The Geomorphology and Sediments of Cockburn Sound

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