Luminescence and radiocarbon chronologies of playa sedimentation in the Murray Basin, southeastern Australia

Playa lakes are important palaeoenvironmental repositories in arid landscapes. As geochemically open and organically poor systems, however, accurate dating of playa sequences is often hampered. This study paired accelerator mass spectrometry radiocarbon (AMS 14C) and optically stimulated luminescence (OSL) ages from four Murray Basin playa lakes to assess the accuracy of the chronometric techniques in these environments. The OSL dating directed the sampling strategy, which targeted sandier sections of the playa lacustrine sequences. These appear to comprise aeolian-derived fine quartzose sand, ensuring the effective resetting of the luminescence signals prior to deposition. In contrast, the 14C ages were on bulk organic carbon and not targeted. Ages are generally in stratigraphic order and span the middle-late last glacial and Holocene. This accords with models of past groundwater hydrology for the playa basins. Only half of the paired samples are in agreement at 2σ precision, however, with five of the 14C ages older than their corresponding OSL ages. Low organic contents of playa sediments increases the susceptibility of 14C ages to contamination, with the incorporation of small amounts of dead carbon into the dated samples possibly accounting for the apparent discrepancies. Obtaining valid chronologies from arid settings requires close targeting of dated fractions. The application of independent techniques is essential for scrutinizing the dating results.