Lake Pontchartrain
Basin: Bottom Sediments and Related Environmental Resources |
Summary of Geological and Chemical Data (cont.)
Organic Carbon
The distribution of organic carbon concentrations in the sediments is
shown in the histograms of figure 15 and in the scaled-dot diagram below (fig. 16).
In figure 15, the 1987 data of Flowers (summary
reported in Flowers and Isphording, 1990), show a smaller distribution
of organic carbon than do surface sediment data of Sikora and Sikora
(1982). The apparent discrepancy may be explained partly by the fact
that the Flowers and Isphording studies incorporated nearshore data,
whereas Sikora and Sikora studied only dredged areas in the central
lake. The data sets of Demcheck and others (1996) and Macauley and
Summers (1998) for Lake Pontchartrain yielded 1.33% and 1.07%
organic carbon, respectively. Taken as a whole, one can estimate the
median organic carbon for the total lake sediments at about 1%. The
hypothesis that the authors' data sets form separate populations is
rejected at the 95% probability level. |
Figure 15. Histogram
distribution of organic carbon concentrations
see appendix B for more source information |
Figure 16.
Organic Carbon
(click for full view)
|
The distribution of organic carbon shown in figure 16
is erratic, as seen by the size of the dots. The reasons for this
variability are not clear. Some of the lowest values occur in the
deeper parts of Lake Pontchartrain, which are characterized by fine,
clayey sediments. The variability is no doubt partly affected by the
disturbance and artificial mixing of sediments by shell dredging in
Lake Pontchartrain before 1990. We note the relative uniformity and
high organic carbon content of Lake Maurepas sediments compared to
those in Lake Pontchartrain. The higher content of organic matter in
Maurepas can be attributed to rich sources of organic matter in the
swampy, organic-rich environments surrounding this water body. |
A large, earlier
study of Lake Pontchartrain sediments, the raw data from which have not been found to
date, is briefly summarized by Steinmayer (1939). These data reported "organic
matter" concentrations up to a mean of 6.7% in the clays. However, these values were
derived from ignition loss determinations rather than from actual organic
carbon measurements.
As pointed out by Manheim and others (1997), such data include CaCO3 and bound
water loss during heating of the clays.
Given the organic-rich, swampy deposits surrounding Lakes
Pontchartrain and Maurepas, it may seem surprising that the sediments do not contain more
organic carbon. This condition may be attributable to the relatively warm waters that help
ensure the breakdown of fresher planktonic organic matter into refractory organic
compounds that make up only a small fraction of the original material. Respiration and
consumption by bottom organisms is very efficient. Normal particulate
matter in the water column of Lakes Maurepas and Pontchartrain can be shown
to have a mean organic matter of more than 40% when not stirred off the
bottom by storms or high winds or brought in by Mississippi River influxes
(see the section on particulate matter and
sediments in the Discussion). The low
concentrations of organic carbon in bottom sediments (1-1.5%) demonstrate
clearly that organic matter in particulates settles to the bottom. The
decomposition to highly refractory materials is known to be particularly
aggressive in warm estuarine or marine waters.
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