THE INTERACTIONS OF VERMETID GASTROPOD REEF DEVELOPMENT AND

SEA-LEVEL FLUCTUATIONS ON LATE HOLOCENE COASTAL

GEOMORPHOLOGY AND ENVIRONMENTAL EVOLUTION OF THE TEN

THOUSAND ISLANDS, FLORIDA

A Thesis

Presented to

The Faculty of the College of Arts and Sciences

Florida Gulf Coast University

In Partial Fulfillment of

The Requirements for the Degree of

Master of Science

By

Nicole A. Fronczkowski

2013

APPROVAL SHEET

This thesis is submitted in partial fulfillment

of the requirements for the degree of

Master of Science

______

Nicole A. Fronczkowski

Approved: December 2013

______

Michael Savarese, Ph.D. Committee Chair/Advisor

______

Joanne Muller, Ph.D.

______

Michael Parsons, Ph.D.

i

Acknowledgments

I owe my deepest respect and gratitude to Dr. Michael Savarese, who has taught me to think outside the box, where to start on such a brave endeavor, and how to persevere through the toughest obstacles. His direction and guidance throughout this project has been invaluable and it is my hope that my work reflects well on him as a mentor. I also must thank Dr. Michael Parsons for his inspiration and insight regarding reef growth through time. His classroom and field instruction has allowed me to design and carry out quality scientific research. Additionally, Dr. Joanne Muller has been an encouraging and supportive team member, whose cutting-edge ideas contributed greatly to the data analysis and final composition of this thesis.

I must thank the Geobiology of Modern and Fossil Reefs class of Fall 2011. Led by Drs. Michael Savarese and Michael Parsons, much of the fieldwork and laboratory analysis within this project was a contribution of my fellow students. It was a pleasure having that group of students as part of my field team, who were genuinely interested and added valuable discussion to the conclusions of the study.

Several organizations must be recognized, for without them, this research would have never been achievable. The Coastal Watershed Institute has provided the time and equipment necessary for the field and laboratory work to be carried out in a professional and thorough manner. Working as part of this lab has been one of the most enjoyable experiences of my education. My funding sources truly made this research possible.

Thank you to the U.S. Department of Education, for the grant awarded to Dr. Michael

Savarese, which got this project running full-force in 2011. Many thanks go to the Marco

Island Shell Club for awarding me a competitive scholarship to support my research. I ii will never forget my friends from this wonderful club. The Office of Research and

Sponsored Programs has supported me countless times, as I had the valuable opportunity to travel around the country, not only to present my research in the scientific community, but also to represent Florida Gulf Coast University and take my graduate education beyond the classroom.

I cannot thank my field help enough, as they all know, it wasn’t easy. To Bob

Halstead, Julie Neurohr, Lacey Smith, Dana Dettmar, Christian Ercolani, Patricia

Goodman, Lesli Haynes, Bryn Foster, and Michael Haas: I owe you my deepest thanks.

Even further, Kim Andres, Fritz (Boch) Hoeflein, and Sasha Wohlpart were not only fantastic field help, but helped me to get acquainted with a new environment, to learn research techniques, and encouraged me in the difficult, early stages of this study. Our geobiology group was an inspiration to me, and I thank Dr. Michael Savarese for organizing and leading us through the years. The discussion and feedback that came from our meetings guided much of my thought process. Thanks also to Ron Echols for the advice, inspiration, and encouragement throughout this journey.

Finally, I owe my most sincere gratitude to my parents, whose support, both financially and emotionally, has gotten me to this point in my education. Without the support of my Mom and Dad, I would never have been able to achieve my goals. My entire life I have been supported by a family that encourages exploration in nature and the joy found from making your own discoveries. This motivation has directed me throughout my education and I hope that I am able to pass this same drive to my own children and family in the future. iii

Abstract

The carbonate-producing coastline of Southwest Florida attributes its geomorphology to oyster and vermetid reef construction, which in turn becomes substrate for mangrove forest progradation. Understanding the unique development process of the mangrove-forested islands is crucial to the management of this ecosystem. This study aims to understand the constraints placed on Holocene vermetid reef development by the physical environment and the living ecology. The historic distribution of oyster reefs landward and vermetid reefs seaward may be a result of a well-defined salinity gradient.

Reef history extends 3,850 yBP into the Holocene, providing potential for sea-level interpretations. Twelve sediment cores were taken with this study. Altogether, they illustrate a transgressive/regressive sequence. Sedimentology and elevation data suggest vermetid reef growth closely mimics the patterns of mid- to late-Holocene sea-level rise

(SLR) and seaward progradation of the shoreline. Vermetus nigricans is the vermetid gastropod responsible for this wave-resistant framework, which often includes incorporation of Crassostrea virginica and barnacles. A geomorphologic study shows that relict vermetid reefs play a unique role in shaping the mangrove islands when compared to the same function of relict oyster reefs as frame-builders. Morphologies of vermetid reef construction through time and space vary as a response to environmental stimuli. Salinity and sedimentology are significant controls on distinct changes in vermetid reef morphology. V. nigricans is a low-intertidal sea-level indicator, and radiocarbon dating of the vermetids reveals no evidence of a local sea-level highstand.

However, evidence is presented for an unsmooth sea-level curve, with a possible draw- down event occurring at approximately 2,500 yBP. iv

Table of Contents

Acknowledgments...... i

Abstract...... iii

Table of Contents...... iv

List of Tables ...... v

List of Figures...... vi

Introduction...... 1

Research Objectives...... 8

Methods and Materials...... 9

Results...... 16

Discussion...... 29

Conclusions...... 45

References...... 49 v

List of Tables

1. Depth zonation for various vermetid species...... 53

2. Sediment core sample names, core locations, and core lengths ...... 54

3. Statistics for surface boundstone densities ...... 55

4. Radiocarbon dates for all samples ...... 56

5. Vermetid reef surfaces relative to Mean High Water ...... 57

6. Vermetid and oyster reef elevations ...... 58 vi

List of Figures

Figure 1. Sea-level curve from Toscano and Macintyre (2003) ...... 59

Figure 2. Sea-level curve from Balsillie and Donoghue (2004), 0-7,000 yBP...... 60

Figure 3. Sea-level curve from Balsillie and Donoghue (2004), 0-25,000 yBP...... 61

Figure 4. Local sea-level curve for Southwest Florida by Savarese and Hoeflein (2012)...... 62

Figure 5. Estuaries of Southwest Florida considered for this study ...... 63

Figure 6. Reference map of the northwest portion of the Ten Thousand Islands...... 64

Figure 7. Models of fringing coral reef development...... 65

Figure 8. Locator map of sites in Ten Thousand Islands and Estero Bay ...... 66

Figure 9. Vermetid boundstone sample from Round Key ...... 67

Figure 10. Stratigraphic diagram of core 1106-3...... 68

Figure 11. Location of radiometric samples from Camp Key ...... 69

Figure 12. Locations of all sediment cores associated with this study ...... 70

Figure 13. Sites of Mean High Water calibration...... 71

Figure 14. Aerial imagery of Camp and Round Keys ...... 72

Figure 15. Vermetid reefs of Camp Key...... 73

Figure 16. Vermetid reefs of Round Key...... 74

Figure 17. Location of boundstone samples from Camp Key ...... 75

Figure 18. Locator map of three elevation transects...... 76

Figure 19. Graphic illustrating profiles of elevation transects...... 77

Figure 20. Stratigraphic column of core 1103-1...... 78

Figure 21. Stratigraphic column of core 1103-2...... 79

Figure 22. Stratigraphic column of core 1106-3...... 80 vii

Figure 23. Stratigraphic column of core 1203-8...... 81

Figure 24. Stratigraphic column of core 1203-9...... 82

Figure 25. Stratigraphic column of core 1203-10...... 83

Figure 26. Stratigraphic column of core 1205-18...... 84

Figure 27. Stratigraphic column of core 1205-19...... 85

Figure 28. Stratigraphic column of core 1205-20...... 86

Figure 29. Stratigraphic column of core 1205-22...... 87

Figure 30. Stratigraphic column of core 1205-23...... 88

Figure 31. Stratigraphic column of core 1205-24...... 89

Figure 32. Fence diagram of Round Key coring transect ...... 90

Figure 33. Map of potential vermetid ranges in the Ten Thousand Islands ...... 91

Figure 34. Fence diagram of Turtle Key coring transect...... 92

Figure 35. Map of potential vermetid ranges in the Ten Thousand Islands ...... 93

Figure 36. Fence diagram of offshore coring transect ...... 94

Figure 37. Hand-drawn sea-level curve using dated vermetid materials from this study ...... 95 1

Introduction

The study of modern day sea-level rise (SLR) is based around documenting changes in historic sea-level trends. These trends are graphed as a curvi-linear function that displays sea-level changes over time, called a sea-level curve. Sea-level curves have been developed for the Holocene using a variety of biological sea-level indicators including fossilized coral heads of Acropora palmata (Toscano and Macintyre 2003) and

Porites lobata (Webster et al. 2004), pollen analysis (Horton et al. 2005), mollusks, and the mangrove genus, Rhizophora (Bird et al. 2007), as well as vermetid gastropods

(Angulo et al. 1999) and the reef-building oyster, Crassostrea virginica (Savarese et al.

