Plio-Pleistocene climate change and the onset of aridity in southeastern Australia

Extreme global climate change in the Late Neogene is well known. In Australia, climate changed from wet conditions in the Late Neogene to the arid conditions that characterize much of the continent today. We constrain the nature and timing of the onset of aridity in southeastern Australia by detailed stratigraphic analysis of palaeo megalake Bungunnia. The demise of the megalake has previously been thought to herald the onset of arid climatic regimes and the sedimentary record of this event has become a proxy-type section for understanding Plio-Pleistocene climate in southern Australia. Our investigations of this sedimentary record reveal regionally extensive and correlatable aeolo-lacustrine quartz silts that represent a major and previously unrecognised step in the aridification of the continent, prior to the demise of the megalake. The age of the aeolo-lacustrine silts is constrained to be around 1.4 Ma to 1.5 Ma, just prior to the middle Pleistocene transition. The silts are preserved in a record of maximum lake fill and not long after their deposition the lake began to contract. Our data show that the youngest lake sediment, the Bungunnia Limestone, is diachronous and preserved on at least 5 distinct terraces, ranging over more than 20 m in elevation. The terraces are interpreted to represent downcutting events during arid conditions, while the sediments on each terrace represent deposition during wet periods. The mineralogy of the limestone preserved on these terraces changes from calcite, aragonite and dolomite dominated at higher elevations to gypsum and magnesite dominated on the lower terraces, providing a unique record of the increasing amplitude of arid climatic cycles. These observations suggest that the onset of aridity in southern Australia was progressive and step-wise, beginning significantly earlier than previously suggested.