Year

2014

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

The Macquarie Marshes (MM), located in semiarid northwest New South Wales (NSW), are a unique wetland system for its inland location, high biodiversity and important role as "sanctuary" or "refuge" for flora and fauna especially colonially breeding waterbirds. However, the high demand for water in this semiarid area especially to support agriculture has led to the decline of the wetlands and their associated wildlife in particular since the 1950s.

This PhD project analyses surface sediments and modern plant samples from the main areas of the northern and southern marshes to assess the most appropriate proxies to be applied to sediment cores to reveal the 'condition' of the MM. Four cores from the northern marshes where organic matter was better preserved were chosen to reconstruct the palaeoenvironmental history of the marshes. Optically stimulated luminescence (OSL) single-grain dating was applied to obtain the chronology; lipid biomarkers particularly n-alkanes and α-phellandrene were analysed to trace the vegetation change in the marshes.

The palaeoenvironmental history of MM in the past ~ 50 ka is reconstructed: the site where the modern Marshes are likely had been inundated since the late Pleistocene and experienced oscillations of dry/wet climate which in turn led to the change of water level and in response the variation of the vegetation types and abundances. The abundance of wetland plants was probably highest during the establishment of the marshes 8-6 ka ago. A dry period at about 2 ka is shown by C4 drought-tolerant plants. It is not until after European arrival (from the 1880s) that terrestrial plants started intruding and gradually replacing the wetland plants. The most striking shift of aquatic wetland plants to more terrestrial plants in this ecosystem occurred in the 1950s to 1970s due to water diversion after the construction of upstream dams. Compared to natural environmental changes, anthropogenic effects have a greater and irreversible impact on the well-being of the marshes. The fact that the MM are free from anthropogenic pollutants (i.e. pesticides from cotton farming and faecal contaminant from the grazing industry) indicates that water loss, rather than pollutants, is the main cause of the decline of the wetlands.

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