sands associated with buried circular engravin gs (pecked cupules) and on thermolumninescence ages of 116-176 kyr for the underlying artefact-bearing deposits. Here we report substantiall y younger optical ages for quartz sand, and ages based on measurements of radioactive carbon in charcoal fragments, from the occupation depo sit. Using conventional (multiple-grain) optical dating methods, we es timate that the base of the deposit is 22 kyr, However, dating of indi vidual grains shows that some have been buried more recently. The sing le-grain optical ages indicate that the Jinmium deposit is younger tha n 10 kyr. This result is in agreement with the late-Holocene ages obta ined for the upper two-thirds of the deposit from radiocarbon measurem ents. We suggest that some grains have older optical ages because they received insufficient exposure to sunlight before burial. The presenc e of such grains in a sample will cause age overestimates using multip le-grain methods, whether using thermoluminescence or optical dating.