Natural and anthropogenic influences upon trace metal geochemistry in sediments of Cleveland Bay and adjacent areas of the Great Barrier Reef Lagoon

Doherty, Gregory Brian (2001) Natural and anthropogenic influences upon trace metal geochemistry in sediments of Cleveland Bay and adjacent areas of the Great Barrier Reef Lagoon. PhD thesis, James Cook University.

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Processes that influence variations in trace metal concentration and their bioavailability in surface and subsurface sediments have been measured in sediments from Cleveland Bay and the adjacent shelf areas of the Great Barrier Reef lagoon from Upstart Bay to Rockingham Bay. The scope of work is consistent with strategies incorporated within the National Water Quality Management Strategy (e.g., the draft Australian and New Zealand Environment and Conservation Council (ANZECC) Guidelines for Fresh and Marine Water Quality). The results can be used as a guide for the management of sediment quality in the study region.

Statistical analysis (Principal Component Analysis, Multiple Regression Analysis) has been completed upon chemical and physical parameters from 1023 surface sediment samples collected in the central section of the Great Barrier Reef Lagoon. In conjunction with spatial analysis the results of statistical analysis have been used to identify influences upon the distribution of whole surface sediment strong acid soluble Cd, Co, Cu, Ni, Pb and Znoncentrations [that is, Zn concentrations]. The predominant influence upon their distribution is the distribution of fine-grained aluminosilicate rich terrigenous sediment. Subordinant natural influences include; changes in the concentration of Co, Cu and Ni in terrigenous sediment being supplied to the lagoon; increases in Cd and Co concentration in sediments due to post depositional early diagenetic processes; and nutrient like behaviour of Cd.

In oder [that is, order] to evaluate anthropogenic influences upon trace metal concentration In Cleveland Bay, statistical analysis (Multiple Regression Analysis, Principal Component Analysis, Linear Discriminant Analysis) and spatial analysis of 122 surface sediment samples and 124 sediment core samples have been supported by temporal analysis of samples from 5 sediment cores using 210Pb radiochemistry. Anthropogenic influences in sediments of Cleveland Bay are distinguishable from natural variations in concentration at sites that have high rates of fine grained terrigenous sediment accumulation, are typically close to source, are limited in spatial extent, and are characterised by simultaneous increases of more than one trace metal.

Temporal trends in enhancement of metals exhibit associations with economic growth and the introduction of specific metal commodities to the region. Enhancement of metal concentrations above a statistically modelled background are generally less than 2 times the modelled background, and are typically less than the draft ANZECC guideline trigger values for the concentration of metals in sediment. The results indicate that identification of anthropogenic influences do not necessarily represent a decline in environmental quality but are diagnostic of urbanisation and industrial development.

To assist understanding processes that influence the bioavailability of trace metals in sediments a mass balance of dissolved and solid phase sulfur concentrations of 5 sediment cores collected from Cleveland Bay was completed. A model of the rate of organic matter decomposition explains between 82 to 90% of the variance observed in the sulfur accumulated in sediments of cores from Cleveland Bay. Therefore the rate of organic matter decomposition is the dominant influence upon sulfur accumulation within individual sediment cores. The mass balance indicates that typical muddy sediments in the intertidal zone are open to the resupply of sulphate, but probably have a high retention rate of authigenic sulphides due to abundant reactive Fe and high burial rates.

Speciation of trace metals in sediment profiles has been evaluated by two operationally defined methods. 1) Speciation determined by wet chemical analyses of metals. 2) Speciation determined by in situ passive sampling devices that use the theory of Diffusive Gradients in Thin-films (DGT). Interpretation of the influences upon metal speciation was found to be highly dependent upon the scale of sampling (e.g., vertical resolution, depth), and method of analysis.

In sediment cores sampled at 20mm intervals, the partitioning of trace metals into 1M HCI soluble (Cd, Pb and Zn) and 1M HCI insoluble (Co, Cu and Ni) species can be attributed to the predominant early diagenetic process occurring in sediments, S0₄ reduction. However, in 300mm deep DGT probes sampled at intervals of 10mm strong vertical zonation of metals with apparent contrasting lability to that deterined [that is, determined] by wet chemical analysis. Lability in DGT probes can be described as DGT available (Co, Ni, Pb and Zn) and DGT unavailable (Cd and Cu) metals. These patterns in speciation are interpreted to be created by interaction of metals with other early diageneitc processes including Fe, Mn and S0₄ reduction.

The comparison of operationally defined speciation methods indicates that the use of a single speciation method (e.g., 1M HCI soluble metals or ΣSEM/AVS > 1) to predict the bioavailability of Cd, Co, Cu, Ni, Pb and Zn may not adequately describe the speciation of some metals in near surface sediments. As a consequence methods used to assess metal bioavailability in sediments should not be used in isolation, or applied as generic indicators of sediment quality. The methods are tools that can be used for the assessment of sediment quality, and require to be evaluated before being integrated within sediment quality management strategies.

Item ID: 28557
Item Type: Thesis (PhD)
Keywords: anthropogenic effects; anthropogenic influences; Cleveland Bay; GBR; geochemistry; human influences; Great Barrier Reef lagoon; marine sediments; metal concentrations; natural influences; Rockingham Bay; sediment quality; trace metals; Upstart Bay
Date Deposited: 13 Aug 2013 00:04
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040305 Marine Geoscience @ 50%
04 EARTH SCIENCES > 0402 Geochemistry > 040202 Inorganic Geochemistry @ 50%
SEO Codes: 96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961104 Physical and Chemical Conditions of Water in Marine Environments @ 70%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 30%
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