the florida state university college of arts and sciences evolution of ...

island. Another is a C-14 date of 2,110±130 years from a shell (Mulinia sp.) exposed 15 cm
above mean sea level on the surface of a beach ridge on the east side of the island on Tahiti
Beach near Mallard Slough (Stapor, 1973). Since this date is based on a single shell that may
have been reworked, it is suspect. The last is the historical record of the closure of Oyster Pond
(location shown in Figure 2.1) 200 years ago for the southern coast of the island, which is based
on historical charts (Stapor, pers. comm.). These ages provide some limited time constraint. The
oldest ages reported on the island are archaeological ages from the oldest ridge sets, comprising
artifacts in paleoindian middens dated at 3,000-3,500 years BP. Based on this previous work, all
ridge ages were expected to fall between 0 and about 4,000 years.
The OSL ages obtained during this investigation (Table 4.6) fall within the expected age
range based on the inferred progradation history of the island (Stapor 1973, 1975). Progradation
rates were calculated for the beach ridge plain based on the dated samples. The calculations and
the state of sea level associated with each calculated rate are shown in Table 4.8. The calculated
rates were then superimposed on the northern Gulf of Mexico sea level curve created by Balsillie
and Donoghue (2004) (Figure 4.42). Between 4,100 and 3,500 years, the progradation rate for
the St. Vincent Island beach ridge plain was 4.3±3.0 m/yr. This time period was one of sea level
fall. Sea level fell from an elevation of 1.0 m to –0.5 m. The progradation rate between 3,500 and
2,500 years was 0.7±0.3 m/yr. During this time interval, sea level was rising. Sea level rose
from an elevation of –0.5 m to 0.5 m. Sea level fell at 2,500 years and the progradation rate
increased to 2.6±0 m/yr. Between 2,500 and 1,200 years, sea level was variable, with both a rise
(from 0.5 to 1.4 m) and a fall (1.4 to –0.3 m) and a short period of sea level stability. At this
time, the island’s progradation rate was 1.0±0.2 m/yr. Sea level was falling between 800 and
400 years and the progradation rate increased to 2.0±0.5 m/yr. At this time sea level fell from an
elevation of 0 to –0.2 m). From 400 years to the present, sea level has remained relatively stable
and the progradation rate of the island has average approximately 1.9±0.5 m/yr. Based on Figure
4.42 and Table 4.8, these results appear to confirm that when sea level is stable or falling,
progradation rates are high. This trend could be related to the change in the nearshore
accommodation space available for sediment accumulation that is related to sea level change.
This could also be a result of an increase in sediment supply. When sea level falls, wave base is
lowered and it is possible to erode more sand from further offshore. Alternatively, when sea
level is rising, progradation are low. These lower rates could be related to an increase of energy
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