Chemical and mineralogical composition of marine sediments, and relation to their source and transport, Gulf of Carpentaria, Northern Australia

Journal of Marine Systems 53 (2005) 169 – 186 www.elsevier.com/locate/jmarsys

Chemical and mineralogical composition of marine sediments, and relation to their source and transport, Gulf of Carpentaria, Northern Australia Micaela Preda *, Malcolm E. Cox School of Natural Resource Sciences, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia Received 23 June 2003; accepted 10 May 2004 Available online 8 July 2004

Abstract The Gulf of Carpentaria is an epicontinental sea, deeply indented in the north of the Australian continent. A selection of 100 samples of the surficial marine sediments collected on a regional grid were analysed for major chemical elements and mineral phases using wet chemistry, X-ray diffraction, optical and electronic microscopy. The surficial sediments of the Gulf are highly heterogenous and consist of both young and relict mineral and carbonate components. The terrigenous fraction is fluvial in origin and consists mainly of quartz (16 – 68%), minor feldspars (0 – 9%) and traces of siderite. The clay species determined include kaolinite, mixed layers of smectite – illite and illite; clays range up to 15% and do not present a clear pattern of distribution. Biologically productive areas of the Gulf, mainly in shallower parts, supply the carbonate component of the sediment. The carbonate material is comprised of aragonite (7 – 30%), low-Mg (5 – 30%) and highMg calcite (7 – 28%), and has variable degrees of alteration caused by sediment transport and/or diagenesis. Such processes are partly reflected in the regional distribution of mineral and chemical components throughout the Gulf. The interpretation of the data set was further refined by cluster analysis (Ward’s method), which separated eight clusters (provinces) of sedimentary material. The eastern side appears to be the main source of both terrigenous and carbonate sediment, which is inferred to be transported clockwise. During this dispersion, physicochemical and mineralogical changes take place; the sediments become finer grained and characterised by more stable species of carbonates. As a consequence, the center and the northwest sections are clay-rich and contain dominantly low-Mg calcite. Ooids are relict components that have been identified in areas in which they were not mentioned by previous studies, notably in the southeast. These carbonate particles consist of concentric layers of aragonite deposited around a nucleus of angular quartz, most likely of fluvial origin. This study indicates a complex history over a short time frame with sediment supply, biological production and current patterns being the main factors that control the sediment character and its regional distribution within the Gulf. D 2004 Elsevier B.V. All rights reserved. Keywords: Gulf of Carpentaria; Sediment transport; Sediment distribution; Major elements; Ooids; Aragonite; Low-Mg calcite; High-Mg calcite; Cluster analysis

* Corresponding author. Tel.: +61-7-3864-1910; fax: +61-7-38641535. E-mail address: [email protected] (M. Preda). 0924-7963/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2004.05.003

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1. Introduction The chemical and mineral character of shallow marine sediments is closely related to the geology and hydrography of the adjacent land areas as well as the local climate. As a consequence, the geochemical and/or mineralogical study of such sediments can provide valuable insight into the regional hydrodynamics including patterns of sediment transport and deposition. Due to their socioeconomic and environmental significance, coastal embayments such as gulfs and bays are among the most studied marine environments. The main objective of the majority of such studies is the estimation of abundance and distribution of sediment components (primarily major and trace chemical elements), which in turn help to determine their source (Ergin et al., 1996; Srisuksawad et al., 1997; Basaham and El-Sayed, 1998; Cho et al., 1999; Kim et al., 1999; Sirocko et al., 2000; Shumilin et al., 2002). In some investigations, the identification of sediment sources and/or the distribution of sediment components are used as tools in the assessment of the dynamics of the study area. For example, lateral variations in sediment character can be related to present oceanographic conditions (Leivuori and Niemisto, 1995; Brunskill et al., 2001; Lin et al., 2002) while vertical changes can be indicative of changing sedimentation conditions over thousands of years (e.g., Sohlenius et al., 2001). Numerous other investigations consider the mineral character of marine sediments, in particular the speciation and distribution of clay minerals, which can help determine sources and depositional conditions of marine sediments (Chauhan and Gujar, 1996; Petschick et al., 1996; Bayhan et al., 2001; Daessle et al., 2002). Literature reporting the chemical character of surficial marine sediments in conjunction with their mineralogy is, however, scarce. In this context, the present investigation addresses both these sediment characteristics. The Gulf of Carpentaria is a large semienclosed water body in northern Australia. It represents a valuable fisheries resource and a wide range of investigations has been conducted including ecology (Long and Poiner, 1994), hydrodynamics (Forbes and Church, 1983) and sedimentary evolution (Jones and

Torgersen, 1988; Chivas et al., 2001). However, only limited information is available on the geochemistry and mineralogy of the sediments and the degree to which these parameters relate to the setting and history of the Gulf. This paper aims to determine (1) the distribution of major and minor elements in surficial sediments, (2) sediment mineralogy and its relationship to the chemical composition, (3) the speciation of carbonate phases and their regional distribution, and (4) the physicochemical character of ooids found in the relict component of the surficial material. Determining such features enables better definition of the sediments and their source, transport and distribution patterns, and the relationship to the regional geology and water circulation.

