Geology Reference
In-Depth Information
environments in southwest Florida (Gelsanliter,
1996) and initiated a 400-year period of recycling
of unstable sediment bodies. This transgressive,
recycling phase triggered rapid resedimentation,
as sediment bodies were reworked and sediment
was transferred to more stable sites. A vast, sub-
tidal to supratidal carbonate fl at, extending for
more than 100 km along the southwest coast,
was deposited. This mud fl at succession is up to
2 m thick and 8 km wide. Coastal mud deposition
choked and blocked the palaeodrainage channels
of the Everglades outfl ow, which had been through
the Lake Ingraham area of Cape Sable (Jackson &
Wanless, 2004). The emergent carbonate marl
ridges that exist today behind the present shore-
line of Lake Ingraham and along the north shore of
Florida Bay were built in response to this 2500 yr
BP
rapid sea-level rise (Gelsanliter, 1996).
More recently, rapid RSL rise (IPCC, 2001) is
again changing the rates and patterns of coastal
response. The present rise is seven times faster
than the average rate of rise during the previous
2400 years, during which littoral and shallow
marine environments shallowed and prograded.
The coastal and shallow marine environments in
southwest Florida are actively responding to the
accelerated rate of rise in sea level, eroding in
some areas and rapidly accreting in others. Cape
Sable, with several natural and anthropogenic
triggering events in the past century, illustrates
the nearly instantaneous response that can occur
in a sediment-rich coastal system.
(a)
0
3
6 kilometres
(b)
−
21.5
Station 3 EBB
Station 1 + 4
EBB
−
22.0
−
22.5
−
23.0
−
23.5
Station 1 + 4
FLOOD
Station 3 FLOOD
Station 1
Station 3
Station 4
−
24.0
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
0:00
Time (h)
Fig. 13.
(a) Stable carbon isotopic (
13
C) values around
Cape Sable from the top 5 cm of sediment cores 37 (
δ
−
26.1‰),
51 (-24.4‰) and 68 (-27.5‰) and
13
C
POC value of
Florida Bay suspended sediment sample (-21.4‰); (b)
δ
Rapid recycling via complex sediment
transport pathways
13
C
POC values for water samples at Stations 1 (ECC South),
3 (HSC) and 4 (ECC West), measured during one tidal cycle
on 16 January 2005 (see Fig. 4 for station locations). Notice
2 hour delay at Station 3 (compared with Station 1
δ
The data presented in this paper suggest that sedi-
ment is recycled and stored within the Cape Sable
coastal system. The resulting shallowing-upwards
sediment package in Lake Ingraham and the
Southern Lakes contains high percentages of TOM
(15-35%) and accumulates with an average rate of
6.2 cm yr
−1
4)
for low and high water (arrows indicate end of ebb and
fl ood). In general, the ebb tide carries isotopically lighter
suspended sediment than a fl ood tide. Station 1 displays
largest isotopic variability (
+
Δ
2‰), Station 4 displays least
isotopic variability (
Δ
1‰) throughout tidal cycle.
2 cm. In comparison, typical carbon-
ate muds in the coastal bays of south Florida do not
exceed accumulation rates of 2 cm yr
−1
(Bosence
et al
., 1985; Wanless & Tagett, 1989; Holmes
et al
., 2001; Strasser & Samankassou, 2003) and
contain 2-10% TOM (Lutz, 1997; Wanless
et al
.,
2005). The high organic content and accumula-
tion rate, acting in conjunction with high erosion
rates observed both in the marine and transitional
freshwater environments (Wanless & Vlaswinkel,
2005), demonstrate the major recycling phase this
coast is undergoing.
±
(DePratter & Howard, 1981; Dominguez & Wanless,
1991) has also been recognized in Florida
(Gelsanliter, 1996) and has dramatically infl u-
enced coastal stratigraphy and sedimentation.
A global cooling starting around 3200 yr
BP
and
peaking at 2800 yr
BP
caused relative sea-level
in south Florida to stabilize at
1.8 m and possi-
bly fall (Wanless
et al
., 1994). A small, quick rise
of RSL (< 1 m) followed between 2500 and
2400 yr
BP
and raised sea level up to about
−
1.2 m
(Wanless
et al
., 1994). This rise destabilized coastal
−
