Oxygen isotopes reveal a significant Middle Pleistocene transition from temperate rainforest to arid desert-like conditions in eastern Australia

Reconstructing terrestrial palaeoenvironmental change during the Middle Pleistocene is essential for understanding ecosystem stability and resilience under long-term climatic forcing. However, well-dated, quantitative records from Australia remain limited. Fossil deposits at Mount Etna Caves, central eastern Queensland, preserve two taxonomically distinct faunal assemblages that have been interpreted as a substantial ecological transition from a diverse rainforest system to an arid-adapted faunal community. Previous interpretations of this transition have relied primarily on faunal evidence, with limited site-specific geochemical validation. Here, we integrate oxygen isotope (δ 18 O) analyses of fossil macropodid enamel with refined in-situ U-Th geochronology to reconstruct changes in hydroclimatic conditions across this transition. New U-Th ages better constrain deposition of the arid-adapted assemblage to a narrow interval between 199 ± 5 and 205 ± 4 ka. Fossils from older deposits (pre-280 ka) exhibit consistently low δ 18 O values (−2.59 ± 0.54 ‰), suggesting relatively humid conditions, whereas the younger deposit is markedly enriched (4.59 ± 1.92 ‰). Random Forest classification models trained on modern macropodine δ 18 O data suggest that pre-280 ka assemblages show strongest similarity with humid temperate forest environments analogous to modern temperate oceanic (Cfb-like) systems of southeastern Australia and highland New Guinea. In contrast, the post-205 ka assemblages show the strongest affinity with arid, hot desert (BWh-like) conditions characteristic of central Australia. This transition coincides with broader Middle Pleistocene climatic reorganisation centred on ∼270 ka, which may have contributed to sustained regional drying. These findings suggest that long-term climatic forcing may push ecosystems beyond hydroclimatic thresholds, resulting in major ecological reorganisation, altered species distributions, and reduced biogeographic connectivity. By providing a directly dated, multi-proxy terrestrial record, this study highlights how substantial environmental change during the Quaternary offers an important analogue for understanding ecosystem vulnerability under ongoing and future climate change.