2. Features of the study area 2.1. Location and bathymetry The Gulf of Carpentaria is located between Australia and New Guinea and represents the largest embayment on the coastline of the Australian continent. This body of water extends from 10jS to 18jS and covers an area of around 230,000 km2. The greatest water depths are approximately 70 m and are located in the eastern section of the Gulf. To the north, there is the shallow Torres Strait, which is only 12-m deep and connects the Gulf with the Coral Sea to the northeast. The Gulf is more open to the northwest and is connected to the Arafura Sea through the Arafura Sill, with depths of up to 53 m (Fig. 1). 2.2. Climate and hydrography The climate of the region is tropical with heavy summer rains, notably in the adjacent coastal and highland areas, and typically has dry mild to warm winters. Tropical cyclones (average of 10 per decade) induce strong currents, which are the main mechanism of sediment transport throughout the Gulf (Harris, 1995). For example, measurements made during one cyclone showed near-bottom current speeds to have an hourly average up to six times more than under normal wind conditions (Forbes and Church, 1983). Near the shoreline, however, tidal currents are stronger and could be

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Fig. 1. Map of the Gulf of Carpentaria showing bathymetry (contours at 20, 40 and 60 m), main geographic features, sample locations and numbers.

more effective in the transport of sediment (Harris, 1995). Diurnal tides enter the Gulf from the northwest and propagate clockwise. The semidiurnal tides have a higher frequency and a more complex behaviour, which leads to the trapping of their energy in the northern half (Church and Forbes, 1981). Seasonally, north-easterly monsoon winds and density-induced currents produce clockwise circulation, while southeasterly winds occurring in winter can trigger weak counterclockwise circulation (Forbes and Church, 1983). In addition, a permanent hydrodynamic feature of the Gulf appears to be a clockwise residual current, which was modelled by Forbes and Church (1983). The combined effect of both sea current and wind patterns can determine the redistribution of sediments

within shallow coastal waters, while the deeper waters of the central Gulf are less dynamic. The above factors have an important influence on the overall distribution of sedimentary material. 2.3. Late Quaternary evolution Seismic reflection profiles, sediment coring and analysis, and carbon dating carried out in the 1980s (Jones, 1986, 1987; Jones and Torgersen, 1988) and in the late 1990s (Chivas et al., 2000; Garcia et al., 2000; Chivas et al., 2001) provided a detailed view of the Gulf’s evolution over the last 200,000 years. Several episodes of sea-level fluctuations resulted in the development of a variety of paleoenvironments

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encompassing lacustrine, deltaic or estuarine settings. A significant episode of evolution was 28 –18,000 BP when the sea level fell to about 140 m. The region was completely isolated and sediment deposition took place under lacustrine conditions (Jones and Torgersen, 1988). The newly formed Lake Carpentaria was around 180,000 km2 in area (as opposed to the present 230,000 km2) and about 15-m deep. This stage was followed by the last marine transgression, which peaked at about 9700 BP (Chivas et al., 2000, 2001). These models and the radiocarbon ages reported for surficial sediments are significant for the present study as they suggest that the material analysed here is late Holocene. 2.4. Surficial sediments Previous grain size studies show that the distribution of surficial marine sediments is related to the Gulf’s bathymetry. Terrigenous shelly sediments are concentrated in shallower waters, which are influenced by fluvial input as well as nearshore ecosystems. Muddy sediments prevail in deeper waters and greater amounts of fine material have been determined in the middle-western Gulf at a depth of 40– 60 m (Jones, 1986; 1987; Somers and Long, 1994). The terrigenous material is supplied by 26 rivers draining the coastal zone, of which Mitchell and Leichhardt Rivers (Fig. 1) contribute 60% and 22%, respectively, of the total runoff of 95,000 GL/year (Australian Water Resources Council, 2001). Another important nearshore component consists of relict sediments: stained and fractured quartz grains, ferruginous or calcareous pisoliths, ooids and abraded foraminifera (Jones, 1986, 1987). The most interesting relict components are the ooids. They are nonskeletal carbonate grains, which develop in shallow waters through the deposition of multiple coatings of carbonate around a nucleus of noncarbonate material (Bathurst, 1975; Morse and Mackenzie, 1990). The presence of ooids in sediments was mentioned by Jones (1986, 1987) and detailed by Jones and Torgersen (1988). The carbon age of ooid samples collected from the central Gulf is around 9000 years, suggesting that they formed under marine conditions and not during lacustrine stages (Jones and Torgersen, 1988). However, their origin is uncertain, the mineral composition is

unknown and there is no evidence of formation under present conditions.

