Fitzroy River Basin, Queensland, Australia. III. Identification of sediment sources in the coastal zone

Abstract

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.