The Geomorphology and Sediments of Cockburn Sound

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The Geomorphology and Sediments of Cockburn Sound Sub-Surface Geochemical Characteristics The comparison of surface and down-core geochemical data suggests that trace metal concentrations of modern sediment are much higher than pre-industrial levels. However, sediment quality appears to have improved at several sites where the sub-surface metal concentrations are higher than at the surface. This suggests that in the past metal contamination of the surface sediments was greater and these sediments have now been buried by sediment that is significantly less contaminated. The improved conditions at some sites are likely to be due to a lowering of industrial pressure, successful pollution reduction strategies and a change in the contaminant transport path. Since 1955 liquid waste products have been discharged into Cockburn Sound from the heavy industrial area on the eastern foreshore of the sound. The first comprehensive study of Cockburn Sound (1976-1979) identified a large variety of contaminants in industrial discharges to the sound (Department of Conservation and Environment, 1979). The Southern Metropolitan Coastal Waters Study (1991- 1994) found that contaminant levels had decreased significantly since the late 1970s, due to large reductions in wastewater discharges from industry (Department of Environment Protection, 1996). This study also found that contaminated groundwater had replaced direct industrial pipeline discharge as the main nutrient input to the sound, which came mainly from the southern part of the Kwinana Industrial Area. Estimated amounts of metals and oil discharged by industry have continued to decrease due to improved waste treatment practices, and are presently about one sixth to one thousandth of those discharged in 1978 (DAL, 2001). In some cases, however, elevated subsurface metal concentrations may indicate the impact of a point source of pollution such as an old spoil dump. Previously reported background metal concentrations for Cockburn Sound are consistently higher than pre-industrial background values observed at depth and reflect low levels of contamination themselves. The down-core geochemical data provides information on pre-industrial metal concentrations in sediments of the same type as modern surface sediment. Therefore, it appears that when compared to these new pre-industrial levels, the level of metal contamination in the surface sediments is greater than previously reported. GEOLOGICAL EVOLUTION OF COCKBURN SOUND Based on the geomorphic, stratigraphic and sedimentological data presented above, Cockburn Sound has evolved into its present form in the following sequence: 1) With the rise of sea level in the early to middle Holocene the inner Rottnest shelf was flooded and the Cockburn-Warnbro Depression, between the Garden Island and Spearwood Ridges, was inundated. This low lying area is part of a large swale that formed between coastal dune ridges that were emplaced in the Late Pleistocene. The depression had a clay soil cover and possibly some karst landforms. 2) As the depression was filled by the rising sea to form the proto-Cockburn Sound there was some reworking of relict quartz-rich sand. These sediments accumulated on the western margin of the sound as muddy quartz sand and along the eastern shore as shelly carbonate/quartz sand (Safety Bay Sands). 3) Since being flooded, the northern, northeastern, western and southern margins of the sound have rapidly shoaled as large volumes of carbonate muddy sand and sand have been deposited in seagrass meadows (the Becher Sand). At the same time the central basin has been partially filled with carbonate sandy mud and mud (Bridport Calcilutite), a large proportion of which has been winnowed from the surrounding seagrass banks. 4) The fine fraction of surface sediments currently accumulating in much of Cockburn Sound has a terrestrial trace element signature and elevated levels of several trace metals. The spatial pattern of these data suggests major inputs of this sediment from the eastern coast, particularly at Kwinana Beach. 45
The Geomorphology and Sediments of Cockburn Sound Conclusions Cockburn Sound is a large, low-energy coastal waterway located on a moderate to high-energy carbonate coast. It has formed in an elongate depression that sits in the lee of a cemented Pleistocene shore-parallel dune ridge. This study provides the following new insights into this coastal depositional environment: 1) Surface sediments in the sound are dominantly biogenic carbonates, with sandy mud and mud in the relatively deep (15–20 m) central basin. The marginal banks (2–10 m) comprise carbonate sand and there is mixed carbonate and quartz sand in the eastern nearshore zone. Maps of the distinctive surface sediment types can be used to ground-truth acoustic backscatter datasets and help explain textural patterns in the acoustic data. 2) Surface sediment Nd/Sr ratios and PCA results indicate that there is a higher proportion of fine terrestrial sediment on the eastern side of the sound and this sediment fraction contains elevated levels of some trace metals. The PCA results and the presence of water-filled cavities within the Tamala Limestone that underlies the central basin sediments suggest there may be submarine discharge of groundwater in a zone that extends from the northeastern coast westwards into the central basin. 3) Maps of surface sediment trace metal concentrations need to be compared with maps of benthic habitat loss, such as seagrass, to indicate whether there is a link between the decline in benthic habitat area and sediment contamination. Cu and Ni levels are higher and exceed trigger values near Kwinana Beach (Ni), Three Fathom Bank (Cu, Ni), northern central basin (Ni), James Point (Cu) and Buchanan Bay (Cu). 4) The most significant geochemical variations, both down-core and between cores, are a result of the dominance of either marine or terrestrial sediments which have unique geochemical signatures. 5) Previously reported background metal concentrations for Cockburn Sound are consistently higher than pre-industrial background values observed at depth and surface metal contamination may be greater than previously reported. However, there is evidence for improved conditions at some sites, mainly due to large reductions in wastewater discharges from industry. 6) The stratigraphy of Cockburn Sound reveals that much of the clay soil that formed on the original calcarenite land surface prior to the Holocene rise in sea level is preserved below the marine carbonate mud that has been deposited in the central basin. The central basin has only partially infilled and there is considerable accommodation space for the continued accumulation of muddy sediment. Up to four distinct lithostratigraphic units were found to comprise the sediment fill and the spatial distribution of these deposits will be better defined in future sub-bottom profiling of the sound. 46
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The Geomorphology and Sediments of Cockburn Sound

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