3. Analytical procedures The Division of Fisheries Research, CSIRO Marine Laboratories, Cleveland, made available a set of refrigerated sediment grab samples from the material collected during its research cruises; 100 of these samples were analysed for major and minor metals (Fig. 1). Selected samples were microscopically examined to identify the major components of the sand fraction. A smaller number of samples were also analysed mineralogically, using X-ray diffraction to identify mineral phases; the method is particularly useful for components, which cannot be characterised by other means, such as clay minerals and carbonates. The selection of samples was based on a preliminary interpretation of the chemical and statistical analyses and it was designed to ensure that the material selected is representative chemically and geographically. 3.1. Chemical analysis The major chemical components of the sediment were determined by digesting 0.2 g of air dried powder (at 80 jC for 10 h) in a mixture of acids [1 nitric (HNO3): 3 hydrochloric (HCl): 16 hydrofluoric (HF)] following a procedure described by Loring and Rantala (1992). Quantification was preformed using a Varian Liberty 200 inductively coupled plasma optical emission spectrometer (ICP-OES). Four USGS, calibrated control standards were used for each batch of analyses. Calibration was conducted every 10 samples to monitor instrument drift and the accuracy of results. Ooid structure and chemical composition were detailed using electron microscopy. The separated ooids were mounted in epoxy and polished to obtain sections, which were then chemically mapped. 3.2. X-ray diffraction analysis The X-ray diffraction (XRD) analysis was performed on powdered samples (sediments and ooids) using a Philips PW 1050 diffractometer equipped

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with a cobalt anode. Clay minerals were determined separately on orientated samples. The identification and quantification of mineral phases was assisted by Jade (search-match program) and Siroquant (quantification program that expresses the composition of the sample in percentages of dry weight of crystalline material). The XRD traces were also used to determine the amount of Mg substitution in the calcite lattice. The analysis followed a method proposed by Goldsmith et al. (1961), which is based on the fact that the presence of Mg in calcite changes the position of the main calcite reflection peak on the XRD pattern. Using quartz as a standard to correct any shift of the XRD trace, the amount of Mg (as mol%) was calculated by matching the experimental data against a chart, modified for Co radiation after Goldsmith et al. (1961). 3.3. Statistical analyses Normalisation of geochemical data sets is widely used to compensate for granular and mineralogical variability in sediments. An approach is to standardise element concentrations to a ‘‘reference material’’ (e.g., the standard shale after Turekian and Wedapohl, 1961); however, such a material may not reflect the local background (e.g., Loring, 1991; Fang and Hong, 1999). In the absence of an appropriate reference material, another approach is to normalise concentrations against ‘‘reference elements’’ such as aluminium, iron or lithium (Windom et al., 1989; Loring, 1990; Loring, 1991; Loring et al., 1995; Trimble and Hoestine, 1997; Schiff and Wiesberg, 1999). The drawbacks of such a method are thoroughly debated by Weijden (2002). Using Al as a common divisor, the author tested several data sets and established a number of criteria for a geochemical data set to be successfully normalised using element/Al ratios. For example, the coefficient of variation (standard deviation divided by the mean) of the common divisor has to be small relative to the values of the other variables; otherwise, the correlations between elements would be seriously affected. The normalisation can also lead to erroneous results when an element is present in two different phases (Weijden, 2002). An example is Mg, which, in the case of the sediments analysed here, is incorporated in both clay minerals and shell fragments.

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In this context, the geochemical data set produced in this study was examined (descriptive statistics and correlations) and normalisation with Al and Fe tested; it was concluded that normalisation produces misleading results due to the high mineralogical heterogeneity of the material and the data are better interpreted and mapped in their raw format. In addition, the chemical data generated in this study are not normally distributed and the presence of outliers is common. Therefore, the Spearman rank correlation (Rollinson, 1993) was employed as it was considered more appropriate than a simple linear correlation. Hierarchical clustering was also used to find ‘true’ groups of data; in this case, groups of sample locations. All chemical elements determined in the study were included in the analysis; the Ward’s method with the squared Euclidean distance provided the best discrimination between observation sites compared to other clustering methods, which were tested on the chemical data set. Similar outcomes were reported by other researchers (e.g., Danielsson et al., 1999; Haynes and Kwan, 2002). All the statistical analyses were performed using SPSS v. 10.0.5.

4. Results 4.1. Major element chemistry The major and minor chemical elements considered in this study include Na, K, Mg, Ca, Sr, Mn, Fe, Al and Si. These elements are analysed on total sample and reported as percentages of oxides with the exception of Sr, which is expressed in mg/kg. Due to the large amount of chemical data produced, the results are summarised and presented as selected descriptive statistical parameters only (Table 1). Silicon and calcium are the main chemical elements found in the material analysed. The concentrations vary significantly within the study area (SiO 2 = 5.7 – 70.2% and CaO = 10.8 – 48.2%) and these oxides are negatively correlated [Spearman rank coefficient (rS) = 0.87, Table 2]. Silicon is likely to represent mineral components especially quartz, while calcium may be mainly derived from shell fragments, which are abundant in some areas of the Gulf. The

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Table 1 Chemical composition of surficial sediments analysed in the Gulf Chemical element

