Diverse Organic Sediments and Stratigraphy in Large Tree-Islands of Shark River
Slough, Everglades National Park
Peter Stone
SC Dept. Health and Environmental Control, Columbia SC
Michael Ross, Pablo Ruiz, and David Reed
Florida International University, Miami FL
The tree-islands (“hammocks”) of Shark River Slough in the southwestern
Everglades form one of the main assemblages of tree-islands in the regional
peatland. There they are seemingly the least disturbed among the large mineralmound-focused major type of tree-island in the peatland. Their importance as
emerged to partially emerged sites in adding vegetational and habitat diversity in
the encompassing marshland is obvious; additionally they have had a notable
archeological and historical role. Important in their own right in the national
park, they additionally likely represent lost landscape features of the eastern
middle Everglades (CA2B southward) where mounds of sand or limestone also
focused the upstream ends of large elongated tree-islands in wide marshes
underlain by peat up to a meter or so thick. Related tree-islands lie to the north in
CA3.
The numerous prominent large tree-islands are elongated roughly N–S, clearly
aligned with the direction of sluggish overland flow in the marshes. The exact
mechanism for this elongation remains unknown, but has been much speculated.
Most of the large elongated (“teardrop” shaped) tree-islands here are twocomponent, with a rounded stand or “head” of more-tropical evergreen trees (true
hammock) on the northern end and a long downflow (downstream) “tail” of
bayhead forest. The hammock lies above a slight mound in the underlying
limestone bedrock and is further raised above flooding by a cap of organic-rich
(to peaty) granular sediment. One hammock had an intervening distinct layer of
granular carbonate-rich marine sediment of pre-Everglades origin (perhaps a
residuum of limestone erosion or perhaps a younger stratum). The bayhead tail
lies upon a slight ridge of peat (and muck) and is seemingly unrelated to the
topography of the directly underlying limestone, but clearly related somehow to
the low mound just upflow. The tail floods in highwater season, though less so
than the surrounding marshes. A dense sawgrass marsh can extend this distinct
elongation still farther downstream (S) as well as slightly laterally of the tail (E–
W) by contrast with the sparser or lower sawgrass or wet-prairie vegetation of the
surrounding marshes. Certain large tree-islands lack the hammock head and by
their uniform bayhead stand resemble the large tree-islands on deeper peats of the
northeastern Everglades. Lacking the mineral mound, their origin and reasons for
their location are even more obscure. The rock-mound focus of the more typical
tree-islands existed before, during, and after the development of the early
Everglades rock and marl marshes and then the peat marshes that now surround
the tree-islands. The unfocused tree-islands and the typical tails of all tree-islands
seem to have formed only later, during the peatland era.
Sediment cores in the head and tail of representative tree-islands, plus a few sites
at the periphery and in nearby marshes, reveal an assortment of sediments (some
unanticipated) in a fairly complex stratigraphy. Peat or peat-dominated sediment
basically prevails (as expected) in the bayhead tails, while peat and a granular
forest humus predominates in the hammock heads. But muck—being finegrained organic- and mineral-rich sediment—is extremely common. It is found
intermixed with peat in midlevel zones beneath the tails. Muck even dominates
some strata, but their upper and lowers boundaries are usually more transitional
rather than distinct. Parts of some muck zones have apparently burned to ash. A
much different sediment, a granular organic-rich debris, and a sediment
modification, incipient cementation, occur in places beneath the hammock heads
and their edges (both are probably archeologically related).
The abundant muck was unexpected. It is not marly peat, enriched merely with
carbonate silt precipitated by periphyton in the marshes. The origin of the
silicate-rich muck is yet unknown. The source and mode of transport of the fine
mineral matter is the most puzzling, but those of the fine organic matter are
unknown too. The muck predominates at midlevels, with purer peat found toward
the present surface and in places underlying the mucky sediment. The muck
formation and transport possibly relate to a hydrologic and perhaps climatic stage.
The emplacement of the muck materials by physical transport very plausibly is
related to the process of elongation and alignment of the tails, important
components in the widespread “sweep” of Everglades vegetation. Muck in any
visible importance was not encountered in the two marsh cores, further adding to
the mystery of its origin.
Apparent ash layers are found in the peat-and-muck sediment zone beneath two
tree-island tails and cover an extensive area in one (occur in two cores 200+ m
apart). No other origin seems plausible for this fine, organic-deficient (lightcolored) sediment that contains a little carbonate but is mostly of insoluble
mineral (i.e., siliceous). These relict layers are very thick (2.522 cm in three
cores) compared to wetted ash from typical Everglades peat fires and must result
from the burning of a muck or muck-rich organic layer instead. Typical peat
would have had much lower mineral content. Such mucky source sediment
underlies or overlies the ash layers, forming an obvious source. Very dry
conditions with a severe fire is indicated in each case and these are possible
contemporaneous in the two tree-islands (precise 14C dating of this shallow-lying
layer is problematic).
The pre-peatland era of Everglades wetland evolution is well represented in areas
of thicker sediment beneath the tails. Thick marsh marl lies beneath peat in some
local solution depressions and in places shows obvious, though subdued, internal
stratigraphy, implying environmental changes.
These sediment profiles represent the long-term environmental records of the treeislands, including at the top those of recent times. Questions under investigation
using paleoecological methods to examine specifics of the floristic and, by proxy,
vegetational and hydrological succession, include the following: Are drainage-era
shifts evidenced? If so, what were their natures and directions? Prior to that,
what was the long-term evolution of these tree-islands? What was the
environment of formation of the muck? Additionally, the origin and
emplacement of the muck and the shaping and elongation of the tails have strong
implication to the whole question of flow and physical transport of solids in the
Everglades.
Peter Stone, Bureau of Water, South Carolina Department of Health and
Environmental Control, Columbia, SC 29291
Phone: 803-898-4151, Fax: 803-898-2893, stonepa@dhec.sc.gov