Palaeoenvironmental proxy development and application of leaf wax n-alkanes in Australia

Abstract

This project explores the signals of plant-derived n-alkanes in plants, soils and sediments in Australia, in order to better understand their usefulness as a proxy for the plants and environments in which they were formed. Leaf wax n-alkanes are useful compounds for analysis because they are highly recalcitrant and ubiquitous in the sedimentary record. As such, leaf wax n-alkanes are an ideal tool for examining past environmental conditions where other macro and micro plant fossils are rare or simply not present. In this thesis, I examine n-alkane chain length distribution as well as carbon and hydrogen isotope ratios through a modern field study, a laboratory experiment, and an ancient cave sediment record. When analysing leaf wax n-alkanes in soils and sediments, the temporal and spatial scales of inputs from plants to soils needs to be understood. Therefore modern day calibrations are required to understand what the ancient signals represent. Here we compare leaf wax n-alkanes in modern soils and the immediate surrounding plant community along a latitudinal transect across Australia. Results show that while n-alkane distributions in surface soils do not correlate with local, current vegetation, they do correlate with proportional grass and tree cover, suggesting they provide a faithful record of large-scale ecosystem structure. Further, the signals observed in sedimentary records are likely to reflect a regional, time-averaged signal that is not heavily susceptible to short-term variability or small-scale spatial heterogeneity in climate. To determine whether the lack of correlation between surface soils and local, current vegetation observed above is as a result of degradation processes occurring in the soils, the effects of post depositional modification to leaf wax n-alkane signals in soils are examined through laboratory experiments. To confirm the reliability of leaf wax n-alkane signals in soils and sediments, it is necessary to understand whether alteration to the signals has occurred. The effects of post depositional modification were isolated by incubating soils mixed with organic composts. Results show that degradation processes significantly decrease leaf wax n-alkane concentrations post-depositionally, and this decrease primarily occurs within the first month of incubation. However, despite a significant decrease in concentration, the average chain length distribution of leaf wax n-alkanes remains unchanged after incubation, providing confidence in this signal in soils and sediments and for use in palaeoenvironmental research. Based on the findings from our modern day calibration work, I examine the leaf wax n-alkanes signals in Blanche Cave sediments from Naracoorte, in south eastern Australia, spanning an age range of 70,000 to 17,000 years old. Isotopic analysis of leaf wax n-alkanes provides information about the vegetation and hydrological conditions before, during and after the mass megafauna extinction event that occurred Australia-wide at around 40ka. Carbon isotope analysis shows that C3 and C4 vegetation abundances vary across this time. Further, hydrogen isotopic analysis provides insight into fluctuating hydrological conditions. The results from this study provide environmental context to the causes and effects of megafauna extinction in Naracoorte.