Minimum

Maximum

Mean

Median

Standard deviation

Coefficient of variation

Na2O K2O MgO CaO MnO Fe2O3 Al2O3 SiO2 Sr

0.59 0.09 0.94 10.79 0.01 0.65 0.70 5.68 449

3.06 1.83 3.81 48.24 0.05 4.31 12.48 70.19 2528

1.44 0.96 2.38 25.80 0.03 2.34 5.74 36.50 1096

1.33 0.90 2.33 25.13 0.03 2.31 5.51 36.72 1003

0.50 0.42 0.68 7.28 0.01 0.99 2.92 11.41 391

0.35 0.44 0.29 0.28 0.33 0.42 0.51 0.31 0.36

%

mg/kg

eastern and southern sections consist mainly of silicon-rich material while carbonate sediments are located towards Torres Strait and in the section deeper than 60 m (Fig. 2a). Aluminium is a minor component of the sedimentary material analysed (Al2O3 = 0.7 – 12.5%). Although related to the Si and Ca distribution, Al is strongly correlated with the finer sediments of the central and northwestern sections of the Gulf. Typically, silicon- and shell-rich sediments tend to have smaller amounts of Al (Fig. 2b). Although a minor element based on concentration (Fe2O3 = 0.7 –4.3%), iron is significant within surficial sediments. While Al can be incorporated in both primary and secondary minerals, Fe is exclusively secondary and it was found to be the main adsorbent element of trace metals (Cox and Preda, 2003). There also are abundant relict mineral and shell fragments, which are coated with Fe oxides and occur in the eastern and southeastern sections. However, the largest amount of Fe is not found in these relict sediments but in the fine sediments of the northwest. This association is also shown by the strong correlation Table 2 Spearman correlation matrix (bold text shows strong correlations) Na2O K2O Na2O K2O MgO CaO MnO Fe2O3 Al2O3 SiO2 Sr

1 0.90 0.82 0.37 0.57 0.90 0.93 0.03 0.46

1 0.73 0.58 0.55 0.89 0.97 0.21 0.63

MgO CaO

1 0.19 0.56 0.84 0.78 0.24 0.35

1 0.11 0.39 0.51 0.87 0.81

MnO Fe2O3 Al2O3 SiO2

1 0.70 0.61 0.15 0.35

1 0.94 0.01 0.52

1 0.13 0.59

Sr

1 0.641

between Fe and Al (rS = 0.94, Table 2). The other elements (Na, K, Mg, Ca and Mn) occur in amounts less than 5%; strontium correlates strongly with Ca (rS = 0.78) and the largest concentrations are found in areas rich in skeletal debris (Table 2). 4.2. Mineralogy of surficial sediments The mineralogy of the material analysed (Table 3) correlates with its chemical composition and highlights differences within the Gulf. The east is dominated by quartz-rich sediments, while in the northeastern section towards Torres Strait, the material becomes predominantly carbonate-rich. The central and northwestern sections have very fine sediments, which explains the large amount of clay minerals found in those areas. Of note is that although not present as coarse biogenic fragments, the carbonate phases are in significant amounts, which range from 30% to 60%. Minor amounts of siderite and feldspars (approximately 1% and 5%, respectively) are also found in the sediments but their distribution does not display a regional pattern. The carbonate fraction consists of aragonite and calcite, and represents recent and relict biogenic debris. The detailed analysis of the calcite XRD pattern enabled the detection and quantification of two phases consisting of low-Mg and high-Mg calcite, which contain 0– 2 and 10 –14 mol% Mg, respectively. The distribution of carbonate phases follows a similar pattern to other minerals. In the eastern section, aragonite dominates over calcite; in respect to calcite, the high-Mg type is greater than low-Mg calcite. In the central-western Gulf, calcite is higher than aragonite and the low-Mg species is dominant (Fig. 3).

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Fig. 2. (a) Distribution of SiO2 (wt.%) compared to CaO (wt.%) in surficial sediments. The regional trends in the distribution of each element are clearly shown. (b) Distribution of SiO2 (wt.%) compared to Al2O3 (wt.%) in surficial sediments.

Clay minerals are present in sediments throughout the Gulf but in higher amounts in the central and northwestern section where they extend to depths of 40 –55 m. Their speciation includes kaolinite, mixed layers of smectite –illite and illite, all of which are typical products of weathering under warm and wet conditions. No obvious regional trends were found for either the distribution of different species of clays within the fine-grained sediment area, or the relationship between the clay minerals. 4.3. Cluster analysis of major elements Understanding the distribution of chemical components was further refined using cluster analysis. The eight groups separated by the analysis are likely to represent as many provinces of sediments each with its particular geochemical character (Table 4, Fig. 4). Clusters (provinces) A, B and C include half of the sampling sites investigated and appear to be closely related in terms of major chemical components such as Si, Ca and Al (e.g., similar mean values and low coefficients of variation). Mineralogically, the material

is fine-grained with high clay (21 – 40%) and carbonate content (40 – 48%) of which at least two-thirds is calcite. Provinces B, C and D are located within the contour of the ancient Lake Carpentaria, while province A extends northwest towards the shallower waters of the Arafura Sill (Fig. 4). Province A has also a much higher coefficient of variation for CaO. The next closest cluster is D, which is characterised by a higher content of CaO compared with clusters A, B and C (Table 4). Most of the samples belonging to this province are located within the deepest section of the Gulf (Fig. 4). Another interesting feature of this province is that the proportion of aragonite is much higher and represents half of the carbonate content; this is usually a feature of shallower water sediments. Provinces E and F form a separate group although the locations of the sites included in these clusters are spread throughout two separate areas, the north- and southeastern sections (Fig. 4). The samples are coarser, with either high quartz (province E) or carbonate (province F), and high coefficients of variation suggesting greater heterogeneity of the material. The last and the most distant group within the cluster tree

