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paleo-hydrology of southern florida?
Paleo-hydrology of Southern Florida?
| The radionuclide distribution in core
Core 97-6-6-46 is typical for the peats of south Florida. The low activity
of Pb-210 in the top 8 cm is the result of increased sedimentation and
a dilution of atmospherically derived Pb-210. (see insert)
The coincidence between the Pb-210 apparent peak and the Cs-137 peak suggests
an date of 1964 for this level. Below this level, the Pb-210
activity decreases exponentially until it is in equilibrium with Ra-226
at about 20 cm. Cs-137 decreases to non-detectable levels below 20
cm. Between 20 and 45 cm, both the lead and radium remain constant. This
lack of any change in lead and radium, and the lack of Cs-137 is evidence
of no diagenetic change or migration occurs within these sediments.
At the bottom of the core, both isotopes increases in activity, with Pb-210
increasing more rapidly. As a results there is disequilibrium at
the sediment/rock interface of about 12 dpm/g. This gives a calculated
disequilibrium age of 1970. The calculated age of the site in Seven
Palm Lake (Pb-210 excess of 6.2dpm/g) is 1955. Again the presence of Cs-137
is consistent with this date. |
(Click on the images below
for a larger version.)
Distribution of nuclides at the rock/sediment
interface
| Short-lived radioisotopes concentrations
in the sediments of the southern Everglades and northern Florida Bay have
elevated activities of Pb-210 at the rock/sediment interface (Figure 3a).
The spatial distribution of these isotopes is correlated with the composition
of the underlying water. Pb-210 activity is highest in sediment overlying
the freshwater zone, as defined by apparent resistivity. Radiochemical
analysis shows that the high Pb-210 activity is far out of equilibrium
with its parent isotopes, radium and uranium. However, in Florida
Bay (Whipray Basin sites), Pb-210 in basal sediments is in equilibrium
with radium and uranium. In the transition areas between the bay
and the Everglades, "the Mangrove Fringe", the Pb-210 disequilibrium lies
between these two extremes. Based on the fact that a disequilibrium exists
in the transition zone, it is hypothesized that the process(es) that created
this disequilibrium ceased within the last 150 years. Further examination
of the process leading to the disequilibrium may lead to a development
of a historical model detailing the paleo-hydrology of southern Florida. |
Excess 210Pb activity at the
rock - sediment interface
(The yellow background indicates
the presence of Cs-137)
Activity ratio of 210Pb
/ 226Ra
Estimated age of last fresh-water contact
Hypothesis
| The carbonate strata that dominates the
substrate of south Florida, is enriched in uranium. The decay of
uranium produces a significant amount of radon, which migrates upward.
During this migration a portion of this material decays to Pb-210, which
is adsorbed on the organic matrix of the overlying peats. It is apparent
that the highest radon flux is in the zone occupied by fresh water. It
is hypothesized, under the influence of the hydraulic head, fresh water
migrates southward overriding the salt water encroachment and it is the
chemical reactions within the fresh water zone that enhances this flux.
With the retreat of fresh water northward due to withdrawal, the zone of
high Pb-210 also migrates northward. What remains is a peat with
excess Pb-210, whose activity is time dependent. The process(es)
that leads to this condition is unknown at this time, but the hypothesis
is easy testable by examining the composition of the ground water along
these interfaces between the fresh and salt waters. |
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Related information:
SOFIA Project: Geochronology in the South Florida Ecosystem and Associated Ecosystem Programs
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