M. Palmer

The provenance of sediments in Moreton Bay, Australia: a synthesis of major, trace element and Sr-Nd-Pb isotopic geochemistry, modelling and landscape analysis

Moreton Bay (MB) is a large semi-enclosed coastal embayment, located on the east coast of Australia adjacent to the Queensland capital city of Brisbane. The MB catchment is ca. 22 000 km2 while the bay itself is ca. 1500 km2. This represents a catchment:bay ratio of ca. 15:1. Within the MB catchment there are over 85 rocktypes with the most abundant, the Marburg Formation, constituting only ca. 12% of the catchment area. Detailed landscape analysis and a reconnaissance soil sampling program in combination with major and trace element and Sr, Nd and Pb isotope geochemistry were used to identify the major sources of sediment delivered to MB. A Bayesian linear mixing model allowed the proportion of sediment sources to be estimated. Model estimates suggest that there are substantial differences in the proportions of sediment being delivered into MB by the two major tributaries, the Brisbane and Logan Rivers despite both rivers containing a similar suite of major rocktypes. Over 50% of the sediment delivered to MB is derived from soils developed on the Marburg Formation. This equates to a catchment area/bay sediment deposition ratio of ca. 5. Basaltic soils of the Main Range Volcanics (Brisbane River catchment) and Lamington Group (Logan River catchment) and sediments from the Walloon Subgroup (present in both catchments) are on average also enriched in MB relative to the catchment by a factor of ca. 2.

Characterizing and Modeling Temporal and Spatial Trends in Rainfall Extremes

Abstract A hierarchical spatial model for daily rainfall extremes that characterizes their temporal variation due to interannual climatic forcing as well as their spatial pattern is proposed. The model treats the parameters of at-site probability distributions for rainfall extremes as “data” that are likely to be spatially correlated and driven by atmospheric forcing. The method is applied to daily rainfall extremes for summer and winter half years over the Swan–Avon River basin in Western Australia. Two techniques for the characterization of at-site extremes—peaks-over-threshold (POT) analysis and the generalized extreme value (GEV) distribution—and three climatic drivers—the El Nino–Southern Oscillation as measured by the Southern Oscillation index (SOI), the Southern Hemisphere annular mode as measured by an Antarctic Oscillation index (AOI), and solar irradiance (SI)—were considered. The POT analysis of at-site extremes revealed that at-site thresholds lacked spatial coherence, making it difficult to ...

Identification of sediment sources to Lake Wivenhoe, south-east Queensland, Australia

Effective management of sediment fluxes in aquatic systems involves, in part, the identification of catchment sediment sources. Lake Wivenhoe (LW), the largest water storage in south-east Queensland, serves two important roles: it supplies 80% of the drinking water to the region and acts as a major flood mitigation feature for the city of Brisbane. Highly developed subcatchments in LW have resulted in declining waterway health, with sediment movement from the catchment to LW of major concern. Although there is considerable hydrological information, only limited data exist on sediment and nutrient fluxes. A detailed lake sediment (128 samples) and reconnaissance catchment soil sampling program (89 samples) was undertaken. Geochemical, Nd-Sr isotopic and statistical analyses were used to identify major sources of sediment to LW. A purpose-built Bayesian mixing model was then used to quantitatively estimate the proportion of sediment from major catchment sources. Approximately 36% of the LW catchment delivers the majority of sediment; enrichment factors for the three major sediment sources (dam to catchment ratio) range from ∼2 to 5. The Esk Formation is the major sediment source comprising ∼10% of catchment area but contributing 50% of the sediment and 33% of the total phosphorus delivered to LW.