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Table 3 Mineral composition of surficial sediments (%) Mineral [Site (cluster)]

Quartz

Aragonite

Low-Mg calcite

High-Mg calcite

Siderite

Feldspars

Kaolinite

Mixed layers

Illite

Grain sizea (% mud)

1 (A) 88 (A) 100 (A) 104 (A) 10 (B) 15 (B) 30 (B) 20 (C) 25 (C) 83 (C) 79 (D) 53 (E) 74 (E) 54 (F) 58 (F) 95 (F) 40 (H) 50 (H) 63 (H) 66 (H) 70 (H) 94 (H)

16.0 19.0 21.4 22.5 31.7 25.8 30.7 23.9 20.5 26.6 18.3 40.2 48.4 12.9 29.1 33.1 50.5 29.6 67.7 53.4 58.7 20.9

12.9 7.7 9.9 7.0 21.3 21.5 19.2 13.8 17.9 17.0 29.9 24.6 13.8 22.8 18.7 24.4 17.9 23.5 13.3 24.2 16.9 23.0

17.1 19.1 14.8 15.1 11.9 8.1 14.3 16.6 14.8 15.4 15.6 8.5 10.3 26.2 24.9 7.7 6.5 29.7 4.7 7.6 5.0 22.2

18.7 10.8 14.9 9.4 12.5 20.3 11.0 17.3 17.0 13.7 14.5 14.2 8.9 28.1 12.6 16.1 13.4 10.3 6.8 8.1 11.8 27.9

0.6 0.6 1.0 1.8 0.0 1.4 1.0 1.4 1.0 0.6 0.6 0.7 0.8 0.9 0.9 1.2 0.8 0.9 0.7 0.5 0.5 1.1

3.0 3.9 6.5 4.0 2.6 1.9 2.0 2.5 2.6 2.6 2.1 1.0 7.1 1.3 1.4 4.7 8.8 1.6 1.0 0.0 5.2 2.0

12.0 12.9 10.7 14.7 8.2 7.8 6.0 7.5 8.3 9.3 9.2 2.7 3.8 1.8 3.0 5.9 0.6 0.6 2.1 1.9 1.8 tr

11.8 10.8 10.3 12.5 7.9 10.5 14.5 12.5 13.4 8.9 9.8 7.8 5.5 5.4 7.1 4.4 1.3 3.6 3.5 4.2 0.0 2.5

7.9 15.2 10.6 13.1 4.0 2.7 1.2 4.4 4.5 5.7 0.0 tr 1.4 0.5 2.1 2.5 tr tr tr tr tr tr

50 – 80 >80 50 – 80 >80 20 – 50 20 – 50 20 – 50 50 – 80 50 – 80 20 – 50 20 – 50 20 – 50 20 – 50 < 20 < 20 < 20 < 20 20 – 50 20 – 50 20 – 50 < 20 < 20

Mixed layers—mixed layers of smectite – illite; tr – traces. a After Somers and Long (1994).

consists of provinces G and H, which present the highest coefficients of variation for all the major components (Table 4). The sites are mainly located in the eastern section and the material is dominantly quartz-rich (Fig. 4). Overall, the heterogeneity of the material increases from province A to H, whereas the clay content decreases gradually from A to H. For example, province A has an average of 35% clay minerals and all the species identified in the region are in comparable amounts. In province H, illite is reduced to traces and mixed layers of smectite – illite dominate over kaolinite. The deeper waters usually contain more calcite; in shallower waters, the amount of aragonite increases while high variability exists in terms of which is the dominant calcite phase.

deeper than 25 m (Jones, 1986, 1987; Jones and Torgersen, 1988). In the present investigation, ooids were not found in areas where they were previously reported, even though the samples selected for microscopic study included two of those areas. In the selected samples examined, abundant ooids were present in sample 42, located in the south, close to the 40-m isobath; less abundant ooids were observed in sample 72 (locations in Fig. 1). The ooids investigated (exclusively from sample 42) consist of aragonite layers deposited around an angular nucleus of quartz. Detectable Sr is associated with the aragonite (Fig. 5). Of note is that the majority of samples (85%) contain less than 1500 mg/kg Sr, while sample 42 contains almost 2500 mg/kg Sr.

