Fitzroy River Basin, Queensland, Australia. III. Identification of sediment sources in the coastal zone
J. Smith A D , G. B. Douglas B , L. C. Radke A , M. Palmer C and B. P. Brooke AA Petroleum and Marine Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia.
B CSIRO Land and Water, Centre for Environment and Life Sciences, Private Bag No. 5, Wembley, WA 6913, Australia.
C CSIRO Mathematical and Information Sciences, Private Bag No. 5, Wembley, WA 6913, Australia.
D Corresponding author. Email: jodie.smith@ga.gov.au
Environmental Chemistry 5(3) 231-242 https://doi.org/10.1071/EN07094
Submitted: 11 December 2007 Accepted: 9 May 2008 Published: 19 June 2008
Environmental context. The Fitzroy River Basin is a major source of suspended sediment and nutrients to the southern Great Barrier Reef lagoon. A reduction in sediment and nutrient loads is necessary to protect coastal reefs and this requires an understanding of the sediment sources. The present geochemical and modelling study provides a quantitative estimate of the spatial and temporal variations in the sources of sediment deposited in the Fitzroy River coastal zone.
Abstract. Sediment sources to the Fitzroy River coastal zone have been identified and quantified using an integrated geochemical and modelling approach. The coastal sediments display little geochemical variation as a result of substantial homogenisation during hydrodynamic processes and indicate a sediment composition consistent with derivation from mixed catchment sources. A lack of substantial temporal geochemical variation in the sediment records indicates weathering regimes and hydrodynamic transport have been relatively consistent throughout the Holocene. Despite this apparent geochemical homogeneity, a modelling approach using a Bayesian statistical model revealed changes in catchment sediment sources over time. Variations in the occurrence and intensity of rainfall events in different parts of the catchment as well as land-use changes following European settlement are likely to have had a substantial effect on the relative contributions of the catchment sources delivered to and deposited in the coastal zone. Additionally, large variations in flow events and variable estuary hydrodynamics result in different catchment soil types being delivered and deposited under different conditions. The present study found that basaltic material is the dominant catchment source in the coastal surface sediments with an estimated enrichment of ~3 relative to catchment and estuary abundances. Basaltic soils present as a more recent and extensive, weathered surficial cover are more readily mobilised than other catchment soils and will be transported further within freshwater flood plumes. It is likely that in large flood events, this basaltic material may reach the coral-dominated outer shelf. Improved land management practices to reduce sediment loads can be targeted to the areas supplying the majority of sediment to the coastal zone.
Additional keywords: geochemistry, modelling.
Acknowledgements
The authors acknowledge the funding support provided by the Cooperative Research Centre for Coastal Zone, Estuary and Waterway Management. Staff at CSIRO Land and Water, ANSTO and Geoscience Australia provided laboratory support. The authors wish to thank David Ryan and Ralf Haese for useful comments on the manuscript. Published with permission of the Executive Director, Geoscience Australia.
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