Holocene climate, fire and ecosystem change on Kangaroo Island, South Australia
Miss Lucinda Duxbury1,2,3, Jonathan Tyler1,3, Linda Armbrecht2, Alexander Francke1, Haidee Cadd4,8, W. Boone Law5,6, Atun Zawadzki7, David Child7, Patricia Gadd7, Geraldine Jacobsen7, John Tibby7, Kieren Mitchell2,3
1Department of Earth Sciences, The University Of Adelaide, North Terrace, Adelaide, Australia, 2Australian Centre for Ancient DNA, The University of Adelaide, North Terrace, Adelaide, Australia, 3ARC Centre of Excellence for Australian Biodiversity and Heritage (CABAH), The University of Adelaide, North Terrace, Adelaide, Australia, 4ARC Centre of Excellence for Australian Biodiversity and Heritage (CABAH), University of Wollongong, Wollongong, Australia, 5Department of Geography, Environment and Population, University of Adelaide, North Terrace, Adelaide, Australia, 6School of Biological Sciences, University of Adelaide, North Terrace, Adelaide, Australia, 7Australian Nuclear Science and Technology Organisation, Sydney, Australia, 8Chronos 14Carbon-Cycle Facility, Mark Wainwright Analytical Centre, University of New South Wales, Sydney, Australia
Fire has long been a familiar and important part of Australian landscapes. However, anthropogenic climate change has heralded major shifts in fire regimes, negatively impacting ecosystems. These effects are expected to worsen in coming years, but there remain difficulties with projecting fire regime trajectories and their environmental impacts, in part due to a lack of data on centennial to millennial timescales. Uncertainties remain concerning the independent effects of climate and human impact on bushfires, and the long term impacts of fire on both terrestrial and aquatic ecosystems.
This study aims to address these uncertainties by returning to Lashmars Lagoon, Kangaroo Island, a site of pioneering palaeoenvironmental research in South Australia that has been overlooked in recent decades. Kangaroo Island presents a fascinating case study, due to the putative abandonment of the island by ancient Aboriginal populations ~ 2,500 years ago. As such, it represents a unique opportunity to study the impact of cessation of Aboriginal management practices on ecosystems prior to European invasion.
We collected a ~ 7.5 metre long sediment core from Lashmars Lagoon, which we expected, from the work of previous studies, to span the past 5,000 years. We are combining multiple traditional and novel palaeoecological and geochemical proxies to infer both fire, catchment and ecosystem variability, augmented by an age model based on Pb-210, Pu isotopic profiling and C-14 dating.
Our preliminary age model determined our core to span the past ~ 7,000 years, an adjustment to previous estimates. Our model is based on the radiocarbon dating of 6 plant macrofossils, 2 pollen samples and 1 shell. The pollen dates concur well with the plant macrofossils. The shell demonstrates an older age than the other samples from a similar horizon, a ‘reservoir effect’ that is expected from the integration of older carbon into the sample. The model also integrates 15 Pb-210 dates from the uppermost 30 cm, verified by the establishment of a nuclear bomb testing peak from Pu isotopic profiling. The broad peak detected in the Pu profile is also worthwhile of mention. Further interrogation of this phenomenon could be useful to infer certain processes the uppermost sediments, such as bioturbation, lateral leaching of Pu in the sediment or disturbance from the coring process. Overall, the age model indicates a constant rate of sedimentation approximately 1 mm/yr across the entire length of the, a somewhat surprising result given the considerable variation in sedimentology.
The development of a robust age model is integral to the interpretation of environmental, geochemical and climatic proxies at Lashmars Lagoon, South Australia. Importantly, the better constraint of our age model improves certainty around the timing of the human exodus from Kangaroo Island and the resultant discontinuation of Indigenous fire management.
Lucinda Duxbury is a postgraduate researcher at the University of Adelaide working on untangling the links between fire, environment, people and climate. She has a background in genetics, palaeontology, ecology and linguistics, and delights in interdisciplinary approaches to knowledge creation.