Quantitative reconstruction of Early Pleistocene climate in southeastern Australia and implications for atmospheric circulation

Today, southeastern Australia experiences a winter-dominated rainfall regime, governed by the seasonal migration of the highly zonal Southern Hemisphere subtropical anticyclone. The late Cenozoic history of this rainfall regime is poorly understood, but it has been widely accepted that its onset was a product of the intensification and northward migration of the subtropical anticyclone, driven by steepening of hemispheric temperature gradients associated with the initiation of extensive Northern Hemisphere glaciation, ∼2.6 million years ago (Ma). Here, we use fossil beetle remains from Stony Creek Basin, a small palaeolake record in upland southeastern Australia deposited over ∼280,000 years between ∼1.84 and 1.56 Ma, to quantitatively reconstruct regional climate during the Early Pleistocene. Climate reconstructions based on coexistence of extant beetle taxa indicate that temperatures were consistently 1-3 °C warmer than present, and rainfall as high as or substantially higher than today, throughout the record. In particular, beetle data indicate that rainfall was similar to today during winter, but 2-2.4 times higher than today during summer. This is consistent with the presence of diverse rainforest pollen also present in the record, and indicates 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. We suggest that the Southern Hemisphere anticyclonic circulation must have been much less intense during the Early Pleistocene than today, rather than shifted meridionally as previously argued.