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Project Poster Abstract Stratigraphic Analysis Methods The late Holocene development of oyster reefs can through time by looking at the stratigraphy and taphonomy A living oyster reef at the mouth of Broad River was reveal the history of estuary formation through from the oyster reef. The paleoecology of the reef and the vibracored in June of 2007 (Hoye 2009) and revealed • The core was divided into 21-10cm horizons. standard stratigraphic and paleontologic analyses. taphonomy of the assemblage can be used as indicators of • Sediment samples were collected from each horizon Reefs located at the terminus of the Florida through AMS radiocarbon dating a continuous 960 yr paleoenvironmental conditions. The hypothesis tested was history of reef development. On casual inspection there is for grain size analysis using a Malvern Mastersizer Everglades’ Shark River sloughway should record that the reef originated under higher energy and salinity 2000. the effects of natural and anthropogenic variability in no visible facies distinctions through the stratigraphic conditions, becoming less energetic and progressively more • All skeletal materials were extracted via washing freshwater delivery. Using the 900-year history sequence. Any paleoenvironmental differences must be recorded in one reef from Broad River, this study brackish through history. The results from this study have distinguished on more subtle indicators (e.g patterns in through 1 and 2mm sieves for subsequent taphonomic determined relative trends in salinity and energy implications for oyster reef response to changes in shell preservation and relative abundances of mud). analysis. through time. The hypothesis tested was that the reef environmental conditions especially in relation to • A faunal analysis was attempted but the horizon- originated under higher energy and salinity hydrological restoration practices and sea-level rise. based assemblages were not diverse and all taxa other conditions, becoming less energetic and than Crassostrea virginica were in low abundances. progressively more brackish through history. A core Study Area • Oysters (Crassostrea virginica) >/= 2cm in length was obtained from the mouth of Broad River were selected from the harvested material for according to standard vibracoring procedures. The taphonomic analysis. 210 cm core was divided into 21 – 10 cm horizons • Each shell was assigned an overall taphonomic grade and its stratigraphy was described. Oyster shells and fragments greater than 2 cm in length from each using the method employed by Wohlpart 2007. horizon were examined to evaluate taphonomic Southwest • Grades range from one (highest preservation) to four grade, percent encrustation, and Cliona sp. boring Florida Coast (worst preservation). occurrence. Sedimentology was described using a • Percent borings (Cliona sp.) and encrustation were Malvern laser particle grain size analyzer. T-tests determined using the point counting method developed were applied to the sedimentologic and taphonomic by Booth 2008. data. Cliona sp., a bioeroder requiring marine • To determine the statistical significance of trends in salinities, and mean grain size decrease up section; grain size, taphonomic grade, borings and encrustation these trends were statistically significant. Although between the top and bottom of the core, values for the statistically insignificant, taphonomic grade showed a weak trend of improved preservation up section while upper 5 and lower 5 horizons were combined and encrustation exhibited a decrease. Results indicate compared using Student T-tests (p= 0.05). salinity has decreased throughout history in Broad River. Commensurate with these changes is a Characteristic Grade I Grade II Grade III Grade IV transition from a subtidal origin of the reef 900 years still articulated with valves held together in life valves in close association Articulation disarticulated ago to its current intertidal position. The subtidal Broad River ligaments in tact position with sediments but not in life position moderate thinning/ Valve Margin complete; unaltered minimal thinning/breakage excessive damage interpretation is based upon the reef’s deep position breakage relative to the sea-level curve we have compiled for minimal microboring and moderate microboring and extensive microboring and Interior Borings absent Southwest Florida. The increased volume of mud in etching etching etching the upper section could indicate a lowering of energy Internal Luster nacreous layer present some nacreous layer present minimal nacreous layer nacreous layer absent present with color and present; loss of some color present; little to no color or absent or only outline Muscle Scar or reflect greater fluvial sedimentation associated luster and luster luster