Palaeodunes as archives of environmental change : a case study from the western Murray Basin (South Australia) based on optically stimulated luminescence (OSL) dating of single and multiple grains of quartz

This study is concerned with the reconstruction of the palaeoenvironmental history of southeastern Australia for the last ~300 ka by establishing a luminescence chronology of dune sand deposition. For this purpose, a study area in the western Murray Basin in southeastern South Australia was chosen which today is characterised by semi-arid climate conditions. Vast fields of palaeodunes, stabilised by vegetation, provide evidence of past environmental change. By dating these dunes, past climatic changes can be deciphered, providing valuable information for predicting future climate change. In total, 97 samples were collected from dune sand layers and the time of their deposition was determined using multiple grain and single grain optically stimulated luminescence dating of the quartz fraction. A significant spread in equivalent doses (De) was noticed on a multiple grain scale. In order to analyse the De-distributions in more detail and to gain information on the sources of variations in equivalent doses, single grain measurements were carried out on all samples. Based on analysis of single grain De-distributions, field observations and laboratory bleaching tests, it was demonstrated that mainly microdosimetric effects and post-sedimentary mixing by bioturbation were responsible for the spread of the De-values. Although some of the dune sands exhibited intensive pre-depositional iron crusts, they were well bleached at the time of deposition. To account for the mixing of sediment layers of different age by bioturbation, De-values were calculated using the finite mixture model (Galbraith and Green 1990). The investigation of the De-distributions of samples not affected by bioturbation demonstrated that microdosimetric effects accounted for ~20% of the observed spread in De-values. This value was used to define the allowed spread of the individual components identified by the finite mixture model. Samples with De-values >200 Gy were shown to yield unreliable equivalent dose estimates due to approaching signal saturation. Phases of substantial dune sand deposition were identified for the periods 11-12 ka, 13.5-14.5 ka, 18-38 ka, 62-73 ka, 79-112 ka, 145-185 ka and 205-215 ka, thus up to Oxygen Isotope Stage 7. The oldest depositional phase demonstrates that dune sand layers of great antiquity are preserved in the western Murray Basin. Holocene events of dune sand deposition, represented by at least two luminescence ages, were recognised at 3-4 ka, 5-6 ka and 7-8 ka. Furthermore, two zero ages and an age of 1939 ± 11 AD were obtained. Comparison of the western Murray Basin dune record with a dune record of arid central Australia (Fitzsimmons et al. 2007) showed excellent agreement between the two chronologies. This implies that similar climatic conditions prevailed over major parts of Australia during the Late Quaternary The aeolian depositional phases of the western Murray Basin broadly correlate with arid conditions recorded in other southeastern Australian sedimentary archives. Phases of very limited dune sand deposition were shown to be associated with more humid conditions. However, the relatively large errors on the luminescence ages (~10%) prevent to resolve climatic fluctuations >70 ka and the identification of older non-depositional phases in detail. The oldest, but unreliable age obtained in the present study indicates that the onset of dune formation might reach back to ~450 ka. Although this estimate may not be accurate, it agrees with previous estimates of the onset of dune formation in this area. Based on a reliable minimum De-value derived from the 2*Do-value of this sample, it is demonstrated that dune formation was initiated at least 370 ka ago. This study demonstrates that dunes in semi-arid areas are excellent sedimentary archives for the reconstruction of palaeoclimatic conditions. Single grain dating is very well suitable for establishing a sound chronology of aeolian depositional phases, and ages up to ~270 ka can be obtained with this method, provided that dose rates are low. Although the dune record might be of discontinuous nature, multiple phases of dune formation and dune stability are recorded in the dunes of the western Murray Basin. Not only peaks but also gaps in this dune record can be used for the reconstruction of aridity and humidity, respectively, provided that the chronology is based on a high number of samples. However, a detailed resolution of past climatic fluctuations in the study area was only provided for the last ~70 ka and the interpretation of the dune record has to be supported by comparison with other well dated proxy data of past climatic change.