DISENTANGLING THE ROLES OF HUMAN COLONIZATION AND CLIMATE ON DISRUPTIONS TO THE FLORA AND FAUNA OF SEMI-ARID AUSTRALIA DURING THE LATE QUATERNARY

Throughout the Quaternary, the flora and fauna of Australia evolved and adapted to the high-amplitude, low- and high-frequency climate changes that characterize ice-age cycles. However, between ~120 and 15 ka unprecedented irreversible changes in flora and fauna occurred across the semi-arid zone, and modern humans established their first firm presence in the landscape. Disentangling the impacts of the first-order trend toward a colder, drier planet through the Late Quaternary from the impacts of human colonization has been challenging, from both the chronological and paleoenvironmental perspectives. We utilize the stable isotopes of carbon and oxygen preserved in near-continuous time series of Dromaius (emu) eggshell from five regions across Australia to provide independent reconstructions of ecosystem status and climate over the past 100 ka. Together, d¹³C and d¹⁸O provide ecosystem status and climate from the same dated sample, reducing correlation uncertainties between proxies. These data collectively reaffirm that Australia generally became increasingly, albeit irregularly, drier from the last interglaciation through to the last glacial maximum. Dromaius eggshell d¹⁸O documents peak aridity between 30 and 15 ka, but shows no evidence of exceptional climate change between 60 and 40 ka. In contrast, Dromaius d¹³C_diet documents an irreversible loss of the majority of palatable summer-rainfall-related C4 grasses across the Australian arid zone between 50 and 45 ka, about the same time that the giant megafaunal bird, Genyornis, became extinct, and coincident with human dispersal across the continent. Our data indicate that changes unique to Australia occurred between 50 and 45 ka that led to a new climate-vegetation relationship and an overall reduction in effective moisture across much of the continent. The large summer-rainfall-dominated lakes of interior Australia failed to re-fill subsequently, despite a wide range of global climate states. An explanation for the mechanisms behind these changes remains elusive, but they are almost certainly related to human agency; a climate change explanation is unlikely, given the lack of evidence for unprecedented aridity between 60 and 40 ka, and the successful adaptation of Australian ecosystems to 2.5 Ma of similar changes.