The Geomorphology and Sediments of Cockburn Sound
The Geomorphology and Sediments of Cockburn Sound
The Geomorphology and Sediments of Cockburn Sound
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<strong>The</strong> <strong>Geomorphology</strong> <strong>and</strong> <strong>Sediments</strong> <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong><br />
Principal Components Analysis <strong>of</strong> Sediment <strong>and</strong> Geochemical Data<br />
<strong>The</strong> PCA factor coordinates <strong>of</strong> the geochemical <strong>and</strong> sedimentological variables are shown in Table 2<br />
<strong>and</strong> the complete geochemical data are presented in Appendix II. <strong>The</strong> first principle component<br />
(PC1, Axis 1 <strong>of</strong> the PCA plot) explains 42.2% <strong>of</strong> variance in the data. Most trace <strong>and</strong> major elements<br />
<strong>and</strong> especially the Nd/Sr ratio have strong positive loadings on this axis <strong>and</strong> are inversely correlated<br />
to CaO <strong>and</strong> Sr <strong>and</strong> to a lesser extent the K 2 O/Al 2 O 3 ratio (Table 2). When the sample site scores for<br />
Axis 1 (Table 3) are plotted on the map <strong>of</strong> <strong>Cockburn</strong> <strong>Sound</strong> (Fig. 11a), the scores appear to define<br />
the source areas <strong>of</strong> trace metal contamination <strong>and</strong> to indicate likely sediment transport pathways<br />
from these sites. On this map, Kwinana Beach is a major location <strong>of</strong> elevated trace <strong>and</strong> major<br />
elements, with other sites on the southwestern margin <strong>of</strong> the Eastern Shoal near Woodman Point <strong>and</strong><br />
east <strong>of</strong> Three Fathom Bank (Fig. 11a). <strong>The</strong> isolated site <strong>of</strong> relatively high contamination northeast <strong>of</strong><br />
Three Fathom Bank (sample site CSG29) does not conform to this pattern <strong>and</strong> may be a spoil dump.<br />
A map <strong>of</strong> the Nd/Sr ratios confirms the pattern <strong>of</strong> distribution <strong>of</strong> fine terrestrial sediment (<strong>and</strong><br />
contamination) identified by PC1 (Fig. 12). This map clearly indicates a major source <strong>of</strong> fine<br />
terrestrial sediment at Kwinana Beach <strong>and</strong> on the Eastern Shoal <strong>and</strong> suggests this sediment is<br />
transported in an anticlockwise direction within <strong>Cockburn</strong> <strong>Sound</strong>.<br />
Axis 2 <strong>of</strong> the PCA explains 13.6 % <strong>of</strong> the variance in the data set. Sediment samples are<br />
differentiated along this axis on the basis <strong>of</strong> the grain size characteristics, from well-sorted s<strong>and</strong>s<br />
with strong positive loadings to poorly sorted mud with strong negative loadings (Table 2). <strong>The</strong>se<br />
sediment physical characteristics do not vary significantly on the first principle component axis<br />
(PC1), which reflects the fact that both s<strong>and</strong>y <strong>and</strong> muddy facies in <strong>Cockburn</strong> <strong>Sound</strong> can comprise<br />
marine-dominated sediment (bioclastic carbonate) <strong>and</strong> terrestrially-dominated sediment (quartz,<br />
feldspars <strong>and</strong> clay minerals). <strong>The</strong> map <strong>of</strong> site scores for Axis 2 (Fig. 11b) shows a good correlation<br />
with the map <strong>of</strong> sample grain size (Fig. 6) due to the strong influence <strong>of</strong> the proportion <strong>of</strong> s<strong>and</strong> <strong>and</strong><br />
degree <strong>of</strong> sorting on these scores. Sites with the lowest scores in the southern end <strong>of</strong> the sound<br />
reflect a higher proportion <strong>of</strong> poorly sorted muds <strong>and</strong> higher proportions <strong>of</strong> most elements. This<br />
trend is also born out in the moderate degree <strong>of</strong> negative correlation between terrestrial elements <strong>and</strong><br />
grain sorting (Table 4).<br />
A further 9.3% <strong>of</strong> the variance in the dataset is explained by the third principle component, PC3<br />
(Table 2). <strong>The</strong> SO 3 variable has the strongest (negative) loading on this axis <strong>and</strong> in association with<br />
the relatively strong negative loadings for Na 2 O 3 <strong>and</strong> Cl may indicate a relative lowering <strong>of</strong> seawater<br />
salt assemblage in these samples caused by the submarine discharge <strong>of</strong> comparably fresh<br />
groundwater. When the site scores for Axis 3 (Table 3) are plotted on the map <strong>of</strong> the sound (Fig.<br />
11c) there is a distinct zonation focused on the northeastern coast <strong>and</strong> extending westwards into the<br />
central basin. This pattern appears to indicate either the source submarine groundwater discharge or<br />
the source <strong>and</strong> pathway <strong>of</strong> distinctive <strong>and</strong> relatively clay-rich terrestrial sediment or a combination <strong>of</strong><br />
both influences.<br />
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