Development and Application of Leaf Wax Geochemistry to Reconstruct Late Cenozoic Climate and Environmental Change in Australia

Abstract

Leaf wax n-alkanes are hydrocarbon compounds that are biosynthesized by higher plants. These compounds can be preserved in the geological record as molecular fossil biomarkers for tens of millions of years. Molecular and isotopic properties of these biomarkers hold great potential as proxy systems for reconstructing various aspects of plant physiology, vegetation structure and climate of the past. Still, uncertainty remains around certain aspects of these proxy systems and their application to past vegetation and 6 climate dynamic reconstructions. The first half of this thesis aimed to provide a more nuanced understanding of aspects of leaf wax n-alkane molecular and isotopic proxy systematics. Major outcomes of this research include an increased understanding of the scale at which n-alkane molecular distributions in modern plants reflect the climate in which they live, with results suggesting population scale responses of n-alkane molecular distributions to climatic conditions. Additionally, carbon isotope systematics of discrete leaf wax n-alkane compounds are found to vary significantly as a function of mixed vegetation group inputs to sedimentary records, particularly in relation to aquatic plant contributions. These results provide new insights into the application of leaf wax n-alkane proxy systems to reconstructions of past vegetation and climate dynamics. The second half 16 of the thesis aimed to apply leaf wax n-alkane molecular and isotopic proxy systems to reconstruct aspects of Australia’s palaeo-environment across the late Cenozoic, focusing primarily on the timing and drivers of the expansion of C4 vegetation on the Australian continent. Across many geographic regions, isotope ratios of plant derived carbon in geological archives indicate that plants using the C4 photosynthetic pathway began to proliferate during the late Miocene and Pliocene. While Australia is today the most C4 dominated continent on Earth, little is known of the history of this important aspect of the Australian vegetation. Using carbon isotope ratios measured from leaf wax n-alkanes in sediments from offshore Western Australia, it is demonstrated that C4 vegetation expanded in north-western Australia much more recently than most other geographic regions, within the last 3.5 million years. n-Alkane molecular distributions indicate C4 proliferation was the final step in opening of the landscape likely related to increasing aridification. These research outcomes suggest strongly regional drivers for C4 proliferation across the globe. Because of the strong links between the distribution of C4 vegetation and warm season precipitation in Australia today, this aspect of the climate is reconstructed in the context of C4 proliferation since ~3.5 Ma. Through measuring hydrogen isotope ratios of leaf wax n- alkanes, an increased amount of summer rainfall in northern Australia since the Pliocene is demonstrated to be a plausible driver for this significant ecological shift on the Australian continent. The research presented in this thesis furthers our understanding of the systematics and application of leaf wax n-alkane molecular and isotopic proxy systems. Late Cenozoic palaeo-environmental reconstructions for the Australian continent using these proxy systems further our understanding of the development of important aspects of modern Australian vegetation and climate.