The Geomorphology and Sediments of Cockburn Sound

The Geomorphology and Sediments of Cockburn Sound
CSG45). Likewise, the Cu values in cores CSV03 (sample depth 0.44-0.45 m) and CSV09 (sample
depth: 0.3-0.31 m) were 394 and 231 respectively compared to nearby surface values of less than 30
ppm (surface samples: CSG13 and CSG29). These results suggest that sediment quality has
improved at sites CSV03, CSV06 and CSV09 in recent years. Interestingly, Nd/Sr ratios are elevated
both in the near surface sample of core CSV03 (sample depth 0.44-0.45 m) and in the nearby surface
sediment sample (CSG13; Nd/Sr ~ 0.0064). This suggests that there has been a sustained source of
terrestrial sediment at this site, despite the observed lowering of metal concentrations between the
surface and sub-surface. In core CSV06, the Nd/Sr ratio of the near surface sample (Nd/Sr = 1.28;
sample depth 0.1-0.11 m) is significantly larger than the nearby surface sediment samples (~0.003)
and suggests there may be a spoil dump at this site. The surface sediments near core CSV09 have
also been identified as an isolated site of relatively high contamination despite the low Nd/Sr ratio in
the near-surface sediments from the core (Nd/Sr = 0.0025; sample depth: 0.3-0.31 m).
Sediments within vibracores CSV04, CSV06 and CSV09 are relatively uniform with depth (Fig. 16
and Appendix III) and the metal concentrations (As, Cd, Cr, Cu, Ni, Pb and Zn) do not vary
significantly with depth (Fig. 19). Interestingly, concentrations of As, Cd, Cr, Ni, Pb and Zn in these
cores are lower than the background concentrations suggested by Talbot and Chegwidden (1983)
based on surface sediment samples collected from across the sound. The background Cu
concentration observed in the vibracores is greater than the anomalous values reported by Talbot and
Chegwidden (1983) but less than the ISQG low trigger value that is based on overseas biological
effects data. On the basis of these results, we suggest that new background values for trace metals in
Cockburn Sound should be adopted (Table 5).
The geochemical composition of cores CSV05 and CSV07 also appears to be strongly influenced by
iron and sulfate reduction, resulting in iron sulfide formation at depth (see Fig 20A-E). The sulfur
concentration increases at a depth of 1.7 m in core CSV05, and below 1.8 m in core CSV07. At these
depths, Fe also increases, and as a result the molar Fe:S ratio remains between 1 and 2 which is
typical of iron sulfides. Further evidence of iron reduction is seen in the Fe:P ratio. The phosphorus
concentration also increases at 1.7 m in core CSV05 and below 1.8 m in core CSV07. Since P is
typically bound to iron oxides, iron reduction causes P to be lost from the sediments and while there
is an overall increase in the P concentrations at these depths, the molar Fe:P ratio actually increases.
At a depth of approximately 1.9 m in core CSV05, the S and P concentrations decrease significantly
but the Fe concentration increases causing an overall increase in the molar Fe:S and Fe:P ratios.
These distinct changes in CSV05 below 1.9 m are related to the distinct change in sediment facies
(Fig. 16 and Appendix III). CSV05 consists of carbonate muddy sands to approximately 1.7 m.
Below this depth there is a 0.1 m layer of dark grey, reduced shelly sandy mud which is where iron
sulfides have been identified from the geochemical data. The lower part of the core consists of
terrestrial sediments, mostly mottled sandy clay, and this is reflected in the marked changes in the
sediment geochemistry.
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