4.4. Ooid distribution and composition

5. Discussion

Ooids have been reported in the central Gulf, offshore from Weipa and on the Carpentaria Rise (north sample 29) and are generally found in waters

Shallow marine embayments act as sediment traps and deposition takes place in bands around river mouths (e.g., Lin et al., 2002) or more randomly

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177

Fig. 3. Comparison of the regional pattern of distribution of the carbonate phases present in selected samples.

around depocenters (Carriquiry et al., 2001); in addition, the sediments preserve the signature of the source (e.g., Kim et al., 1999; Sirocko et al., 2000). In the case of the Gulf of Carpentaria, the spatial variation and relationships between chemical elements in the surficial marine sediments suggest the existence of specific controls over their distribution, such as proximity to source and bathymetry. For example, shallow waters have coarser and poorly sorted material, while deeper waters with low-energy levels act as depocenters for clayey and/or carbonate material. However, the overall distribution of elements shows that there is also a regional control, which is independent of bathymetry. The sediments of the eastern and southeastern Gulf differ from those of the center and northwestern sections; an example is the relationship

between two of the major components Si and Ca (Fig. 6). Such regional trends were further investigated for Mg content, which can be included in clay minerals as well as in biogenic calcite (Fig. 7). Both of those examples include outliers, some of which do not integrate within a specific regional trend, but show local characteristics. The presence of outliers is a result of the chemical variability and complexity of processes within this extensive area. For example, large quantities of material can be moved at one time by storms or tidal currents, the latter being especially the case in the southern section. Those events, along with local microtopography and ecosystems, can influence the character of a small area but cannot significantly change the regional chemical and mineralogical character of the sediments.

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Table 4 Ranges of chemical variation within the provinces identified by cluster analysis Cluster/Province (no of sites)

Major and minor oxides (%)

Minimum

Maximum

Mean

Median

Standard deviation

Coefficient of variation

A (n = 25)

Na2O K2 O MgO CaO Fe2O3 Al2O3 SiO2 Na2O K2 O MgO CaO Fe2O3 Al2O3 SiO2 Na2O K2 O MgO CaO Fe2O3 Al2O3 SiO2 Na2O K2 O MgO CaO Fe2O3 Al2O3 SiO2 Na2O K2 O MgO CaO Fe2O3 Al2O3 SiO2 Na2O K2 O MgO CaO Fe2O3 Al2O3 SiO2 Na2O K2 O MgO CaO Fe2O3 Al2O3 SiO2

1.82 1.67 3.69 16.85 3.72 9.42 36.18 1.45 0.97 2.67 28.10 2.55 6.32 39.74 1.94 1.19 2.95 25.99 3.26 7.56 37.89 1.66 0.95 3.02 42.00 2.63 5.77 25.96 1.06 0.46 1.82 24.42 1.36 3.19 45.50 1.28 0.45 2.20 24.91 1.87 4.14 15.24 0.69 0.15 1.09 12.19 0.75 1.20 9.76

3.72 2.22 4.72 41.32 5.36 15.43 55.24 2.12 1.36 3.39 41.61 3.35 8.55 54.15 2.92 1.81 4.49 41.09 4.52 11.04 56.27 2.34 1.34 4.13 58.80 3.61 8.36 41.00 1.40 1.04 2.68 45.16 1.95 4.56 66.58 1.96 1.24 4.52 70.65 4.03 7.04 63.06 1.81 1.02 3.87 82.99 2.63 3.46 79.32

2.62 1.95 4.13 25.28 4.68 12.33 48.06 1.80 1.21 3.01 34.74 3.05 7.42 47.92 2.33 1.53 3.67 32.65 3.79 9.53 45.75 1.93 1.21 3.34 50.25 2.96 7.16 32.61 1.18 0.68 2.19 34.94 1.74 3.79 55.01 1.58 0.91 2.75 40.05 2.44 5.17 46.76 1.06 0.57 1.96 35.94 1.31 2.42 56.66

2.58 1.95 4.12 23.01 4.71 12.36 50.34 1.83 1.22 3.03 34.54 3.14 7.32 48.62 2.30 1.52 3.70 31.86 3.68 9.33 31.86 1.86 1.22 3.24 50.66 2.93 7.25 32.11 1.14 0.64 2.15 36.24 1.75 3.69 53.27 1.53 0.95 2.55 37.80 2.25 5.07 48.01 0.94 0.53 1.70 28.84 1.15 2.53 64.20

0.35 0.16 0.20 7.42 0.41 1.43 5.59 0.23 0.12 0.20 4.66 0.27 0.70 5.13 0.27 0.16 0.40 4.06 0.35 1.03 4.46 0.20 0.13 0.34 5.44 0.28 0.76 4.46 0.12 0.20 0.31 8.33 0.18 0.48 8.15 0.19 0.22 0.62 12.44 0.56 0.67 12.31 0.30 0.25 0.79 20.25 0.47 0.70 20.90

0.13 0.08 0.05 0.29 0.09 0.12 0.12 0.13 0.10 0.07 0.13 0.09 0.09 0.11 0.12 0.10 0.11 0.12 0.09 0.11 0.10 0.10 0.10 0.10 0.11 0.09 0.11 0.14 0.10 0.29 0.14 0.24 0.10 0.13 0.15 0.12 0.24 0.22 0.31 0.23 0.13 0.26 0.28 0.44 0.40 0.56 0.36 0.29 0.37