2004).

The most widely accepted curve for the Atlantic was developed by Toscano and

Macintyre (2003) using both Acropora corals as a shallow subtidal sea-level indicator and mangrove peats as a supratidal indicator (Figure 1). Similar to other local curves

(Parkinson 1989, IPCC 2007, Savarese and Hoeflein 2012), this curve shows that sea level was rising rapidly above 5 mm per year in the early Holocene, 12,000 to 8,000 years before present (yBP). Then, around 8,000 yBP, the rate of SLR declined to 1.5 mm per year. Around 3,500 yBP, the late Holocene exhibited the lowest rates of rise throughout the Holocene transgression, under 1 mm per year. These curves are smooth, decelerate through the Holocene, and do not exhibit any evidence for a Holocene-age sea- level position above current mean sea level (MSL). The most rapid rates of SLR occurred in the early Holocene, but reduced considerably thereafter until reaching a minimum rate until just before present. 2

Other sea-level curves for the Holocene provide evidence that SLR can occur in rapid pulses of significantly high rates of rise, followed by a stillstand or draw-down in sea-level position (Angulo et al. 1999, Balsillie and Donoghue 2004, Bird et al. 2007).

The timing of such events can vary from region to region, but Balsillie and Donoghue

(2004) present compelling data for Holocene sea-level highstand events in which the vertical position of sea level rested higher than MSL today (Figure 2). The overall trend in their curve is similar to Toscano and Macintyre’s (2003) curve, with higher overall rates of SLR in the early Holocene and a gradual deceleration of SLR moving through the middle and late Holocene to the present. However, peaks and valleys representing rapid

SLR events and draw-down events are present within the Balsillie and Donoghue (2004) curve (Figure 3). Overall, Holocene rise occurs in both respective curves, but only the latter curve offers evidence for an unsmooth sea-level curve over time.

A local sea-level curve (Savarese and Hoeflein 2012) for Southwest Florida used radiocarbon ages of supratidal mangrove peat, intertidal Crassostrea virginica oyster reefs, and subtidal carbonate mollusks to denote the changing position of sea level throughout the middle to late Holocene (Figure 4). This regional curve does not present evidence for any rapid rise or highstand events as proposed by Balsillie and Donoghue

(2004) for the Gulf of Mexico, but rather follows a smooth deceleration of SLR rates moving from mid to late Holocene, similar to that of Toscano and Macintyre (2003).

This deceleration in the rate of SLR from around 6,000 yBP to present has allowed for the complex mangrove forest construction of much of the Southwest Florida coastline

(Parkinson 1989). 3

Southwest Florida’s low-elevation coastline will be particularly sensitive to future

SLR, as the unique coastal geomorphology seen today is a consequence of subtle sea- level changes in the late Holocene. The deceleration of SLR rates from 3,500 yBP to recent, to as low as 0.93 mm per year (Toscano and Macintyre 2003), has allowed for the mangrove island formation and progradation of this coastline (Parkinson 1989). This lower rate of SLR allows the red mangrove, Rhizophora mangle, to accumulate peat and for quartz and carbonate sediments to accrete up to sea level, contributing to an increase in land area despite SLR. In the Holocene sediment record, this phenomenon is termed a transgressive/regressive sequence and has been observed in other regions of the world during this time in geologic history (Bird et al. 2007). From 3,500 yBP to present, the slowing down of SLR has allowed for mangrove progradation, which has created the ideal environment for intertidal reef-builders, Crassostrea virgnica, and the vermetid gastropod, Vermetus nigricans, by limiting exposure to the open shore and trapping fresh and marine water. Mangroves and oysters are confirmed sea-level indicators for

Southwest Florida, but vermetids have yet to be used to determine paleo-sea levels, though they may provide data to enhance existing sea-level curves.

Vermetus nigricans is a member of the family Vermetidae Rafinesque, 1815

(Morton 1965). Other genera in this family that are common in subtropical marine waters around the world are Petaloconchus and Dendropoma. All vermetids are limited to mid- and sub-littoral depths (Laborel 1986), making them useful paleoenvironmental indicators (Table 1). The littoral range for the local Vermetus nigricans population in

Southwest Florida has not been adequately defined, a necessary component when using this species for the study of sea-level history. 4

Aggregates of vermetids commonly form large, widespread platforms, resembling pavements, a common growth formation among vermetid gastropods (Safriel 1975,

Antonioli et al. 1999, Silenzi et al. 2004). Vermetid morphology also changes depending on environmental parameters like substrate, water depth, sedimentation, turbidity, wave energy, and food availability (Laborel 1986, Schiaparelli and Cattaneo-Vietti 1999). It has been documented that morphological plasticity within the same species of reef- building vermetids reflects various environmental conditions and sea-level positions during time of growth (Gould and Robinson 1994). This phenomenon has also been documented for reef-building corals (Toomey et al. 2013).

Vermetids build reefs right up to the sea surface (Morton 1965), another behavior commonly seen among corals, accreting vertically until constrained by water depth

(Goreau et al. 1979). Additionally, vermetids can display ridge cap development at the top of the reef, signifying water depth limitations when rates of SLR have decreased

(Adey 1978).

In the Ten Thousand Islands National Wildlife Refuge (TTINWR) (Figure 5), the relict reef surfaces created by the species Vermetus nigricans Dall (Bieler and Petit 2011) are found below MSL on the rims of offshore islands but are exposed at very low tides.

There has not been any documentation of a living population of Vermetus nigricans on the Southwest Florida coast in earlier studies (Shier 1969, Parkinson 1989), nor have any living individuals been observed during field explorations for this study. All vermetid reefs in the TTINWR are remnants of a once thriving population, and the characteristics of these relict surfaces are the focus of this study. 5

The morphology of vermetid reefs, as first described by Shier (1969), varies in the

TTINWR, and the reef surfaces are dissimilar across the landscape. Variations in tube density and reef thickness exist, causing the appearance of the reef surface to fluctuate.

In aerial and field observations, there are also notable differences in the morphologies of islands with vermetid reefs.

Understanding the unique geological history of the TTINWR and the relationship between its development and past sea level fluctuations offers insight into the expected response of such an ecosystem during future SLR. The ability to predict environmental changes is necessary for proper management during continued climatic alterations.

During a changing climate, the future of these estuaries is extremely dependent on the rate of SLR. In fact, the very development of these estuaries was dependent on the relatively low rates of rise during the late Holocene (Scholl 1964, Parkinson 1989). The response of coastal ecosystems to these changes must be considered during imminent

SLR, sea-surface warming, increased storm activity, and other climatic threats as predicted by the IPCC (IPCC 2007).

Florida’s low-lying coastline is vulnerable to the predicted SLR expected to accompany global climate change (Leatherman et al. 2000). Erosion caused by SLR affects a coast more severely than the increase in water levels alone; this effect is exacerbated by low-slope elevations present across the state. However, the presence of a dynamic and productive mangrove forest does provide Southwest Florida the ability to respond positively to a rising sea. Through peat production and sedimentation, growth of

Rhizophora mangle, the red mangrove, can raise the elevation of land above the reach of low water. In addition to mangrove forests establishing growth upward, they have the 6 ability to increase land area over time with growth directed seaward, termed progradation. The coastline has exhibited significant progradational trends during times of decelerated rates of rise experienced over the past 5,000 years (Parkinson 1989).

However, SLR, as predicted by the IPCC (IPCC 2007) over the next century, may reach

3.8 mm per year. This aggressive rise may overwhelm the ability of coastal habitats to self-sustain through mangrove-island growth and progradation. Rapid SLR imposes a burden upon policy makers and land managers. These stakeholders are now challenged with managing a region not only affected by anthropogenic alterations to the ecosystem but also long-term effects of SLR.

Islands in the TTINWR are present up to 5 kilometers seaward of the mainland, and individual islands vary greatly in shape and size. Those located offshore (Figure 6C) are generally larger in size and more lobe-shaped and circular than those further inshore

(Figure 6B). Conversely, landward islands to the north are thin, having formed dendritic and branching chains across the landscape. The drivers of this shift in morphology between offshore and inshore islands have not yet been documented in the literature.

The environmental mechanisms and drivers that controlled these morphological variations in the local Vermetus nigricans population of the late Holocene are unknown.

