Luminescence dating of sediments and ceramics from Angkor Borei, southern Cambodia

A programme of luminescence research and dating has been operating in conjunction with archaeological survey in southern Cambodia since 2001. At the Reno 2002 meeting OSL dating results were presented from one of a series of ancient canal traces in the vicinity of the ancient city of Angkor Borei in the Mekong Delta. Quartz OSL dates were obtained from canal sediments from the first millenium AD, which provided the first absolute ages for an ancient canal trace in this area. The canal work has been extended in 2004 with new fieldwork. A larger canal was excavated and sampled for dating. In the original work primary sediments near the canal base showed evidence of severe mixing with the substrate, leading to ambiguous quartz OSL results from these positions. Feldspars from these layers appeared to be more homogeneous and it was suggested that differential bleaching in turbid tropical waters may be occurring as a result of attenuation of the UV components in daylight. Field experiments have been conducted using a fibre optic spectrometer to study illumination spectra as a function of water depth, and using irradiated quartz and feldspar discs to record belaching rates from OSL, IRSL and TL. The results have confirmed the postulated spectral shifts which compound intensity-attenuation in severely reduced bleaching rates of both mineral systems at depths of 0.5-1.5 m. The extent to which quartz and feldspars are reset under these circumstances is being further examined in conjunction with excitation spectrometry in the laboratory. Meanwhile, work is also underway to investigate a series of brick monuments associated with the so-called "Funan" period. The boundary wall at Angkor Borei, a structure that may contain 106 bricks was sampled in 1999 and 2001. TL and OSL were investigated from a series of rice tempered bricks. Both reduced and oxidised fabrics were encountered, and quartz-OSL have been developed which appear to be capable of dating the bricks. Luminescence ages obtained from the eastern wall range from the late 1st millenium BC to mid 1st millenium AD, and exhibit slightly more age scatter than can be readily accounted for by the laboratory uncertainty budget. This may be the result of the re-use of bricks from older monuments in construction of such large scale structures. A series of smaller brick monuments are currently being investigated using similar approaches. Preliminary laboratory work is also underway to investigate the effectiveness of thermal exposure in resetting quartz OSL signals.