B (n = 10)

C (n = 15)

D (n = 12)

E (n = 8)

F (n = 16)

G + H (n = 15)

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Fig. 4. Distribution of clusters representing groups of sampling sites, which were separated using the Ward’s method of hierarchical clustering. Clusters or provinces of similar chemical character show regional trends. Provinces A, B, C and D are located in the central and northwestern sections and consist of fine-grained well-sorted material. Provinces E, F, G and H are located in the shallow waters of the eastern and southern sections; this material is highly heterogenous with either high carbonate or silica. Dotted line shows the estimated extent of the ancient Lake Carpenteria (after Jones and Torgersen, 1988).

The regional character appears to be mainly controlled by large-scale processes, such as wind and current patterns. The importance of the hydrodynamics in the movement and chemical character of marine sediments has been recently studied for other similar epicontinental bodies such as the Gulf of California (Shumilin et al., 2002), Arabian Sea (Basaham and ElSayed, 1998), Dead Sea (Herut, 1997) or Yellow Sea (Kim et al., 1999). However, for the Gulf of Carpentaria, the overall water movement is highly significant, as it has been a marine environment for only the last 10,000 years. The existence of such a geologically

short episode offers an estimation of the time necessary to imprint a particular geochemical character to surficial marine sediments. Clay speciation is often used for dispersal and circulation studies (e.g., Chauhan and Gujar, 1996; Petschick et al., 1996; Daessle et al., 2002) but its investigation was not particularly useful in this study as no clear patterns of clay occurrence have been observed. Overall, the interpretation of mineralogical data accompanied by nonnormalised chemical data that were further refined using cluster analysis, provided a

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Fig. 5. Scanning electron microscope back-scattered image and element mapping of ooids, showing particle size and chemical composition. The X-ray diffraction analysis showed that the ooids are composed of aragonite, which explains the presence of Sr within the wall of the ooid. The nucleus is an angular grain of quartz.

base for a regional assessment of the sediments and enabled the inference of sources and transport paths. Considering also the asymmetry of the Gulf with respect to bathymetry (Fig. 1) and the overall clockwise movement of water, several transport paths can be inferred (Fig. 8).

explain the poorly sorted character of the sediment. As expected for this deep section, there is abundant fine material, but there also are numerous large and angular shell fragments, which suggest limited reworking.

5.1. Northeastern Gulf

The eastern section extends over water depths of 20 –55 m and consists mainly of quartz-rich sediments (province H). This material is directly derived from the continent and brought in by several rivers draining the eastern side. Some of the material could move quickly down the ‘steep’ eastern slope and be trapped in the deepest area of the Gulf.

The carbonate material of the northeast (mainly province F according to the cluster analysis) is part of the Torres Strait tidal delta (as defined by Jones, 1987) and is likely to be moved southwards and trapped in the deepest area of the Gulf (province D). This would

5.2. Eastern and southern Gulf

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Fig. 6. Plots of Si – Ca showing a strong negative correlation (rs = 0.87) between these major elements. There is no apparent grouping based solely on bathymetry (a); however, outliers exist and indicate regional trends (a and b). Ca-rich extremes are recorded in the NE (Torres Strait) and to a lesser extent in the south (sample 39). The SE section has the largest amount of Si, which is mostly as quartz; the centre and NW sections are dominated by fine sediments in which Si occurs within the clay minerals.

In the south, there are complex current patterns that create large variability in terms of sediment character, such as the occurrence of siliceous areas (province H) relatively close to carbonate areas

(province F). The siliceous character is common in water 20 –55-m deep and away from the very shallow carbonate-productive areas. The terrigenous character of that area can be, in part, due to the clockwise

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not as readily detectable shell fragments as in other areas. The overall size of the material suggests transport and reworking. Considering the general patterns of sediment transport, that area is the ultimate destination for material sourced from the entire eastern side of the Gulf. Further study is necessary to establish whether some of this material is of lacustrine origin or not. 5.4. Carbonate phases

Fig. 7. The relationships between the Mg and Ca or Al reveals the existence of several populations. Panel (a) shows a positive correlation between Mg and Al, which is stronger in the centre and NW; in this area, Mg is mostly incorporated in clays. The E and SE population is more widely spread and the correlation with Al is much weaker, indicating other sources of Mg apart from clays. The positive correlation between Mg and Ca for the sediments of the E and SE section (b) suggests that Mg is mainly associated with carbonates.

current movement, which brings in sediments from the east. 5.3. Central and northwestern Gulf This region is dominated by fine well-sorted sediments, which occur in waters ranging between 40- and 60-m depth (provinces A, B and C). In some places, such as north of Groote Eylandt, the sediments are very muddy, including shallow areas close to the shoreline. In the center and northwest, the carbonate fraction, although significant, is present as fine silt,