Fitzroy River Basin, Queensland, Australia. I. Identification of Sediment Sources in Impoundments and Flood Events

Environmental Context. The Fitzroy River Basin is a major contributor to the loads of suspended sediment and nutrients reaching coastal areas in the southern Great Barrier Reef. Cost-effective investment in improved land, vegetation, and water management to lower these loads requires an understanding of the sources and movement of sediments within the basin. This multidisciplinary geochemical and modelling study provides for the first time a quantitative estimate of sediment sources and spatial and hydrology-related variation within the Fitzroy River Basin. Abstract. An integrated geochemical, modelling, and reconnaissance soil sampling approach has been used to identify the sources of sediment in the Fitzroy River Basin (FRB). The composition of sediment in weirs and dams within the FRB indicate that in the southern and central FRB the Dawson River contributes only a small basaltic component and the inputs are dominated by soils from the Surat and Bowen Basins. Rivers from the central FRB carry variable amounts of basaltic soils. In contrast, basaltic soils constitute the majority of sediment transported during flood events. Surat Basin soils form a minor component of flood events with little contribution from soils of the Bowen Basin despite it constituting the majority of the area of the central FRB. Soils from the Thomson Fold Belt constitute a substantial proportion of the sediment transported by, and retained in, impoundments in the central FRB and also dominate sediment delivered from the western FRB. This study will inform cost-effective investment by government to target remedial actions to reduce sediment and nutrient loads within the FRB that may be ultimately transported via the Fitzroy River Estuary to the southern Great Barrier Reef.

Fitzroy River, Queensland, Australia. II. Identification of Sources of Estuary Bottom Sediments

Environmental Context. The Fitzroy River Basin constitutes a major source of suspended sediment and nutrient fluxes to the southern Great Barrier Reef. Improved land management practices to ameliorate these catchment loads require an understanding of the sediment sources and dynamics. This multidisciplinary geochemical and modelling study provides for the first time a quantitative estimate of sediment sources delivered to, and their degree of retention in, the Fitzroy River Estuary. Abstract. Sources of sediment deposited in the Fitzroy River Estuary (FRE) have been identified and quantified using an integrated geochemical, modelling and reconnaissance soil sampling approach. A companion paper (this volume) identifies the major sources of sediments in impoundments on the major river systems and sediment sampled from flood events in the Fitzroy River Basin (FRB). Sediment within the FRE may display distinct longitudinal variation with little basaltic material retained. Sediments derived from the Bowen Basin, which occupies the greatest portion of the FRB, and from the Surat Basin display the greatest longitudinal variation. All FRB soils have a similar total phosphorus (P) concentration. Thus, in considering P export from the catchment it is the total sediment flux which is of major importance, rather than the relative proportions of individual catchment soils. This research provides crucial new regional scale information on the sediment sources deposited within the FRE.

Delineation of sediment sources to a coastal wetland in the Great Barrier Reef catchment: influence of climate variability and land clearing since European arrival

Environmental context. Undisturbed sediments provide a record to past events in a catchment. In this study we examine changes in sources of sediment and their variation over the past century due to changes in climate and extensive modification of the catchment after European settlement. We also highlight how multiple lines of forensic evidence acquired from the sediments can be used to reconstruct catchment history over a range of timescales. Abstract. Enhanced delivery of sediment and nutrients to the Great Barrier Reef has the potential to profoundly influence ecological processes in this natural icon. Within the Fitzroy River Basin (FRB) of north-eastern Australia, natural impoundments such as Crescent Lagoon provide an invaluable archive of accumulated sediment that can be dated using multiple techniques to reconstruct the history of sediment export. During the last century, net rates of accumulation of sediment remain similar; however, large variations in sediment sources are apparent. A major sedimentary and geochemical discontinuity is present between ~45 to 29 years before present. Within this time interval a redox front is preserved corresponding to a change in organic matter influx; C3 plant detritus derived from the onset of broadscale agriculture within the FRB provided an assimilable carbon source resulting in more reducing conditions within the sediments. Statistical correlations demonstrate a notable correspondence between some sediment fractions supporting the notion of a short-lived disturbance to the sedimentation regime in the 1960–70s.

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

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.