Exploration of these structures should provide insight into the changing environment of the late Holocene during SLR and the effect of coastal ecosystem development in the

TTINWR. Additionally, variations in growth forms of vermetid reefs may reflect changing environmental parameters in a similar way that coral reefs do. Phenotypic plasticity with various environmental conditions is a well-documented phenomenon in scleractinian corals (Todd 2008). For instance, fragments of Montastrea annularis 7 exhibit a flattened morphology when light levels are low; Acropora formosa shows prominent branching when transplanted to shallower depths as its counterparts deeper in the water column shows no branching morphology; and Acropora aspera produces denser skeletons when exposed to high wave energy (Todd 2008). In addition, accommodation space limitations of the air-water interface restrain the vertical growth of

Acropora framework, so growth must then be redirected laterally (Figure 7) (Kennedy and Woodroffe 2002).

Sea level is the absolute maximum height for reef building by corals and vermetids, so when relict reef populations exhibit seaward growth, accommodation space could have been a limiting factor. There may be evidence of such occurrences preserved in the growth form of relict reef structures. Studying the different morphologies exhibited by the Vermetus nigricans population provides insight into this reef-builder’s role in coastal development. Moreover, understanding this relationship offers paramount conclusions about past sea-level activity for the eastern Gulf of Mexico. Finally, the validity of Vermetus nigricans as a sea-level indicator should be considered and tested against other commonly used local indicators, to allow for the proper use of vermetids in future study of sea level.

Two hypotheses were tested by this research. The first hypothesis explored was that the presence of vermetid reefs controlled the geomorphology of the coastline within the TTINWR. The shape of these islands is attributed to the vermetid reef structures that lined the outer coast. Second, changes in vermetid growth form drove variations in coastal geomorphology. The resulting geomorphology was a function of accommodation space. Investigation of these hypotheses offered insight into the vermetid reef 8 development within TTINWR, how these reefs reflect late Holocene sea-level history, and further, how the relationship between reef growth and sea level has driven coastal development.

Research Objectives

The principal goals of this research were to fully explore the history of mangrove island development in the TTINWR, the potential of vermetid reef substrates to influence island construction, and to determine the sea-level controls on reefal growth form. As this exploration is of critical interest in determining the relationship between SLR and coastal evolution, the following research objectives were established:

1. Characterize the structure of vermetid reefs in the TTINWR. The variations in

reef structure across both the temporal and spatial landscape must be observed

and documented qualitatively and quantitatively. In this study, two distinct

regions of the study area are identified for comparisons. These regions were

denoted as: 1) northwest islands, on and near Camp Key and 2) southeast islands,

on and near Round Key and Nursery Island (Figure 8).

2. Determine the age of exposed relict surfaces. Radiocarbon dating determines how

much time has passed since the Vermetus nigricans population was a thriving,

reef-building community.

3. Interpret the geologic history of these reefs and observe any paleoenvironmental

or successional trends. Characterization of the sediment record within the

TTINWR provides insight into environmental changes that occurred during the

middle to late Holocene. The transitional period from marine transgression 9

through island progradation of this region must be well understood to determine

the role of reef-builders for coastal geomorphic development.

4. Determine the position of the actively accreting reef surface relative to mean sea

level. Relating the position of relict vermetid reefs to other local sea-level

indicators (e.g. red mangrove leaves, living oyster reefs) provides comparison to

modern day reef-builders that shape today’s coastline. Further, the species

Vermetus nigricans and its littoral range must be quantified for its use as a sea-

level indicator.

Methods and Materials

The main study area is located in Southwest Florida, along the coastline of the

Gulf of Mexico (Figures 5 and 6). The TTINWR receives marine water from the Gulf of

Mexico and freshwater from the numerous tributaries to the north (Palm, Blackwater,

Whitney, Pumpkin, Little Wood, Wood, Faka Union, Fakahatchee, East, Fergusen, and

Barron Rivers). This estuary is minimally altered and its mangrove construction controls the geomorphology of the study area.

I. Quantifying Reef Structure a. Fieldwork

Methods were developed to compare the structural differences between vermetid reefs in the northwest and the central regions of the TTINWR. Topographic profiles were taken across the exposed reef structures to investigate the possibility of environmental gradients controlling the growth and development of the vermetids. The 10 orientation of each transect, seaward to landward, was meant to capture the effects of varying wave energy. On Camp Key, a 15-meter transect was positioned perpendicular to the reef’s long axis. Another 15-meter transect was positioned across a “dome-shaped” vermetid reef on the seaward edge of Round Key. This second transect was positioned along the dome’s diameter. A third transect was set up on Nursery Island, located just inland of Round Key (Figure 8A). This transect was only 6 meters in length, limited by the width of exposed reef surface. Elevation and GPS coordinates were recorded along each transect at one-meter intervals using a Trimble RTK 8 GPS unit.

Samples of exposed reef boundstone were collected at both the Camp and Round

Key localities and brought back to the lab for further study. Extreme care was used during the cleaning process to prevent disaggregation of the individual vermetid tubes composing the loosely aggregated boundstones.

b. Laboratory Analysis

Because the density of vermetid shells within boundstones influences the growth and morphology of a reef, it should vary between sites. Consequently, skeletal density was measured from three samples at each location, Camp Key, the eastern shoal of Camp

Key, and Round Key, for a total of nine samples. Density was measured using a point count-intersection method along a 20-cm cross-sectional length of boundstone (Figure 9).

A transparent sheet marked with a 20-cm line was laid across the boundstone. For every vermetid individual intersected by that line, a mark was made on the transparency. The number of intersections per centimeter was determined and reported as a percentage.

One-way ANOVA tests were used to determine significance between sites. 11

In addition to surface samples, down-core densities were obtained for fossil vermetid boundstones. (The coring methods will be explained in detail below.) Ten samples from four cores (1103-01, 1106-03, 1203-08, and 1205-18) were analyzed for density comparison. The vermetid boundstone facies within each core was identified and divided into horizons based on the relative age of the boundstone. Densities in each facies were recorded at the initiation, middle, and termination of the reef history. This was carried out for all mentioned cores with the exception of cores 1103-01 and 1205-18, whose reef growth was too short lived to include a “middle” subdivision; in these cores, only density from initiation and termination phases was recorded. The sampled material was cleaned carefully with deionized water and a soft-bristled toothbrush to avoid disaggregation of the individuals. Skeletal density was measured as above but along a 7- cm transect (the diameter of a core tube). Final counts were expressed as vermetid individuals per 7 cm. One-way ANOVA tests were used to determine significance between sites.

II. Determining the Age of the Exposed Reef Surface

Samples of Vermetus nigricans shell were sent to Beta Analytic in Miami, FL for

AMS radiocarbon dating. Samples representing reef initiation and termination were selected from cores 1203-8 and 1203-18. Both cores contain an extensive and time- persistent vermetid reef facies with an obvious initiation and clearly defined termination.

Additionally, samples of relict surface boundstones at two locations in the TTINWR were dated from Round Key and Nursery Island. Finally, a subsurface sample from the Round

Key reef was taken from core 1106-3 (Figure 10) to estimate an accretion rate for this 12 particular “lobe-shaped” reef structure. Corresponding stratigraphic heights relative to mean high water (MHW) and the NAVD88 datum, provided by the Trimble, were determined, allowing these data to be integrated into a sea-level curve. The samples were then carefully collected in life position.

Two surface samples, 2D and 3B (Figure 11), taken from Camp Key’s sandy shoal were dated to determine the direction of reef growth through time, whether prograding seaward or transgressing landward. These two samples were measured before the elevation study was proposed and are thus are the only dated specimens that are not associated with precise elevations.

Dating specimens in life position ensures the fossils are not reworked, and therefore accurately represent the time of reef formation. Samples were cleaned with deionized water in a heated sonication bath, dried, and packaged in plastic bags for shipping. Samples from cores were separated from surrounding substrate and other carbonate contaminants and prepared the same way as the surface samples. All radiocarbon samples were calculated using 2-sigma calibrations and a delta-R value of

33+/-16.

III. Evaluating Paleoenvironmental and Successional History

Twelve sediment cores, along four north-south transects, were collected in the

TTINWR from strategically placed locations (Figure 12). These locations were chosen to maximize the variety of reef form, depth of reef facies, habitats, spatial locations, and time intervals of vermetid reefs along the Southwest Florida, late Holocene shoreline.

The cores were taken across the salinity gradient to capture the transition, both at and 13 below the surface, between oyster reef-dominated islands and vermetid reef-dominated islands. Coring transects were positioned along the shoreline moving from the northwest to the southeast islands to capture the transition between the different morphologies displayed by the reefs at the surface.

Coring methods were designed to fulfill three objectives. First, oyster reefs are a principal frame-builder for mangrove island construction. Cores taken further inland will show oyster reefs’ persistence in serving that role throughout the Holocene. Second, coring transects moving from seaward to landward will capture the transition from oyster to vermetid frame-builders. Lastly, these transects will reveal any differences in vermetid morphology and abundance across the salinity gradient.

