Year

2021

Degree Name

Doctor of Philosophy

Department

School of Earth, Atmospheric and Life Sciences

Abstract

The thesis presents a multi-proxy study of lunette dunes and explores their significance as palaeo-archives, that is, their potential for recording past environmental change and for improving our understanding of the drivers of past change. In particular, the work presented in this thesis provides new insights on the palaeo-history and past water availability in the Murray-Darling Basin (MDB), Australia, with the aim of contributing to long term reconstruction of palaeo-environmental change in this critical agricultural region for Australia.

The work pertains a multi-site investigation of lunette dunes within the MDB and consists of the use of different investigative strategies to examine how lunettes record/respond to change. These include a statistical analysis, used co-jointly with qualitative analysis of lunette sediments, designed to improve the characterization of lunettes facies and the palaeo-information inferable. The results of this component of the thesis allowed for the identification of major lunettes facies, which reflect specific processes in multiple locations. However, results also showed that these facies were characterized by a more complex sedimentary history than previously described. Such a history has resulted in intra-facies heterogeneity ascribable to the occurrence of different combination of processes occurring at a local (site specific) scale. This highlighted the importance of using records of multiple lunettes at different sites to reconstruct palaeo-environmental change and the value of using adjacent/connected geoarchives to explore the role that the geomorphic context plays on lunettes evolution. Based on this, a broader landscape analysis was undertaken. This included the development of chronology for lunettes themselves and the sedimentary and geochronological analysis of the lake basins adjacent to the lunettes, as well as adjacent fluvial deposits. This analysis demonstrated that the response of the MDB lunettes to palaeo-change depends upon the degree of connectivity between the drainage network and lunettes hosting lake basins through time. Therefore, even close lake-lunette systems may record a different set of palaeo-information. Despite this, broad-scale environmental changes can be recorded in lunettes sequences, particularly in geomorphically active periods, such as in the Last Glacial Maximum in the MDB, which this thesis demonstrated was characterized by powerful rivers co-existing with enhanced aeolian activity.

The multi-site investigation of lunettes across the Murray-Darling Basin shown distinctive lunette building phases during the late Quaternary. These were clustered at 50-45 ka, 26-21 ka and 17-14 ka, and were associated with enhanced river discharge by comparison to the present day, and by increased landscape instability related to the establishment of glacial or de-glacial conditions in the headwaters of the MDB. These results imply that these periods were likely associated with shifts in the position/intensity of the synoptic climatic systems that influence the MDB, including the position and strength of the mid-latitude westerlies and the supply of precipitation to the northern rivers of the basin from the tropics and sub-tropics. The 50-45 ka lunettes accretion phase coincided with broad-ecological changes in Australia, including the megafauna extinction. Similarly, the last phase of lunettes accretion at 17-14 ka coincides with the last recorded glacial advance, while the relative absence of lunettes accretion through the Holocene aligns with ameliorated environmental conditions.

FoR codes (2008)

0402 GEOCHEMISTRY, 0403 GEOLOGY, 0499 OTHER EARTH SCIENCES

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.