Climate, humans, fire and megafauna - key drivers of Australian subtropical vegetation change

Abstract

The timing and cause of megafauna extinctions across Australian, and indeed around the world, have been strongly debated particularly since Martin (1967) first implicated human agency as a major factor in megafauna disappearances. The cause of the demise of the megafauna has been the focus of many studies, yet no work to date has developed independent environmental and climate reconstructions from a single Australian location in relation to megafauna extinctions. Sedimentary records from wetlands provide a particularly powerful archive to examine terrestrial ecosystem change in response to internal and external drivers across a variety of spatial and temporal scales. The presence of coprophilous fungi spores, such as Sporormiella, preserved in wetland sediments can indicate the local presence of large herbivorous, including extinct megafauna. Records of megafauna presence can then be coupled with palaeoecological and palaeoclimatological proxies to disentangle the influence of climate on terrestrial ecosystem change and the timing of megafauna disappearance. Sedimentary records that extend beyond the Holocene in Australia are rare. Rarer still are long, continuous, high-resolution records that extend beyond Marine Isotope Stage 3 (MIS3; 57 – 29 ka), a period of substantial significance in Australia that encompasses human arrival and megafauna extinctions. In this thesis I integrate a range of proxies from an 80,000 year old, well-dated, continuous sedimentary sequence from Welsby Lagoon, North Stradbroke Island to investigate the climate and environmental variability of subtropical eastern Australia from MIS4 to present. In this thesis I present, for the first time at a single location, records of inferred megafauna presence, local fire occurrence, vegetation change and independent local climate variability. Understanding the development of the wetland system and identification of changes in depositional environment at ca. 28 ka provide a robust basis for interpretation of proxies. During MIS4 and the Holocene fire is an important component of the surrounding landscape and drives vegetation change, with a more limited influence during MIS3 and MIS2. The largest changes in the vegetation around Welsby Lagoon occur between 55 – 40 ka, in the absence of frequent fire and coincident with the timing of widespread human migration and megafauna extinction. The shifts in vegetation during this period are predominantly driven by changes in climate, as inferred from the δ13C of bulk sediment and the δ18O of aquatic cellulose. The climate of this period displays a high level of variability as well as a shift to drier climates at ca. 54 and again at 43 ka. In addition to driving changes in vegetation dynamics, Sporormiella disappeared from the record at this time, during a shift to drier climate conditions. Within chronological uncertainty, the changes in vegetation composition and hydroclimate at ca. 43 ka at Welsby Lagoon are concurrent with abrupt changes in the Darling River region and central Australia as well as five vegetation records from across the central and eastern Australia. The data suggest that climatic change was major contributor to megafauna and vegetation change during Marine Isotope Stage 3.