Each of the twelve sediment cores (Table 2) was taken using standard geological coring methods, either by vibracoring or push coring. Cores 1103-1, 1103-2, 1106-3, and

1106-4 utilized a vibracore machine to ensure Pleistocene-age bedrock was reached, which can be as deep as 5 meters below modern sediments. Having a baseline depth to bedrock is necessary for reconstructions of Holocene sedimentology. The remaining cores were taken by hand to capture between 2 and 3 meters of the sediment column. A two-meter depth was critical to ensure that the entire history of a vermetid reef was captured.

Before extraction, the water depth and nearby mangrove leaf heights were measured to obtain an accurate elevation for the surface of the sample. To account for compaction of the sediments during the coring process, the height of the sediment outside the tube was measured and then subtracted from the height of the sediment inside the core tube. The seven centimeter wide aluminum pipe, filled with sediment, was topped 14 with water to create a vacuum, capped with a plastic lid, sealed with electrical tape, and then extracted either with a custom-built tripod when vibracoring, or by hand when push coring. Cores were transported back to the laboratory on the FGCU campus and immediately split lengthwise for sediment, facies, and radiocarbon analyses.

Cores were split in half by a circular saw, photographed, and one half was archived. The remaining half was used for facies descriptions, interpretations, and sampling for radiocarbon dating and grain-size analysis. Facies were described using texture, substrate type, grain size, faunal composition, and sediment color. Contacts between facies were also defined as sharp versus gradual, their stratigraphic position was recorded, and evidence for disconformities noted. The texture-by-feel method (Thien

1979) was performed primarily in descriptive analysis for each facies. Following description, a sediment sample 3 cm deep into the core half was taken from each distinct facies with a clean scoopula and stored for later grain-size analysis. If there were noticeable changes in content or texture within a facies, or if the facies extended for longer than one meter, two to three samples were taken to be sure the sediments sampled in that facies were representative of the entire package. Great care was taken to avoid sampling the sediment near the aluminum barrel to avoid contamination due to compaction from adjacent facies. Grain size analysis of sediments was conducted using a

Malvern Mastersizer 2000 particle size analyzer to confirm results from the texture-by- feel test and to provide further details of grain size distribution for environmental interpretation.

Graphic depictions of each core and its characteristics were drafted. Each core was homogenously decompacted using a simple algorithm. The thicknesses of all facies 15

were equitably stretched to bring the top of the core flushed with the sediment-water interface.

Shelly fauna from every facies was identified to the lowest taxonomic level possible using multiple shell identification resources (Mikkelson and Bieler 2008, Leal

2011). In any facies that contained shell hash, sediment was extracted from the core and washed through a 63 micron stainless steel sieve. This was done to determine if vermetids were present in the sample. If vermetid hash was present, it was assumed that a vermetid boundstone facies was located near the chosen site.

IV. Relating the Position of Vermetid Reefs to Local Sea-level Indicators a. Mean High Water Calibration and Elevation Study

Elevations of lowest mangrove leaves and modern oyster reefs were also taken to provide a height comparison for the relict reefs. The heights of lowest mangrove leaves were measured with the Trimble unit at two separate locations. Four total elevations from red mangrove leaves were taken, two from Lindland Reef in the TTINWR and two from Horseshoe Keys in Estero Bay (Figure 13). In Estero Bay, tidal ranges and magnitudes are the same as compared to those in the TTINWR, so mangroves are expected to grow similarly in relation to mean high water. The range of mangrove leaf heights was minimal between the four points from the TTINWR and Estero Bay, so arguably, the lowest mangrove leaves do accurately represent the same height in each estuary. The resulting four elevations were averaged to achieve the approximate height of mean high water (MHW) for Southwest Florida estuaries. Heights measured in this 16 study relative to both NAVD88 and local MHW allow for easier interpretations and comparisons to similar studies elsewhere.

To compare relative heights of relict vermetid reefs to modern oyster reefs, oyster reef elevations were measured at four locations. Elevations of highest, intermediate, and lowest bounds of living oyster distribution were measured at Lindland Reef, Camp Key, further inland at Blackwater Bay in the TTINWR, and in the Horseshoe Keys of Estero

Bay (Figure 13). Because the eastern oyster, Crassostrea virginica, has a relatively wider tidal range than vermetids, the entire range of productive oysters was considered.

Knowing the elevations of the relict reefs relative to modern living structures permits the calculation of the rate of SLR since the time of vermetid reef termination.

Additionally, these heights can be used to confirm or disprove any possibilities of a sea- level highstand in this region as proposed by numerous authors (Missimer 1973, Stapor and Mathews 1983, Donoghue et al. 1998).

Results

I. Characterization of Vermetid Reef Structures

The vermetid reef structures to the northwest, just south of Cape Romano, exhibit a different growth form than those reefs to the southeast (Figure 14). Aerial imagery shows two linear sand spits oriented along the tidal gradient extending seaward from mangrove islands on the outer coast. Surface surveys revealed a vermetid reef structure exposed at low tide; its crest sits in positive relief relative to surrounding sediment

(Figure 15). The composition of the boundstone is predominantly individual skeletons of deceased V. nigricans and some intergrowth with C. virginica and barnacles. Most 17 vermetid tubes are oriented upward in life position. Boundstone volume is dominated by interskeletal porosity or quartz-rich detrital sand.

The majority of the relict vermetid reef structures flanking the open-ocean shoreline have a different morphology than that of Camp Key’s region. Most other reef structures exhibit a lobate or circular pattern of growth, most notably at Round Key. The circular morphology of the boundstones commonly occurs at three hierarchical scales.

First, close examination of the reef surface reveals a platter-like formation of numerous intergrowing vermetid individuals (Figure 16). At a larger scale, many platters aggregate to form a larger circular structure, herein termed a “dome”. Further, many domes come together to form a “lobe”, whose circular structure is the common feature of many mangrove islands in this area.

At Round Key, there is little to no evidence of intergrowth with any other frame- building species, and the vermetid densities within the boundstone are significantly higher than those of Camp Key (Figure 16A). With significantly higher tube density, the detrital sediment component of these boundstones is very small. Also, unlike the Camp

Key sites, Round Key vermetids grew mostly along a horizontal plane, extending laterally as opposed to the upward growth observed in the northwest sites. These characteristics will be described in greater detail in the upcoming sections.

To account for the percentage of vermetid tubes within the surface structures at three sites, density counts were performed. Surface boundstone samples taken from the shoal just east of Camp Key (Figure 17) were the least dense of the three, with a mean of

31% vermetid composition (N=9), and densities ranging from 16.5% to 68.5%. Nine samples from the shoal just seaward of Camp Key (Figure 17) had a mean density of 18

48.9% vermetid, ranging from 26% to 70.5%. Considerably different from these two shoals were the densities of vermetid individuals comprising the boundstones at the surface of Round Key. Mean density at Round Key was 85.6% for nine samples, with a minimum density of 64% and a maximum of 98.5% vermetid composition (Table 3).

One-way ANOVA confirmed that all three sites had density values that were significantly different from one another (P < 0.001). Both LSD and Games-Howell post-hoc tests were performed and confirmed that each site had significantly different densities from each other site; all P values from post-hoc tests were ≤ 0.080.

Also observed during the field investigation was a distinct reef crest along the length of the shoals in the Camp Key area, with the highest relief in the center (Figure

15A). Alternatively, the vermetid reef structures near Round Key form a flat, table-like structure, with little relief. The position and shape of 3 topographic profiles (Camp Key’s eastern shoal across a lee to stoss gradient, Round Key across the seaward to leeward gradient, and on Nursery Island landward of Round Key also across a seaward to leeward gradient) can be seen in Figures 18 and 19.

The results of the three profiles were consistent with respect to maximum height achieved by the reef surface (Figure 19). In each profile, the highest recorded elevations were 0.66, 0.71, and 0.70 meters below MHW on the eastern shoal, Round Key, and the island behind Round Key, respectively. The maximum elevations recorded from each site all fell within a range of 0.5 meters.

II. Radiocarbon Ages 19

Dated samples were associated with precise GPS and Trimble elevation points

(Table 4). The only samples without precise heights are samples 0106112D and

0106113B which were taken from the Camp Key shoal. At Camp Key, the sample taken from the north end of the island (most proximal to shore) is dated at 1315 cal. yBP, while the sample taken from the southern tip of the island (moving seaward) is 1240 cal. yBP.

Radiocarbon dating was also performed on samples taken from within cores, well below the sediment surface. The oldest sample found from a core taken at Nursery Island

(8-194; Figure 20) sits 2.8 meters below MHW and dated at 3850 cal. yBP. The youngest sample found was taken from a boundstone on the surface of Round Key, dated

550 cal. yBP and sits 0.73 meters below MHW.