For the Gulf as a whole, the carbonate fraction is the most complex component of the sediment as it can represent different sources; for example, molluscs can provide the aragonitic component, foraminifera produce dominantly low-Mg carbonate while echinoderms have a high-Mg test (Bathurst, 1975). However, the fine size of the shell fragments often makes identification of the organism impossible. The correlation between the distribution of carbonate phases and the local skeletal organisms is difficult without a detailed study of the marine species that are likely to have provided the carbonate. Even with the support of such a study (e.g., Long and Poiner, 1994), the interpretation could be difficult due to a range of events (e.g., diagenetic transformations and storm events) that could readily change the chemical and physical composition of the material. For example, aragonite and high-Mg calcite are more soluble than calcite and low-Mg calcite, respectively. In addition, the carbonate material mobilised from high-Mg calcite can reprecipitate as low-Mg calcite (Bathurst, 1975). High variability in terms of carbonate speciation and distribution has been reported for a large range of environments from Mediterranean to very cold climates (e.g., Karageorgis et al., 1998; Marinoni et al., 2000). Within the Gulf of Carpentaria, it is concluded that the occurrence and regional distribution of carbonate species is controlled not only by local ecosystems and water temperature (e.g., Mackenzie et al., 1983; Nelson, 1988) but also by the availability of other sediment components such as quartz and clay minerals. On the eastern side, aragonite dominates over calcite and high-Mg over low-Mg calcite. That feature may relate to a number of regional characteristics such as biological production, the siliceous character of the sediment and the occasional large fluvial input.

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Fig. 8. Potential sediments transport paths influencing the character of the material. The clockwise circulation within the Gulf is well reflected. The northeast and the deepest section of the gulf is carbonate-rich and sourced from the Torres Strait area (provinces E and F). The eastern side is supplied with quartzose terrigenous material (province H). The southern section is heterogenous due to complex wind and current patters (provinces E and F). The centre and the northwestern section predominantly consist of fine sediments representing reworked material brought mainly in from the eastern Gulf (provinces A, B, C and D). Overall, the sediments originated from terrigenous (provinces E and G + H) or carbonate sources (province F) and evolved into fine-grained clay-rich material with higher (province D) or lower (provinces A, B and C) carbonate content.

In the central and northern sections, calcite is dominant over aragonite and low-Mg over high-Mg calcite. That speciation is rather due to diagenetic transformations of the carbonate material. The fine grain size of the material, its location in a low-energy environment and the absence of low-Mg productive organisms (e.g., foraminifera) all support this assumption. In addition, carbonate speciation suggests a difference in sediment age; the eastern side consists

of younger sediment, while the western section is dominated by older, reworked and diagenetically transformed material. 5.5. Ooids The early Holocene age of the ooids suggests that their formation was triggered by the last marine transgression. The angularity of the quartz nuclei is

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consistent with a fluvial source for the grains. Considering also the dominant clockwise water movement in the Gulf, the ooids could have formed anywhere within the eastern section using fluvial quartz as nuclei, and then migrate southwards and become dispersed. The predominantly unidirectional water movement may in part explain why some ooids were found where they were not previously reported. Another explanation for this apparent inconsistent distribution could be that the ooids analysed in this investigation are much younger than previously thought. Until recently, it was considered that only freshwater cementation is rapid (e.g., Halley and Harris, 1979; Dravis, 1996). However, Friedman (1998) observed that rapid lithification is possible under marine conditions as well, which raises the possibility that some of the ooids may be hundreds or possibly tens of years in age.

layers of aragonite deposited around a nucleus of quartz of fluvial origin and may be much younger than previously thought. Although the Gulf is a young water body, the sedimentary history is complex due to a variety of processes. Sediment supply, biological production and current patterns are the main factors that control the sediment character and its regional distribution.

Acknowledgements Division of Fisheries Research, CSIRO Marine Laboratories, Cleveland, Queensland, are thanked for providing the samples and their locations. Sharyn Price performed the chemical analyses and Tony Raftery helped with XRD analyses and pattern conversion for assessing Mg calcite. Dr Greg Webb is thanked for numerous discussions and constructive ideas.

6. Conclusions References The surficial sediments of the Gulf of Carpentaria have a marked heterogenous character due to a complex evolution and changing depositional environments. The material consists of fresh as well as relict components. The main source of sediments is fluvial and therefore the terrigenous component is dominant (quartz-rich with minor feldspars and traces of siderite). Biologically productive areas supply the carbonate component of the sediment, which consists of material with variable degrees of alteration caused by transport and/or diagenesis. Those processes are partly reflected in the distribution of the three species of carbonates that have been identified in the sediments analysed: aragonite, low-Mg calcite (0 – 4 mol% Mg) and high-Mg calcite (10 – 14 mol% Mg). The eastern side is the main supplier of terrigenous and carbonate material (provinces E, F, G and H), which is then transported clockwise and dispersed throughout the Gulf. During this process, physicochemical and mineralogical changes take place; the sediments become finer grained and characterised by more stable species of carbonates as they reach the center (provinces A, B, C and D). Ooids have been identified in areas in which they were not previously reported such as the southeastern section. These carbonate grains consist of concentric

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