Reconstruction of Early Pleistocene climate in southern Australia, and implications for Southern Hemisphere atmospheric circulation

Southern Australia today experiences a winter-dominated rainfall regime, governed by the seasonal migration of the Southern Hemisphere subtropical anticyclone. The history of this rainfall seasonality regime is poorly understood, because well dated, late Neogene climate records are scarce, both in Australia and in the Southern Hemisphere more generally. However, it has been assumed that the initiation of this rainfall regime was linked to the onset of extensive Northern Hemisphere glaciation, around 2.6 Ma. Here, we use fossil beetle remains from an upland southeastern Australian palaeolake record to quantitatively reconstruct regional climate during ~280 kyr of the Early Pleistocene, between 1.84-1.56 Ma. Our results, based on coexistence of extant fossil beetle taxa, indicate that temperatures were consistently 1-3°C warmer than present, and annual rainfall as high as or higher than today, but summer rainfall was 2-2.4 times higher than today, throughout the period of our record. This is consistent with the presence in the associated pollen record of diverse rainforest plant taxa, many of which are now extinct regionally or Australia-wide. Our results indicate that the modern, winter-dominated rainfall regime was not yet in place by 1.5 Ma, at least one million years later than previously thought. In as much as the modern climate regime is a function of the intensity of atmospheric circulation driven by hemispheric temperature gradients, this implies that Southern Hemisphere anticyclonic circulation may have been much less intense than today during the Early Pleistocene, and that a major regional climate reorganisation (Ravelo et al., 2004) occurred sometime after 1.5 Ma, perhaps associated with further growth of the Antarctic cryosphere (Raymo et al., 2006).