Vol.1 part 4-5 - Department of Invertebrate Zoology

Following the approach of Trefry and Presley
in the present study, leachable trace metal concentrations
were compared to that of iron in the sediments surrounding
the platforms . Appendix C1 contains computer-generated
scatter plots of metal concentrations
against Fe concentration using all samples collected during
Cruises I and II . Regression lines, regression equations,
and correlation coefficients (r) for each cruise
have been included . An r value greater than 0.2144 is
significant at p = 0 .05 . These data (metal concentrations
normalized to Fe) are summarized by platform and station
in Appendix C2 .
The plots of trace metals against iron can be
used to determine if the data from sediments surrounding
a production platform follow a trend consistent with
the data collected on a more regional basis (Trefry and
Presley, 1976a ; Shokes, 1978) . Outliers of any trends
that might be observed would clearly suggest pollutant
concentration . One P4 sample from Cruise I had unusually
high Fe concentration and is not included in these
scatter plots . It probably represents contamination from
iron imputs such as metal debris in the sediments or
shipboard contaminants . Iron releases from the platforms
obviously could invalidate the comparison of present
data with others using the approach just described .
It is unlikely, however, that the rates of releases of iron
and other trace metals from the platforms are constant
and in the same proportions as their consolidation in regional
sediments . Therefore, the approach seems reasonable
.
For those scatter plots for which the information
is available, regression lines, equations and r or rz
values from Shokes (1978), Tillery (19806), and Trefry
and Presley (1976a) have been included for comparative
purposes . Although different digestion solutions were
used in each of these studies, the results should be comparable
. The stronger digestion solutions would only
shift the data points further to the right along the regression
line .
(1) Barium/Iron Ratios
Figure 1 in Appendix C1 is the scatter
plot of Ba against Fe . Ba is an alkaline earth metal, and
of the metals studied is geochemically the least similar to
iron . It is not surprising, therefore, that the correlation
between Ba and Fe is poor for both cruises . Natural barium
in sediments (i .e ., not from anthropogenic inputs)
is probably better correlated with the CaC03 fraction of
the sediments . Calcareous sediments usually contain
higher concentrations of Ba because of its chemical similarity
to Ca . However, the correlation of the Ba concentration
to the CaC03 content (%) of the sediments
was rather weak (r = 0 .372, significant at p