recognizable with greater freshwater input. Alternatively, the finer Pallial Line distinct indistinct nearly absent absent sediments could be due to greater oyster productivity Color all color retained significant loss of color traces of color retained mostly absent no encrustation on interior minimal encrustation on significant encrustation on extensive encrustation on Encrustation resulting in the trapping of more mud. These results surface interior surface interior surface interior surface have implications for the management and Figure 3. Stratigraphic column for core 0706-15 illustrating the restoration of hydrologic flow through Everglades representative facies sequence for this region. Radiocarbon dates (1 National Park. Because freshwater delivery is vital to sigma calibrated results) derived from oyster shells and peat help oyster reef development, significant flow alteration define the timing of reef development. could affect coastal geomorphic change and Figure 1. Maps showing location of Broad River core site (indicated estuarine ecology. by X) within Everglades National Park, Southwest Florida. I Introduction II III IV Overall Grade I II III IV Excellent preservation, no Good Preservation, slight Fair Preservation, Poor Preservation, The late Holocene development of oyster reefs can or minimal alteration alteration moderate alteration extensive alteration reveal the history of estuary formation through Figure 5. Scale used to apply a taphonomic grade to oyster standard stratigraphic and paleontologic analyses. shells. Grades range from one, indicating that shells have 0706-15 Broad / subtidal oyster reef? Reefs located at the terminus of the Florida Best fit hand drawn curve At reef base undergone few taphonomic processes, to four, indicating extremely poor preservation. Multiple criteria were assessed for Everglades’ Shark River sloughway should record 0706-15 each shell. In addition, an overall preservational grade was Broad / less subtidal oyster reef? the effects of natural and anthropogenic variability in At middle reef assigned freshwater delivery. All of Southwest Florida’s modern oyster reefs and virtually all its fossil oyster reefs are intertidal with oyster productivity and survival dropping drastically below the shallow subtidal contours. As sea-level rise rates accelerate, and as humans continue to manipulate the amount of freshwater into the system, these oyster reefs may begin to decline. Determining the developmental history of the reef will allow for a better Figure 2. Sea-level curve produced using many radiocarbon dates from all understanding of how the reef evolved and provide over Southwest Florida. Red and purple circled data points represent more accurate predictions of future responses to dates obtained using oyster shells at 193cm and 96 cm within the Broad environmental change. Using the 900-year history River core (Figure 3). These oysters date back 960 yrBP and 750 yrBP respectively and both fall beneath the sea-level curve. This leads to the recorded in one reef from Broad River, this study assumption that the reef must have been subtidal at these points in Figure 6. Oyster valve with transparent grid. Grids were used in determined relative trends in salinity and energy Figure 4. Stratigraphic column character legend point counting method to determine extent of Cliona sp. borings history. and encrustation Discussion Taphonomic Analysis Percent Number of Oysters Encrusted 100 Percent Mud throughout • Results of taphonomic and sedimentologic analyses support the Oyster Mean Taphonomic Stratigraphic Column hypothesis that the reef in question originated subtidally under 4 Grade 80 higher energy and salinity conditions, becoming less energetic and % Mud progressively more brackish through history. 60 3 0 20 40 60 80 100 • The occurrence of Cliona sp. borings is used as a proxy for salinity 0 40 as this genus requires marine salinities. Based on the trend of decreased borings up-section, salinity is inferred to decrease 2 through recent history. 20 20 0 • Trends in taphonomic grade and percent encrustation, although 1 40 statistically insignificant, mirror the results observed in Cliona sp. % Number of Oysters Encrusted borings (which is a factor contributing to taphonomic grade). These results suggest that quality of preservation increases up-section. 0-20 cm 0 30-40 cm 40-50 cm 60-70 cm 70-80 cm 80-90 cm 20-30 cm 50-60 cm 90-100 cm 60 100-110 cm 100-110 cm 110-120 120-130 cm 130-140 cm 140-150 cm 160-170 cm 170-180 cm 180-190 cm 190-200 cm 200-210 cm 150-160 cm Average Taphonomic Grade (+/- 1 SE) Taphonomic Average • The higher percentage of mud up-section may be explained by a 1-20cm Horizon 20-30cm 30-40cm 40-50cm 50-60cm 60-70cm 70-80cm 80-90cm number of mechanisms. Lower energy
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