Rapid recycling of organic-rich carbonates during transgression: a complex coastal system in southwest Florida - Perspectives in Carbonate Geology - page 93

coast of Florida had undergone signifi cant
fl ooding over the past 5000 years. In this area,
a transgressive facies succession is present, and
consists of basal freshwater peat, overlain by tran-
sitional mangrove peat and marine carbonate mud
(Spackman et al ., 1964; Scholl & Stuiver, 1967).
Scholl et al . (1969) generated the fi rst Holocene
sea-level curve for south Florida with an infl ec-
tion point between 3500 and 3200 yr BP . An initial
rapid rise, which averaged about 26 cm 100 yr −1 ,
was followed by a much slower rise of about
4 cm 100 yr −1 . Parkinson's (1987) subsurface
work in the coastal zone of southwest Florida
focused on evaluating the sedimentological suc-
cession deposited as a result of this changing
rate of sea-level rise. He recognized a 6-m-thick
Holocene package in the Ten Thousand Islands
region, in which he defi ned a lower transgres-
sive and upper regressive sediment package,
and interpreted this shift to RSL rise. The lower
sediment succession refl ects shoreline retreat
and subsequent accumulation of subtidal sedi-
ments. The upper sequence consists of biogenic
shallowing-upwards buildups and coastal man-
grove peats thicker than the present tidal range.
As the rate of rise slowed, the rate of biological
sediment production and accumulation began
to outpace the rate of sea-level rise, initiating
shoreline stabilization and island emergence.
More recent studies along the southwest coast
of Florida (Frederick, 1994; Gelsanliter, 1996),
northern shore of Florida Bay (Huang, 1990) and
within Florida Bay (Cottrell, 1989) have all docu-
mented a regressive package as a direct result of
a declining rate of RSL rise.
Roberts et al . (1977) improved the under-
standing of the regional transgressive-regressive
succession, identifying the major sedimentary
units in a transect across Cape Sable and Lake
Ingraham (Fig. 2). Sediment thickness of car-
bonate mud on the seaward portion of the tidal
plain is 3.5-4.0 m. The carbonate facies switch
landward to peat, which is underlain in some
places by freshwater calcitic mud above bedrock.
Roberts et al . (1977) also dated the late Holocene
progradation of the shoreline and the three sandy
capes after which Cape Sable is named. The old-
est (most inland) ridge is dated at 2280

Roberts et al . (1977), and extensively by Gebelein
(1977). Gebelein's work focused entirely on the
surface geology, as he analysed the sedimento-
logical and biological processes and patterns in
the subtidal, intertidal and supratidal carbonate
sediments in and around Lake Ingraham.

METHODS AND MATERIALS

Sediment sampling

Thirty sediment cores, ranging in length from
80 to 150 cm, were collected within Lake Ingraham,
the Southern Lakes and the interior marsh to docu-
ment the stratigraphic succession, including its
texture, composition and sedimentary structures.
Sediments are classifi ed using Dunham's (1962)
carbonate textural terminology for rocks.
Between May 2004 and January 2005, 34 sedi-
ment reference markers were deployed on the
intertidal mudfl ats of Lake Ingraham and the
Southern Lakes to measure in situ sedimentation
rates. A series of four, 40 40 cm wide carpet tiles

were pinned down on the sediment bed during low
tide, acting as artifi cial marker horizons. One car-
pet tile from each location was removed after one,
two, four and six months (Fig. 3). The sediment
weight on top of the carpet was converted to verti-
cal sedimentation rate using bulk densities of the
wet mud. Precision of the weight measurements
is

×

50 g.
Several 12-hour experiments were carried out
throughout ECC to determine the spatial vari-
ability of suspended sediment concentration in
the water column: water samples (250 mL) were
collected 1 m below the water level each hour at
Stations 1, 3 and 4. Earlier experiments, in which
sediment concentrations were measured at 50,
100 and 150 cm above the sediment bed, dis-
played a homogeneous vertical turbidity profi le.
The carbon isotopic value of suspended sediment
was measured simultaneously at the same three
stations throughout one full tidal cycle in January
2005. Stable carbon isotopes (

±

13 C) are widely
used to differentiate organic matter sources in
estuarine sediments, because there is a signifi cant
difference in the 13 C content of the primary pro-
ducers in question (Zieman et al ., 1984; Kennicutt
et al ., 1987; Fleming et al ., 1990; Chmura &
Aharon, 1995). Suspended sediment samples and
several sediment reference marker samples were
analysed with an ANCA GSL mass spectrome-
ter to determine the isotopic composition of the

δ

100 yr BP
( 14 C years) (A in Fig. 1) and the youngest marl
ridge at 2000

±

80 yr BP (B in Fig. 1). Each one
of these ridges was interpreted by Roberts et al .
(1977) to be an ancient shoreline.
The major depositional environments in the
Cape Sable region were briefl y described by

±