How ancient are Australian landscapes and soils?

Old soils on ancient landscapes in a dry and fire-prone continent is how Australia is portrayed by its citizens and by others across the globe. This message, initially developed by various researchers, has entered the public domain by a variety of pathways but especially by books and articles in popular journals and magazines. Mary White’s After the Greening: the Browning of Australia and Tim Flannery’s The Future Eaters are prime examples. But how old are its landscapes and soils, and how might this be tested? Recent work using terrestrial cosmogenic nuclides and OSL dating provide valuable insights on this since they allow dating of exposed rock and mantled rock providing that a suitable soil production function can be determined (Wilkinson and Humphreys 2005). These methods display similar results to denudation rates established by other means (e.g. river loads, hillslope processes, catchment averages etc). In SE Australia, denudation rates are mostly between 5-50 m/Ma and these results are similar to those from other regions, across a variety of lithologies including basalt, granite, sandstone, and limestone. On this basis a 1 m mantle, a typical maximal thickness in upland SE Australia, is formed (renewed) in 20–200 ka. In drier parts of Australia denudation rates decline to 1–2 m/Ma. Even at these rates and when considered over long periods of time a landscape might expect at least c. 600 m of lowering since the beginning of the Palaeozoic. Truly ancient landscape remnants, if they exist, will be rare. Equally, truly ancient soils that preserve their essential quality despite residing at the surface of a landscape for long periods will be very special indeed.