Geochemical characterization of estuarine depositional environments of Moreton Bay, southeast Queensland, Australia

Abstract

Moreton Bay in southeast Queensland, Australia, is a complex ecosystem and an ecologically significant peri-urban area that is experiencing fast population growth and significant urban development. Grain size statistical parameters were developed and used in this study as proxies to characterize Holocene depositional environments across Moreton Bay, and to reconstruct past depositional environments and identify the influence of sea level during the Holocene. Extensive experiments were carried out to establish the impact of sample preparation and organic matter content on the reliability of grain size and geochemical analyses, and both existing and newly-developed grain size statistical parameters were tested and calibrated against the modern estuarine bay environments. Fluvial, aeolian, and intertidal-marine depositional environments were identified in this study. Aeolian deposits dominate the sedimentary record in North Stradbroke Island and also in the Beachmere coastal area. Fluvial deposits were predominant in the Tinchi Tamba wetlands, Burpengary estuary and Beerburrum forest, while intertidal-marine deposits were identified at different depths in all sites excluding North Stradbroke Island. Rare earth elements and yttrium (REY) were tested to further identify and constrain the distinctive geochemical features of the different depositional environments. Each environment showed distinct REY patterns which are largely inherited from their source lithologies. The highest REY concentrations and the least fractionated REY patterns were observed in fluvial deposits with the highest percentage of mud. In contrast, aeolian deposits composed of almost 100% quartz sand had the lowest concentrations of REY, and their patterns can be described as enriched in HREE. A unique LREE- enriched pattern was detected in the intertidal deposits at Burpengary. Radiocarbon dating was used to constrain the timing of main sea level fluctuations during the Holocene. At least four periods of sea level rise have been recognized. The first occurred at ~6800 years BP shortly after the initial deposition of the Holocene sequence in the Tinchi Tamba wetlands. This was followed by a second episode at ~5360 - 5100 years BP. Two other minor oscillations may have occurred at ~3477 years BP, and ~2005 - 1880 years BP, however these are very poorly constrained. Detailed geochemical investigations showed that the different types of deposits defined using grain size statistics and sedimentary features also have distinct REY patterns. Thus, and although it was demonstrated that REY in sediments are mostly inherited from their source rocks, REY abundances and patterns are also partly controlled by depositional processes (e.g., mineral fractionation during transport and sorting) and the distribution and chemical and physical stability of organic matter. Despite the complex nature of these processes, this study demonstrated that REY are a powerful geochemical indicator of depositional environments even in a complex and dynamic setting such as a tidal estuary.