There is no stratigraphic disordering among dates of the samples 112, 611, 8-77 and 8-194 from core 1203-8, and 18-85 and 18-121 from core 1205-18. All samples show a trend of upward growth, with younger samples stratigraphically above older samples. Growth rates for three locations, Round Key, Nursery Island, and Turtle Key, were calculated based on radiometric dating of shell from the first and last appearance of vermetids within each respective core (1106-3, 1203-8, and 1205-18). Round Key exhibited a growth rate of 1.25 +/- 0.35 mm/yr. 2,000 years prior, nearby Nursery Island showed similar growth rates at 1.22 +/- 0.045 mm/yr. The Turtle Key reef facies developed more slowly, at 0.85 +/- 0.05 mm/yr.

III. Core Stratigraphy and Sedimentology a. Camp Key Region 20

Cores 1103-1 and 1103-2 were both taken from Camp Key (Figure 12). 1103-1

(Figure 19) starts with a muddy sand, rich in highly degraded organics in Facies I. Up core, Facies H is marked by the loss of mud but is still rich in large roots and other organic materials. The contact with Facies G is sharp. G is characterized by interlaminations of sand and mud. Near the top of G, a degraded peat is present within a sandy mud. The contact with F is sharp with an abrupt change to mud containing whole shells and 25% gravel. No organics are present here. The F facies fines upward; shell content is lost in the middle of the facies; and oyster shells make their first appearance at the top. Chione elevata shells, characteristic of marine environments (Mikkelson and

Bieler 2007), are found at the top of F. The contact with Facies E is gradual and denoted by the loss of whole valves of marine shells and fining sediment. E is composed of fine silt with shell hash laminations in the lower 30 cm and again in the upper 20 cm. The sharp contact between E and D is marked by the first appearance of vermetid boundstone.

The presence of oysters persists, and the sediment is sandy mud. A shell gravel layer exists in the lower portion of the facies. The C contact is marked by the abrupt loss of vermetid shell boundstone, but the facies contains abundant vermetid gravels. Oyster shells remain prevalent, and the sediment is very fine sand. Root fibers are present moving up core. The contact with B is gradual, marked by a progressive development of a loose peat. Organic materials are degraded. This facies transitions to A with a reappearance of poorly sorted medium sand, vermetid gravels, fragmented vermetid individuals, and oyster shell.

The sediment history for core 1103-2 (Figure 21), positioned landward of the previous core, is less complex but shows the same overall trend of increasing shell 21 materials moving up core coinciding with a loss of woody and peaty organics. Facies E at the base of 1103-2 is composed of sandy clay with wood fragments. Iron oxides are present in facies E. Vertical roots persist through the contact at facies D, which is marked by a loss of sand. At the top of facies D, a well-developed peat is present. The contact with C is gradual, but marked by the loss of peat. Whole marine shells and gravels appear in the top of C and all organics are lost below the contact with facies B.

Facies B is fine muddy sand with a prominent shell gravel component, with shell fragment size decreasing up core. Facies A is marked by the abrupt transition to sandy sediment with fused Vermetus nigricans individuals and gravels. Shell gravels are still prominent but dominated by the vermetid gastropods.

1103-1 and 1103-2 are similar in sediment trends; however, the vermetid boundstone sediment package is thicker in 1103-2 than 1103-1. Also, the more seaward core, 1103-1, contains facies of loose oyster boundstones above, within, and below the vermetid boundstone facies, which are lacking in 1103-2.

b. Round Key Region

The Round Key transect from seaward to landward, contains cores 1106-3, 1203-

8, 1203-9, and 1203-10 (Figure 12).

Facies C, at the bottom of 1106-3 (Figure 22), consists of fine sand, rich in whole marine shells of poor taphonomic grade and degraded gravels. Grain size coarsens up- facies, with the contact at B marked by coarse sand. Facies B contains organic material and root fibers, numerous marine shells, and oyster fragments. Dense shell hash layers occur at 110 cm and 90 cm. The contact with facies A is recognized by loss of all shell 22

fragments and organics, as well as a drastic grain size shift to medium silt. The entire facies consists of very dense vermetid boundstone up to the present-day surface.

1203-8 (Figure 23) was taken landward of 1106-3 on Nursery Island. The base of the core, facies E, contains organic-rich sand and mud with many root casts and horizontal root fibers. A sharp transition to shell-rich muddy sediment occurs at the contact with facies D. Mud and organics are gradually lost upsection in D, and the sediment is mostly very fine sand with abundant oyster shell fragments. 1203-8 also has a dense vermetid facies, C, but sits stratigraphically lower than the vermetid-rich facies A of the previous core. Facies C is marked by a sharp transition to well-preserved vermetid and oyster boundstone with vermetid fragments. The sediment in this facies is medium silt. Grain size increases in facies B2 and B1. The high-density vermetid and oyster shell material from facies C is reduced to occasional appearances in facies B2 and B1. The B group is organic-rich with many root casts and vertical roots, and the few oysters within are very well preserved. From B1 to facies A, dense organics are lost and grain size increases to fine sand. The only organics left at the surface of facies A are highly degraded leaf litter.

Core 1203-9 (Figure 24) is located landward of 1203-8 and begins its stratigraphy with a fine sand and shell gravel-rich facies B. Some whole marine shells, Chione elevata and razor clams, are present, and a well-preserved vermetid cluster was found at

250 cm. Facies B coarsens upsection and increases in the occurrence of shell gravels.

The contact with facies A is marked by the presence of oyster shells and a fining-upward sediment, from medium sand in B, to coarse silt in lower A, and to medium silt for upper 23 facies A. Shell gravels are still prominent throughout this facies, and the upper portion of

A contains significant organics and many small, articulated bivalves and gastropods.

Similar to 1203-9, 1203-10 (Figure 25) has a basal facies rich in whole shells.

Facies B is a coarse silt, finer in texture than in 1203-9. Grain-size analysis confirms sediments increase in size moving up-facies. Whole marine shells, Cerithium sp. and

Chione elevata, are found throughout. Shell hash layers exist at 95 cm in B and 75 cm, at the contact with facies A. Like in 1203-9, facies A is marked by a fining in sediment to coarse silt and an abundance of oyster shells. Moving further upward toward the sediment surface, whole gastropod shells reappear, as well as organic material like degraded leaves and root fibers.

1106-3 and 1203-8 both contain significant vermetid boundstones. However, the facies above and below these boundstone facies are different within each core. In 1203-

8, the boundstone facies sits over 1 meter lower than the boundstone facies to seaward from core 1106-3. 1203-9 and 1203-10 are almost identical in stratigraphy, however

1203-10 is the only core in this transect lacking evidence of vermetid boundstones or gravels. Also, 1203-8 is the only core from this transect that exhibits any peat development, while the others are composed of quartz sand and shelly muds and clays.

c. Turtle Key Transect

Cores 1205-18, 1205-19, and 1205-20 were aligned seaward to landward starting at Turtle Key (Figure 10). At the bottom of the most seaward core, 1205-18 (Figure 26), is facies C, a poorly sorted coarse silt with abundant oysters and fine shell gravels.

Moving up-facies, oysters become more fragmented and shell gravel increases. Also, 24 small root fibers are prominent from 200 cm to the contact with B. Facies C transitions to B gradually with a slow initiation of vermetid growth (first observed at 200 cm) within the oyster facies. Throughout facies B, vermetid individuals increase in density. The sediment remains coarse silt, but skeletal gravels do not occur. Vermetid density is moderate, but not as high as that seen within facies from the Round Key region. The transition to facies A2 is abrupt, marked by an increase in organic material and a decrease in grain size to medium silt. Moving up-facies, vermetid clusters are lost and oyster shells reappear, but in a fragmented state. Within the upper 50 cm of the core, a peat develops, and shell gravels are prominent once more. A2 gradually transitions to A1, with the most notable change being the loss of peat and other firm organics. Quartz sand is present, and vermetid gravels are thick within very poorly sorted coarse silt.

Fine sand marks the base of core 1205-19 (Figure 27), just landward of 1205-18.

Shell hash layers are common in facies C2 and C1. One oyster shell and one vermetid cluster were found in facies C2, but facies C1 only contains small marine shells and shell fragments. Moving up-core into facies B, the sediment becomes finer and vermetid gravels are prominent. Marine bivalves remain a component of the sediment package, and a rich shell hash layer is present at 165 cm. The transition to facies A is noted by the loss of shell gravel and a finer grain size. The base of facies A is composed of oyster boundstone with a coarse silt matrix. At 100 cm, shell gravels reappear and another shell hash layer exists at 40 cm. Oyster shells are dense throughout facies A, continuing up to the surface.

The most landward of the Turtle Key cores is 1205-20 (Figure 28). Again, the lowest facies starts with fine sand, however this facies C is rich in organics and contains a 25 well-developed peat with large mangrove root casts. The transition to facies B is sharp, with the loss of all peat and organics and the appearance of sand and shell gravels. The sediment of facies B fines upsection, and shells are no longer gravel-sized but remain whole. Bivalves Chione elevata and Tellina lineata were identified within this facies.

