Insights on the Cenozoic climatic history of Southeast Australia from kaolinite dating

Australia hosts some of Earth's oldest regoliths. They were preserved through geological time thanks to exceptionally low erosion rates affecting large parts of this continent. Accurate chronological constraints about their formation and evolution are abundant in its northern and western regions. However, they remain underexplored in its southern and eastern parts, limiting our knowledge about the extent and impact of Cenozoic hot and wet paleoclimatic events over this continent. To address this question, we provide a new set of chronological data obtained from a tropical regolith from southeastern Australia, close to the western side of the Australian Great Escarpment (Syerston–Flemington, New South Wales). The Syerston regolith, known for its anomalous Sc content, exhibits a thick duricrust (18 m deep) comprising an in situ and a detrital component. The thickness of these indurated horizons and the presence of a detrital duricrust protecting the lower parts of the profile from secondary weathering events and erosion were favourable conditions for the conservation of a long-term record of paleoweathering history. Electron Paramagnetic Resonance (EPR) dating of phyllosilicates allows the discrimination of two generations of kaolinite formed at 10.1 ± 2.3 Ma and 3.8 ± 0.9 Ma, trapped within the in situ duricrust. In parallel, the detrital horizon hosts phyllosilicates with an age of 8.2 ± 2.8 Ma and 1 ± 0.3 Ma close to its top. Such ages confirm the hypothesis that, following the end of the tropical Middle Miocene period, the Late Miocene climate in southeast Australia was still favourable to the development of tropical weathering covers. These results, combined with other dating studies from the literature, suggest that a large part of the extended coastal regions of Australia suffered a warm and wet climate from the Mid-Miocene to the beginning of Late Miocene. However, this hypothesis, as well as the extent of hot and wet climate over Central Australia during the Late-Miocene, period of the onset of the Australian aridity, still need to be constrained by complementary paleoregolith dating studies.