A 210,000-year reconstruction of subtropical climates from Fern Gully Lagoon, North Stradbroke Island (Minjerribah), Australia

Abstract

Climates during past interglacials may be an analogue for future climate change. Robust reconstructions of past interglacial climates are important for providing these analogues, as well as for better understanding ecosystem responses that are likely to accompany an increase in global temperatures. Records of interglacial climates are similarly required to understand the influence of long-term Australian climates on biota, including past extinction events, as well as long-term (~200 thousand year; kyr) patterns in Australian climate drivers. However, records of Australian palaeoclimates beyond the Last Glacial Maximum (~20 ± 2 thousand years ago; ka) are rare, limiting detailed analysis of long-term climate patterns and associated responses. This thesis presents a review of Australian climate records beyond the last glacial stage and a new late-Quaternary multi-proxy climate record from Fern Gully Lagoon, North Stradbroke Island (Minjerribah), through the following components: • A review and synthesis of hydroclimate data from 40 Australian marine isotope stage 3 (MIS 3; 57 – 29 ka) records, allowing a continental record of moisture availability to be created, and moisture availability during peak Australian megafauna extinction to be identified (Chapter 2). • A chronology for a new wetland sediment record from subtropical Fern Gully Lagoon, using radiocarbon (14C) and optically stimulated luminescence age determination as well as estimated hiatus locations as priors for Bayesian agedepth modelling (Chapter 3). • Scanning micro-X-ray fluorescence geochemical analysis of Fern Gully Lagoon sediments, enabling a record of inorganic dust flux to be created and regional moisture availability for subtropical eastern Australia to be inferred for the past ~210 kyr (Chapter 3). • Pollen, stable carbon isotope and lipid biomarker records as indicators of wetland vegetation assemblages during recent interglacial complexes and the Holocene, allowing reconstruction of relative moisture availability, changes in fire frequency and approximate mean annual air temperatures (Chapters 4 and 5). Relatively high average Australian moisture availability for mid-MIS 3 indicates that continental climate change is unlikely to be a major driver of Australian megafauna extinction. As such, human impact rather than climate change is more likely to have driven peak megafauna loss at ~45 ka. Fern Gully Lagoon records of aeolian flux, indicative of wind deflation, provide a record from which regional moisture availability can be inferred. Regional (pollen and charcoal) and local (stable carbon isotope and lipid biomarker) climate records were then used to refine the overall multi-proxy record, as well as determine changes in regional temperatures, fire regimes and local wetland evaporation. Along with findings related to vegetation succession and climate proxy recovery methods, the multi-proxy record indicated relatively similar moisture availability at Fern Gully Lagoon during MIS 7a–c and early MIS 5, but notably drier climates during the Holocene. The Fern Gully Lagoon palaeoclimate record supports an interpretation of drier Holocene climates in subtropical Australia compared to recent interglacials, with increasing El Niño – Southern Oscillation variability a possible cause. As such, this study indicates that records of Australian climates during recent interglacials provide invaluable analogues of possible future climates not available during the Holocene.