PALEOSOL RECORD OF THE MIDDLE MIOCENE THERMAL MAXIMUM IN THE AUSTRALIAN OUTBACK

Paleosols of the middle Miocene thermal maximum reveal changes in soils and vegetation during a period of climatic warmth, comparable to that expected during coming global warming. Lake Palankarinna in the Tirari Desert of northern South Australia is one of the most productive Australian localities for Miocene and Pliocene mammal fossils, and its thick sequence of fluviatile paleosols has been dated by paleomagnetism, biostratigraphy, and radiometric dating. Paleosols in the late Oligocene-early Miocene Etadunna Formation and Pliocene Tirari Formation have shallow gypsic horizons in both outcrop and borehole cores, indicating an arid climate comparable with that forming the modern playa lake. The Mampuwordu Sand, a fluvial cut-and-fill sequence between the older Etadunna and the younger Tirari Formations, is likely middle Miocene (16 Ma) in age based on its mammal fauna and paleomagnetically normal mineral orientation. Unlike geologically older and younger paleosols in the sequence, its calcareous nodules and clayey subsurface horizon are more like soils of the dry woodland called mallee in Australia. Eucalypt leaf fossils compatible with this interpretation have been found in the correlative Wipajiri Formation at nearby Lake Ngapakaldi. Mallee vegetation now grows no closer than 1200 km to the southwest. With middle Miocene climates warmer and wetter than at present, this may have allowed this range extension of mallee into the Australian outback. Middle Miocene warmer and wetter climate is also indicated by paleosols elsewhere in Australia: thick brown coals (Histosols) in the Latrobe Valley, Victoria; laterites (Ultisols) at Long Reef, New South Wales; and kaolinite deposits (Ultisols) around Gulgong, NSW.