School of Geography - Theses
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ItemSpeleothem-based explorations of millennial-scale climate change in southern Australasia( 2018)Understanding the way Earth responds to rapid climate change is critical for understanding future climate scenarios. The best natural examples of rapid climate change are found in millennial-scale climate events recorded in Greenland ice cores over the Last Glacial Period (120-12 ka). These occur concurrently with similarly-paced, gradual warming events recorded in Antarctic ice cores. Understandings of the transition between Greenland-like and Antarctic-like millennial-scale climate events are limited by a lack of appropriate records from the southern mid-latitudes. However, calcite cave formations (speleothems) have the potential to record high-resolution millennial-scale climate change in this region. This study looks at three southern mid-latitude cave sites, develops or improves palaeoclimate reconstructions from each, compares these to external records of millennial-scale climate change, and assesses the suitability of each site for future millennial-scale palaeoclimate reconstructions. Palaeoclimate reconstructions were produced based on U-Th dating, stable isotope analysis and trace element analysis techniques. The first ever high-resolution palaeoclimate record from Naracoorte, Australia from the Last Glacial Period was produced, which suggested that millennial-scale climate change here was influenced by changes in the activity of the southern westerlies. The first ever palaeoclimate record from Wombeyan, Australia was produced, which suggested that millennial-scale climate change here was confounded by both tropical and mid-latitude climate effects. An existing palaeoclimate record from Nettlebed was improved upon and reinterpreted, which supported previous findings that millennial-scale climate in Nettlebed is influenced by the intensity of the southern westerlies. Naracoorte and Nettlebed demonstrated good potential for future millennial-scale palaeoclimate reconstructions, although Naracoorte is limited by a lack of speleothem samples from the Last Glacial Period. Wombeyan demonstrated poor potential for future millennial-scale palaeoclimate reconstructions due to its confounded climate signature, and high U-Th age uncertainties due to low speleothem uranium concentrations. These findings have implications for the future study of millennial-scale climate change, by presenting brand new millennial-scale palaeoclimate reconstructions and demonstrating how future millennial-scale palaeoclimate reconstructions can be developed from a critically under-sampled region.
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ItemDirect and indirect effects of long-term climatic change on terrestrial-aquatic ecosystem interaction in Tasmania( 2018)Climate influences aquatic ecosystems through two important pathways: (1) directly through temperature or changes in the precipitation/evaporation balance and/or (2) indirectly mediated by changes in the terrestrial environment. However, the indirect impacts of climate on aquatic ecosystems are poorly understood. The aim of this thesis is to better understand how aquatic ecosystems respond to past climate change, using two lakes in western Tasmania as case studies. Palaeoecological research on two multiproxy lake sediment records (Paddy’s Lake and Lake Vera) were used to reconstruct chronology (radiometric dating, i.e. 14C); fire regimes (charcoal); vegetation dynamics (pollen); nutrient dynamics (C%, N%, C/N, δ13C, and δ15N); catchment geochemistry (µXRF scanning); and aquatic response (diatoms and cladocerans) to determine the impact of climate change on these aquatic ecosystems. Results from Paddy’s Lake reveal long-term changes in the cladoceran community are indirectly driven by climate through changing vegetation productivity and available 14N altering the trophic status of the lake. Following the invasion of sclerophyll vegetation caused by increased fire frequency, the indirect climate influences on the aquatic system break down and the cladocerans appear complacent to changing vegetation productivity. At Lake Vera, diatoms respond indirectly to climate through changes in the acidity and dystrophic conditions of the lake with catchment peat formation. An increase in climate variability at ca. 5 ka caused declines in lake level resulting in a shift to a direct response in the diatoms to climate. During a period of increased drying at ca. 2.4 to 0.7 ka, increased fire activity adversely impacts the aquatic system causing a non-linear transition in the diatom community. The findings from this thesis show aquatic ecosystems of Tasmania are predominantly indirectly driven by climate through the formation of thick organic peats. Shifts in vegetation composition alter the surrounding soils and catchment dynamics impacting aquatic ecosystems trophic status and pH. Fire is another important driver of aquatic ecosystem response that causes changes in vegetation composition, altering the nutrient profile of soils and increasing erosion and sediment delivery. Aquatic ecosystems respond with increased pH, disturbance taxa and a shallowing of lake mixing depth in the diatom community. These terrestrial-aquatic ecosystem interactions have the potential to be more widespread across Southern Hemisphere biomes and temperate peatlands worldwide that share similar vegetation-soil dynamics.