Geology Reference
In-Depth Information
calcareous algae such as
Penicillus
, formed in
Florida Bay (Stockman
et al
., 1967; Macintyre &
Reid, 1992). Gebelein (1977) also suggests some
negligible
in situ
production of carbonate skeletal
mud by
Penicillus
and
Udotea
in Lake Ingraham.
The second peak, around 6 μm, consists mainly of
carbonate aggregates. Sediment with size fraction
around 50 μm is identifi ed mainly as skeletal
fragments, organic particles, diatoms and foramin-
ifera. The local 'carbonate factory' of Florida Bay
provides several different sizes of carbonate, both
calcite and aragonite, that are produced not only by
calcifying green algae, but also by cyanobacteria,
benthic foraminifera and epibionts on seagrasses
such as
Thalassia
(Ginsburg, 1956; Bathurst, 1975;
Nelsen & Ginsburg, 1986; Bosence, 1995). Besides
these local sources, the shallow shelf to the west
of Florida Bay is also considered as a signifi cant
source of sediment input (Roberts
et al
., 1977).
The organic matter in the sediment samples is
composed of varying mixtures of mangrove peat
detritus, fresh-brackish water marsh peat detri-
tus, algal and cyanobacterial material, marine
plankton and diatoms (SEM observations). The
decay of the former freshwater marsh environ-
ment releases remnants of sedge organics, part
of which has been recycled to microbial organics
(cyanobacteria and fungi) and dissolved organics
(Gunderson, 1994). Likewise, mangroves release
remnants of root-peat organics and decomposed
leaves, as well as microbial organics and dissolved
organics (Twilley, 1985). The mangrove organics
are interpreted to be provided by both shore ero-
sion of mangrove peats and post-hurricane decay
of peats beneath collapsed mangrove forests in
the interior of Cape Sable. The algal or cyanobac-
terial organics are provided to the sediment pool
by algal/bacterial mats growing on decaying and
recycled peat deposits, and from recycling the
organic portion of older carbonate/organic muds
and marls.
(a)
(b)
Fig. 8.
(a) Average sedimentation rates derived from sedi-
ment reference markers. Highest rates are on mid-channel
banks in the entrance of side creeks connecting to East
Cape Canal. Within Lake Ingraham, the lower parts of the
delta are accumulating most rapidly (5-7 cm yr
−1
), whereas
the oldest (and presently highest) parts of the delta are
accreting at 3-4 cm yr
−1
; (b) Total organic matter (TOM)
in weight percentage derived from sediment reference
markers and selected cores. On the intertidal mudfl ats
TOM values range from 15 to 35 wt%. Values in the inte-
rior marsh range from 31 wt% at 10 cm depth (marker 39)
to 85 wt% at surface (marker 37) (Image: Ikonos-2005 @
GeoEye).
Sediment dynamic processes
An integrated hydrodynamic and geochemical
approach links the observed sedimentation pat-
terns and composition to process dynamics. In the
area of interest, ECC is the main artery connecting
Florida Bay to Lake Ingraham and the Southern
Lakes. Water level and current observations were
carried out at different locations throughout the
canal (Fig. 4). At neap tide, maximum fl ood and
ebb (depth-average) velocities are approximately
70 cm s
−1
; at spring tide, maximum fl ood velocities
size is less than 22 μm and the maximum grain
size is 100 μm, and with velocities of ~0.5 m s
−1
all sediment is transported as suspended load.
Scanning electron microscopy (SEM) observa-
tions illustrate that the smallest peak around
0.8 μm refl ects organic matter and aragonite
needles. The needles, blunt-ended prisms,
are believed to originate mainly from green