Mud snails and one Turritella acropora were also found high in the facies near the contact with A. Facies A is rich in oyster shells and contains coarse silt. Shell gravels and fine organics reappear in the middle of the facies. Shell gravel decreases moving towards the surface, and the sediment becomes finer still, a medium silt. Oysters are prominent up to the sediment surface.

Facies A was nearly identical in 1205-19 and 1205-20, except for the lack of shell gravel or hash layers in 1205-20. Each of these cores shows evidence of significant oyster boundstone development.

d. White Horse Key Transect

Cores 1205-22, 1205-23, and 1205-24 were positioned just behind White Horse

Key (Figure 12) and oriented landward to seaward. 1205-22 (Figure 29), the most seaward core of this transect, has a basal facies rich in sand and marine shell gravel. At the lower portion of facies C, vertical root casts are present, as well as interlaminations of light sand and darker organic-rich silt. Immediately above this, mottling of the two sediments occurs, with the resulting grain size being a poorly sorted fine sand. Sediment fines upsection in facies C, becoming a very fine sand. A prominent shell layer occurs at

250 cm and contains whole marine shells of Chione elevata and Cerith sp. Shell gravels increase up-facies, and thin sand lenses and laminations mark the transition at the contact 26 with B. Facies B contains very poorly sorted fine sand with some vermetid shell gravels.

More prominent are well-preserved whole gastropod shells belonging to the species

Conus sp. and mud snails as well as the bivalve Tellina lineata. Shell gravels decrease in abundance upsection, causing the sediment to fine further. The contact at A is marked by a complete loss of gravel and the first appearance of oyster shell. The lower portion of facies A is coarse silt. Sediment becomes medium silt at the top of the core, containing well-preserved oyster shells and organic matter.

Similar to 1205-22, 1205-23 (Figure 30) begins with vertically oriented root casts in facies B2. The sediment here is a very fine sand. Shell gravel is prominent, resting above the organics. Small gastropods and bivalves are common throughout B2, and an articulated Tellina lineata was identified at 170 cm. Vermetid clusters and gravels are found, though scarce, upwards to the facies B1 contact. The contact at B1 is gradual but marked by a loss of marine bivalves and gastropods as well as the first appearance of oysters. The sediment here becomes finer - medium silt - and the percentage of shell gravels within the sediment decreases upsection. Oysters are absent from the middle to top of facies B1, and vertically oriented roots and other organics are present from 60 cm upwards to the contact with A. Sediment at the A contact is coarse silt, and organics are markedly more prominent in this facies. Like in core 1205-22, oysters reappear at the sediment surface.

Vertical roots are also found in facies D at the base of core 1205-24 (Figure 31), within fine sand. Mottling of quartz sand and organic-rich, very fine sand marks the contact with facies C. Facies C is rich in organics and has a poorly developed peat that becomes more organic-rich upsection. Other sources of organics, like mangrove root 27 sheaths and root casts, are common within C. Interlaminations with sand occur at the contact with B3, which is also marked with the first appearance of oysters. Organics are more prominent in the lower portion of this facies, but as root materials phase out up- facies, vermetid gravels appear. Shell gravels, rich in fragmented vermetids, occur at the contact of B2. There are no oysters in facies B2. The contact with B1 is marked by the reappearance of oyster shell, now very dense within much finer sediment. Horizontal root fibers also reappear and persist throughout the facies. Also, vermetid gravels still comprise a large component of the fine silt sediment package. The contact with facies A is abrupt, and all organics are lost. Vermetid gravels remain prominent but other shell gravels reappear as a constituent of the sediment. The grain size of facies A is coarse silt.

Large vermetid gastropod clusters are common at the surface of this facies.

IV. Relating the Position of Vermetid Reefs to Local Sea-level Indicators a. Mean High Water Calibration

All measurements taken with the Trimble RTK unit were calibrated against the

USGS NAVD88 datum and our local MHW height. Elevations were taken with +/- 15-

21 mm precision. Mean high water (MHW) was calculated by averaging all elevation measurements taken of the lowest mangrove leaves. The two leaf measurements from

Blackwater Bay were +0.27 and +0.23 meters relative to NAVD88 datum. The two measurements from Horseshoe Keys were +0.39 and +0.32 meters relative to NAVD88 datum. These four mangrove leaf measurements have a mean of 0.302 meters. This value represents the established height of MHW, and all subsequent elevations are relative to this newly defined datum. 28

b. Elevation Study

Elevations of the relict Vermetus nigricans reef surfaces in the TTINWR and in modern oyster reefs in the TTINWR and Estero Bay were compared. The topographic profile of the vermetid reef at Camp Key (Figure 19) has vermetid boundstones sitting at elevations between -0.66 and -1.28 meters relative to MHW. The maximum elevation here is consistent with the maximum elevations recorded in the topographic profiles from

Round Key and Nursery Island, whose highest documented positions were -0.71 and -

0.70 meters relative to MHW, respectively (Table 5). The lowest elevation observed from the Round Key profile was -1.30 meters. The range in reef elevations of the exposed structures on Camp Key and Round Key was similar, with Camp Key’s reef varying in height by 61.9 cm and the reef at Round Key spanning 59.3 cm. The minimum elevations for the Nursery Island exposed vermetid reef was -1.26 meters MHW. The exposed structure on Nursery Island ranged 55.9 centimeters, again consistent with the height differentials from the other sites.

Oyster reefs measured at Horseshoe Keys in Estero Bay reached a maximum elevation of -0.44 meters MHW (Table 6). The oysters at the low tide line measured -

0.70 meters MHW. The highest elevation of productive oysters at Lindland Reef near

Camp Key (Figure 13B) sits at -0.48 meters MHW and at -0.47 meters for the reef in

Blackwater Bay (Figure 13C). The range of productivity for the oyster reef at Horseshoe

Keys was measured from the maximum elevation to the low tide line and is calculated at

26 centimeters. Below the tide line, the lowest productive oysters in Blackwater Bay were measured at -1.11 meters MHW, so the range of productivity here, measured from 29 highest to lowest elevations of oyster productivity is a height differential of 64 centimeters. The same measurements on Lindland Reef revealed a range of oyster productivity of 65 centimeters. The mean of the upper bound of living productive oysters in all three tested locations is -0.464 meters MHW. The mean lower bound for productive oysters on Lindland Reef and Blackwater Bay is -1.122 meters MHW.

The youngest surface of relict vermetid reef lies 67 centimeters below MHW while the youngest oyster reef of modern age sits 47 centimeters below MHW. The maximum heights of vermetid reef surfaces consistently sit 30 centimeters lower than the upper bound of modern-day productive oysters.

Discussion

I. Characterization of Vermetid Reef Structures

Island geomorphology in the TTINWR varies by location. Aerial imagery of the northwestern islands shows vermetid reefs forming a linear geomorphic form (Figure

14A), while imagery of mangrove islands in the southeast display a lobe-shaped island morphology (Figure 14B). The mangrove islands that formed on vermetid substrate in the area near Round Key mimic the shape of the vermetid platforms that lie beneath.

The morphological differences between the northwest and southeast reefs could be attributed to a variety of factors. Wave energy, sedimentation rates, interspecific competition, and sea-level position all have control on the growth strategies of individual vermetids (Shier 1969, Laborel 1986, Schiaparelli and Cattaneo-Vietti 1999). For example, the vertical, upward growth exhibited within the vermetid boundstones from the

Camp Key region generates lower overall densities and suggests a purpose for the growth 30 form (Figure 15). This area has relatively low wave energy due to its position in the lee of Cape Romano and experiences significantly more siliciclastic sedimentation than the southeastern region (Parkinson 1989). These higher rates of sedimentation could serve as a stressor to the vermetid population, necessitating upward growth for survival. Failure to redirect growth would result in burial or could limit access to food. Lower energy for this region in general can limit food independent of sedimentation rates. As sessile mucus net feeders, vermetids must rely on wave or current energy to provide food, and growth is often redirected to achieve an optimal feeding posture (Schiaparelli and

Cattaneo-Vietti 1999). However, Schiaparelli and Cattaneo-Vietti (1999) also noted that

Dendropoma does not produce erect feeding tubes when exposed to a high-energy environment, with little chance of sedimentation inhibiting the animal’s access to food.

This recumbent growth form is reflected in the reef morphologies on Round Key, but the behavior is not evident within the Camp Key reefs.

Competition for space has the potential to control growth rates at the Camp Key location, as many samples were found intergrown with oysters and barnacles.

Intergrowth is more prevalent in the northwest region of the TTINWR, as the salinity here is more suitable for oyster growth (Soderqvist and Patino 2010). The tidal channel directed south from Blackwater River dispenses large amounts of freshwater, attracting oyster larvae. In reference to all southeast boundstone samples, there was only one sample (on Round Key) with evidence of oyster recruitment, but no oysters were larger than 10 mm, revealing that those individuals never reached maturity.

