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  • Remote tree-ring proxies: methods, opportunities, and limitations for reconstructing South Pacific climate
    (2022) Higgins, Philippa
    Thesis
    Increasing population and resource demands, a changing hydroclimate, and increasing risks of extreme events means that sustainable water management is more important now than ever before. Water planners are increasingly recognising that short instrumental records are insufficient to understand fully natural trends and variability in climate. High resolution paleoclimate proxies, like tree rings, can provide long time series of observations prior to the instrumental period, to better understand instrumental and pre-instrumental variability, the occurrence, trends, and drivers of extreme events, and provide insights into possible future hydroclimatic scenarios. However, tree-ring proxies are not evenly distributed in the landscape, and the South Pacific has very few high-resolution paleoclimate proxies to develop detailed reconstructions of climate variability. This thesis explores whether the relationships between tree-ring proxies in regions with strong teleconnections to the Pacific (i.e., ‘remote’ tree rings) can be exploited to reconstruct hydroclimatic indices across eastern Australia and the South Pacific Islands. Methods for hydroclimatic reconstruction are investigated, considering the unique challenges of the region: strong inter-annual and inter-decadal variability, very short data records, data gaps, and potential non-stationarities in climate teleconnections. Existing methods for tree-ring reconstructions have been successfully applied in the South Pacific (Chapter 2); however, overcoming the challenges posed by very short and non-continuous records required adaptations to existing methods (Chapter 3) and the development of new methods (Chapter 5). In the final two chapters, the thesis focuses on how catchment-scale tree-ring reconstructions can be most useful to water managers. In these chapters, methods of identifying, explaining, and representing extreme event frequency, return periods, and trends are explored, as are methods for using paleoclimate data along with climate model projections to help contextualise future risks of climate change. Overall, this thesis highlights the enormous potential of remote tree-rings for improving our understanding of past climate in the South Pacific. The reconstructions consistently demonstrate that the instrumental period underestimates the full range of natural climate variability and shows how century-long records provided by tree rings can help us better understand past climate drivers, contextualise the instrumental period, and refine estimates of future climate risks. This thesis builds upon a growing body of work that demonstrates the considerable value of tree-ring based reconstructions for current and future water resource decision making, most notably in remote regions that are highly vulnerable to climate change but where there are limited instrumental records. Maximising the potential of tree-ring data for water management will require ongoing collaboration between dendrochronologists and water managers.
  • The processes determining dissolved organic carbon (DOC) character and concentration in groundwater in different geological environments
    (2022) Oudone, Phetdala
    Thesis
    Dissolved organic carbon is stored and processed in groundwater in three ways. It is stored on minerals by adsorption, it is biologically processed through biodegradation, and it also undergoes a process to return to groundwater called desorption. This biophysiochemical research shows that the groundwater system is therefore a vital part of the global carbon cycle and carbon sink. This research fills a gap in the existing understanding of how to calculate the global carbon budget, as does not yet include the dissolved organic carbon that is stored in groundwater. This thesis exclusively explores processes determining dissolved organic carbon character and concentration in groundwater in different geological environments. This is new, useful knowledge to describe the biophysiochemical process. This research did not examine human interference in adding carbon to groundwater. This research found how dissolved organic carbon is stored and processed in groundwater due to biodegradation and desorption, and how it is adsorbed onto sediment surface. This research explored the characteristics and concentration of Dissolved organic carbon in groundwater by using Liquid Chromatography-Organic Carbon Detection, and other techniques, to examine dissolved organic carbon in terms of its fractions: humic substances, hydrophobic organic carbon, biopolymers, building blocks (BB), low molecular weight neutrals and low molecular weight acids. There were several key findings. First, the results showed that both semi-arid inland low sedimentary organic carbon environments – i.e., Maules Creek and Wellington – were a carbon source; while the high rainfall temperate coastal peatland environment of Anna Bay was a carbon sink. Secondly, another key finding was that dissolved organic carbon was not processed as a whole chemical compound, but it was processed by its fractions where each fraction was processed distinctly. For example, humic substances were only adsorbed/desorbed in groundwater; while low molecular weight neutrals were only consumed by microbes in the biodegradation process in groundwater.