Holocene climate, fire and ecosystem change on Kangaroo Island (Karti), South Australia

Abstract

The Earth system – a complex interplay of climate, landscapes, fire, ecosystems, and people – is on the precipice of a geologically significant crisis. Examining Earth systems interactions in the past offers important context for contemporary changes and helps to predict and plan for the future. In this thesis I present a multi-proxy reconstruction of the past 7,000 years of climate, fire and ecosystem change from the lake sediments of Lashmars Lagoon, Kangaroo Island (Karti), South Australia. Kangaroo Island (Karti) is uniquely positioned to fill a regional gap in Holocene palaeoenvironmental reconstructions and has a singular history of human habitation ideal for investigating the long-term implications of losing Aboriginal land stewardship. A robust chronology combining 14C dating with 210Pb analyses and Pu isotopic profiling was established for the site from a new ~7.5-metre-long lake sedimentary record. Changes in catchment processes and hydrology were inferred using X-ray fluorescence elemental scanning combined with mineralogical analysis through X-ray diffraction and bulk organic geochemistry. Past fire and vegetation history was reconstructed using macroscopic charcoal, which augmented microscopic charcoal and pollen data analysed by another researcher. Finally, ecosystem responses to fire – with particular focus on the Viridiplantae (green plants) – were inferred from the analysis of sedimentary ancient DNA (sedaDNA) using shotgun metagenomics, in one of the first studies of this kind in Australia. In this thesis, fire activity was inferred to have increased around Lashmars Lagoon at ~3.3 ka, providing improved chronological precision for a similar finding in a previous study. The validity of this result was reinforced by the absence of a clear correspondence with changes in depositional processes previously implied to have biased the palaeofire record at this site. Pollen and sedaDNA analyses provided ecological context for the fire regime change. The pollen record shows an increased diversity of mid-story shrubs at ~3.3 ka. The sedaDNA record also records compositional changes at this point, as well as increased relative abundance of Viridiplantae DNA after ~3.3 ka. All evidence considered, it appears that the change to more frequent and or bigger fires on Kangaroo Island (Karti) at ~3.3 ka was the combined result of a change in land use and climate. The loss of Aboriginal land stewardship sometime after ~4.3 ka may have led to the development of a dense shrubby mid-story landscape, priming it for more fire with increased fuel loads. Ultimately, late Holocene climate change – namely increased aridification linked to an apparent intensification of the El Niño Southern Oscillation (ENSO) – may have tipped the balance of an unmanaged system into a novel fire regime. Critically, the palaeoenvironmental record from Lashmars Lagoon may provide a rare analogue to contemporary fire regime changes on mainland Australia – and thus an opportunity to understand the potential long-term consequences of current events. This thesis paves the way for future palaeoenvironmental research on Kangaroo Island (Karti), encouraging a more detailed assessment of the human occupation history of the island and additional work to distinguish between more informative aspects of past fire regimes including fire intensity, frequency and biomass burned.