Among the vermetid boundstones collected on Round Key, only one of nine exhibited the presence of another reef-building species. The lack of interspecific 31 competition here could be attributed to the direct exposure to gulf wave and storm activity. Contrary to the relative low-energy in the sheltered Camp Key region, the higher wave energy conditions in the Round Key region also support the increased density observed in the vermetid reefs. With stronger waves and greater exposure to storm energy, sedimentation here is significantly less than what Camp Key experiences.

With low to moderate sedimentation rates, the chance of burial is lessened and energy can be focused towards denser growth. Additionally, the densely populated and tightly packed vermetid reefs of the southeastern islands provide a resistant framework to repetitive exposure to strong wave energy. Also, the outward-directed growth resulting in the high-density platter-shaped aggregates can effectively dissipate wave energy. The radial symmetry of platter and dome formation observed at Round Key is the more common growth morphology observed worldwide, particularly in coastal environments exposed to consistent wave energy (Ginsburg and Schroeder 1973, Safriel 1975, Logan

1988, Antonioli et al. 1999).

Ginsburg and Schroeder (1973) and Antonioli et al. (1999) have documented mushroom-shaped reef formations where vertical growth is limited by the species’ intolerance to subaerial exposure. Further, once the species-specific vertical limit is reached, growth is redirected laterally (Safriel 1975), and this mechanism is presumably responsible for the platter structures within the TTINWR. This dominant morphology is always found in the surf zone along regions of the coast with the most significant wave energy (Antonioli et al. 1999).

Topographic profiles also reveal reef morphologies that differ between regions

(Figure 19). The upward growth at Camp Key represents a population expending energy 32 to accrete vertically, contrary to Round Key and Nursery Island whose profiles exhibit a vertical limit to reef accretion. Camp Key reefs grew vertically without any apparent limitation of accommodation space; Round Key reefs appeared to direct growth laterally, suggesting accommodation space limitation was reached (Figure 16). Two common causes of this phenomenon among carbonate reef builders are sea-level limitations and increasing water agitation (Kennedy and Woodroffe 2002). The population will either initiate at that elevation, or accrete up to it, but as the water column limitation is reached, growth halts or is redirected outward.

Another environmental condition that could control reef morphology is the tidal gradient characteristic of the Camp Key region. Two parallel sand bars seaward of Camp

Key, as seen in Figures 11 and 14A, were created by the tidal flow northward from

Gullivan Bay (Holmes and Evans 1963). The vermetids opportunistically utilized these sedimentological structures as substrate for reef development. The instability and dynamic nature of these sub- and intertidal shoal environments near Camp Key could be the root cause of the sub-optimal reef morphologies and overall lower densities when compared to other vermetid reefs to the southeast.

Alternatively, reefs in the Round Key region are not secondary structures but rather stand-alone biogenic structures. With little to no detrital sedimentation within the matrix of Round Key boundstones, the reef structure is composed entirely of vermetids.

This starkly contrasts the content of Camp Key boundstones, with significant sediment composition and intergrowth with oysters and barnacles. The greater reef density in the southeast is associated with the hierarchical, lobate geomorphology. Because the reef formation in this area was not influenced by high sedimentation rates and directional tidal 33 flow, the more typical vermetid boundstone morphology is achieved. During reef growth, environmental conditions were ideal at Round Key and Nursery Island. Wave energy was likely balanced between too rough, which causes erosion not tolerated by the vermetid gastropods, and too little, causing detrimentally high rates of sedimentation

(Sanlaville et al. 1997).

The shape of progradational mangrove islands that formed on top of vermetid reefs mimics the original shape of those reefs. Aerial imagery of islands in the southeastern region shows numerous circular and lobate mangrove-forested islands

(Figure 14B). Consequently, the lobate growth form has had tremendous influence on the geomorphology of the TTINWR, as the relict vermetid reefs provided substrate for mangrove recruitment, island formation, and progradation. Further, as those lobe-shaped mangrove islands continue to flourish, the merging of islands dramatically increases land area and the potential for peat development and sequestration of carbon through burial.

This type of high-density vermetid growth generates firm substrate more effectively than the vertically oriented, low-density reefs, as the latter results in more highly mobile sediments. The denser the reef substrate, and thus the greater establishment of mangrove-forested islands, the greater the coastal protection offered to inland Florida against extreme storm events.

II. Radiocarbon Ages

The relative radiocarbon ages obtained along the Camp Key shoal support a seaward progradation of the vermetid reef structure. Boundstones have older dates in the more landward locations. The dates are consistent with the interpretation of these sand 34 bars as tidal deposits. Sands originating from the Cape Romano Shoals enter the

Blackwater River tidal system (Holmes and Evans 1963, Parkinson 1989) and are reworked as tidal bars affixed to the mangrove-forested islands.

Vermetid boundstones near the main island of Camp Key date from 1315 yBP, while the boundstone found 700 meters seaward dated 1240 yBP. This suggests a progradation rate of 9.3 meters per year. This rate, however, does not reflect the rate of vermetid boundstone growth, but rather the rate of sediment deposition associated with tidal bar formation. The progradational rates obtained at Round Key (discussed below) reflect vermetid growth and boundstone accretion, and are therefore significantly lower.

The radiocarbon dates taken from relict reef surfaces 2.80 and 1.44 meters below

MHW at Nursery Island and dates from 1.13 and 0.73 meters below MHW at Round

Key, further seaward, can be used to measure island progradation in the southeast islands.

The rate is expressed as distance the reef moved seaward over time: progradation here is significantly slower than on Camp Key, 2.5 meters per year. This is due to the nature of the boundstones in the southeast region; the high-density, biogenic boundstones take significantly more time to accrete.

Geochronologic data in this area also provide the opportunity to calculate vertical accretion rates of the vermetid reefs at the Nursery Island and Round Key sites. The vermetid facies at both sites exhibit similar accretion rates, 1.22 mm/yr on Nursery Island and 1.25 mm/yr on Round Key. The Nursery Island reef surface is approximately 175 years older than the surface of Round Key. The similarities in accretion rates between the two time periods suggest that parameters controlling reef growth from 920 to 550 yBP remained unchanged. 35

Similar high vermetid skeletal density boundstones are found 3,000 years earlier,

2 meters below the relict reef surface at Nursery Island. This suggests that environmental parameters from 3850 to 2940 yBP were similar to those from 920 to 550 yBP on the

Round Key and Nursery Island surfaces, and were comparable and ideal for high-density reef accretion.

III. Paleoenvironmental Interpretations and Temporal Trends

Graphic stratigraphic columns of all 12 cores from the study region reveal trends of marine transgression followed by mangrove progradation through the late Holocene.

Fence diagrams, presented below, are used to explain such trends and spatial relationships among facies.

a. Camp Key Region

Cores 1103-1 and 1103-2 exhibit marine transgression, starting with upland soils and rich, charred organics, as evidenced by the charcoal around vertical roots at the base of both cores. The transgressive paleo-shoreline is marked by a few environmental indicators within cores 1103-1 and 1103-2 (Figures 20 and 21). First, marine shells and sand denote a nearshore subtidal facies that is superimposed upon high intertidal and supratidal peat facies. Further, laminations of mud and sand indicate the onset of tidal influence. Marine inundation drowned young and underdeveloped mangrove forests, and a sandy and shell-rich subtidal environment was established. At this time, approximately

3,000 yBP, sea level was rising, and because intertidal facies are often not preserved during transgressions, a disconformity exists in the stratigraphy. In both cores from the 36

Camp Key area, the successive facies resting above the subtidal sands represent a low- intertidal and clay-rich reef environment. This marks the onset of the regressive progradational phase in island development. Additionally, at this time, sedimentation rates were increasing, due to both the decrease in SLR rates and increased biogenic production. Biologic production of these additional sediments is attributed to the skeletonization of reef-building vermetids, the peloids formed through their suspension feeding, and added allochthonous sediments from the nearby Cape Romano shoals. As

SLR rates decline, sedimentation rates begin to exceed SLR, effectively reducing water depth, thus supporting progradation. This process provides for rapid growth and recruitment of the common eastern oyster, C. virginica, and the vermetid gastropod, V. nigricans. There is ample accommodation space for the low-intertidal vermetid facies to extend upward and contribute to the 230 cm of reef boundstone in core 1103-1 (Figure

20) and the 125 cm of boundstone and gravels in core 1103-2 (Figure 21). This reef- building phase, paired with the mangrove progradation characteristic of the late Holocene in the Ten Thousand Islands (Parkinson 1989), allows for further entrapment of local sediments. The increased sedimentation results in the distinct vertically oriented morphologies of the vermetid communities of the Camp Key region.

Comparison of cores 1103-1 to 1103-2 reveals that the vermetid facies in 1103-2 persists for longer in history but lacks a boundstone. As this core is landward of 1103-1, which contains significant boundstone development, it is probable that the vermetid facies of 1103-2 represents post-mortem transport of nearby vermetid reef debris. The gravels in facies A of this core consist of both large vermetid clusters, signifying proximity to the reef source, and gravel-sized bioclasts, indicative of material that 37 traveled a greater distance or of material reworked longer. The sediment surface around core 1103-2 is rich in sand and shell gravels of various sizes, indicating that the island and its relict reef surface help to trap those sediments and further expand land area, by supporting mangrove recruitment and progradation.

b. Round Key Region

Even though cores 1106-3, 1203-9, and 1203-10 contain sediment packages with different characteristics than 1203-8 (Figure 32), the paleoenvironmental interpretations remain the same for all four of these cores. All cores exhibit a transgressive sequence within the older facies. However, similar to the two cores in the Camp Key region, sediments become fine silt and represent a shallowing upward regressive trend through the upper portion of the cores (Figure 32). This younger stratigraphy, in which marine environments are brought up into intertidal depths, is due to both mangrove progradation and slowing of SLR around 3850 yBP, which allowed V. nigricans and C. virginica to establish reefs and enhance sediment production and accumulation. These events have caused the Ten Thousand Island’s shoreline to move seaward, and the estuary to expand and become more productive.

Steady rates of SLR during the late Holocene allowed the vermetid facies of cores

1106-3 and 1203-8 to accrete rapidly (1.25 and 1.22 mm/yr), from 3850 yBP until 550 yBP. V. nigricans reefs were prolific at this time because the moderate wave energy from the Gulf of Mexico and lower sedimentation rates relative to the Camp Key region created the ideal conditions for vermetid gastropod growth. Radiocarbon dating confirms that the vermetid facies from 1203-8 is older than the vermetid facies of 1103-6. Because 38 reef development initiated over 1 meter deeper than in 1106-3, and because core 1203-8 sits further landward than 1106-3, the following conclusions can be made: (1) sea-level rose in the last 3,000 years (the span of time between initiation at Nursery Island and termination at Round Key); and (2) that 3850 yBP represents the approximate time of shift from a transgression to progradation due to the slowing of SLR. Further, a peat found in core 1203-8 constrains a maximum sea-level position at 2940 yBP.

Additionally, the densities represented by the vermetid boundstones at the top of the facies are high, consistent with the densities of the surface structures. High densities at boundstone facies’ termination support the interpretation that SLR rates decreased dramatically, reducing available accommodation space, causing vermetid growth to be directed laterally. Further seaward and over 2,000 years later in the Holocene, the same high densities are observed in core 1106-3, but at a one-meter higher stratigraphic position. These sequential observations (dense vermetid growth, peat development, and another dense vermetid facies) suggest again that sea level is not always slow and continuous, but rather exhibits pulsed rise events followed by a slowing rate of SLR, or possible stillstand. The stillstand would have been relatively short lived, as there was approximately one meter of SLR over 2,000 years that allowed the Round Key V. nigricans domes to develop.

More recent times of mangrove progradation are supported by findings in the upper portions of cores 1203-8, 1203-9, and 1203-10. Progradation is evidenced by sediment becoming finer up-core, as protection from the high environmental energy of the open coast facilitates the accumulation of fine sediments on landward islands. These silt-sized sediments support the intertidal oyster reefs observed in facies A of 1203-9 and 39

1203-10. The development of islands seaward of these locations created a protected estuarine environment that allowed the silt-rich intertidal facies to persist through time to the present. While extensive vermetid development seaward is controlling outer island geomorphology, the oyster reefs control island geomorphology inshore. The Round Key transect delineates the geographic range for V. nigricans in the Round Key region and distinguishes the point along the landscape where geomorphic control shifts to oysters

(Figure 33).

c. Turtle Key Transect

Data from the Turtle Key transect support the findings from the Round Key transect discussed previously (Figure 34). The same facies sequence of a peat overlying a vermetid boundstone exists. Again, dates from core 1205-18 signify that around 2270 yBP, sea level could have reached a stillstand or a dramatic decrease in rate of rise. At this time, the vermetid facies exhausts all accommodation space, reef growth is directed outward, and the reef surface becomes substrate for mangrove recruitment.

The three cores taken from the Turtle Key transect also revealed the same trend of vermetid facies seaward progradation as seen in the Round Key coring transect. Facies B from both 1205-18 and 1205-19 consists of either vermetid boundstone or vermetid-rich gravel. This facies, however, sits a half-meter lower in the inland position, core 1205-19, than in the offshore position in core 1205-18. This further supports an interpretation of reef progradation with SLR.

This transect extends the boundary line marking the geographic extent of reef- building by V. nigricans (Figure 33). Core 1205-18 confirmed the hypothesis that the 40 island lobe shapes are influenced by vermetid reef development. The vermetid facies lies below a one-meter thick mangrove peat, representing a lobate mangrove island mimicking the shape of the reef beneath. The presence of vermetid boundstones in the

Turtle Key cores shows that high-density V. nigricans reefs drive coastal geomorphology in this area, in addition to the southeast region.

Overall grain size decreases among the cores moving landward along the transect.

The finer sediments are characteristic of protected estuarine environments experiencing lower energy than the outer coast. This environment supports the formation of oyster- dominated boundstones, which also facilitates the recruitment and progradation of mangrove islands. The geomorphology of these islands is different than those founded on vermetid boundstones. The form of these mangrove islands mimics the shape of the oyster bars below, forming the typical thin and linear island morphology seen landward.

The mangrove islands located near core 1205-20 and further inland do have a suspicious lobate shape, but without stratigraphic or sedimentologic evidence of vermetid reefs, it cannot be confirmed that the circular shapes are caused by vermetid growth. It has been proposed that the roundness of the islands in that area is due to successful mangrove recruitment (Shier 1969). The diameters of these circular islands also appear to be shorter when compared to the lobe-shaped islands formed primarily above vermetid facies, such as those in the Round Key region (Figure 14B). Shier (1969) also observed the transition in aerial appearance when comparing oyster-bar islands to mangrove- developed islands in the Chokoloskee Bay area. However, it is possible that the most inland core from this transect missed catching the reef facies and further coring of this region may reveal a boundstone facies after all. Still, it cannot be confirmed that lobate 41 structures of the inland islands are due to vermetid growth, the dashed and dotted lines in

Figure 34 denote the uncertainty of the position of this change. At some point between these two limits, the main driver of geomorphology shifts from vermetid reefs to oyster bars.

d. White Horse Key Transect

Cores 1205-22, 1205-23, and 1205-24 from the White Horse Key transect lack a vermetid boundstone facies. Core 1205-24 contains large vermetid clumps at the sediment surface. These fused tubes, forming pebble-sized reef rocks, were likely transported from a nearby island, as clusters showed no preferred orientation, were bioeroded, and rounded. Lack of in situ boundstones captured in the cores, however, does not rule out V. nigricans reefs as the principal frame-builder for offshore islands in this centrally located region of the TTINWR. Field observations confirmed that vermetids proliferated in the area; the beaches surrounding White Horse Key contain abundant vermetid reef rubble and shell gravel.

Regardless of the lack of a boundstone facies, the stratigraphy of this transect still reflects the transgressive-regressive sequence observed from the other transects. Lower portions of the cores display flooding of the upland sand and mangrove peat at the base, indicating marine transgression. Above this, intertidal oyster reefs lie stratigraphically above the subtidal marine sands, signifying regression. Additionally, the sediment fines upsection in these cores, as in the other transects, further supporting coastal regression, through progradation.

42 e. Offshore Cores

In order for a successful vermetid gastropod to settle, hard substrate must be available (Shier 1969). In the case of three offshore cores (1205-18, 1205-22, and 1106-

3) with extensive vermetid reef facies, all reefs are founded upon older sediment either composed of relict oyster reef or hard-packed sands with abundant marine shells (Figure

36). Because intertidal sands are prone to reworking and relocation, these sandy facies must have been subtidal in nature, providing stability for the reef to form. This mode of vermetid colonization within a subtidal environment confirms that late Holocene SLR made way for the prolific reef growth seen at these offshore sites. Substantial marine waters carrying food and supplying ample space overhead allowed vermetids to accrete rapidly up to intertidal depths. Further, with the establishment of hard reef structure, a new habitat is created within the existing ecosystem, not only allowing for mangrove recruitment but also creating viable habitat for migratory birds, juvenile fishes, and other intertidal and benthic fauna. As SLR rates decreased in the late Holocene, vermetid reefs were faced with limited accommodation space and redirected growth outward to form lobe-shaped reefs, thus initiating control over the geomorphology of the outer coast.

IV. Relating the Position of Vermetid Reefs to Local Sea-Level Indicators

The lowest elevations of exposed relict vermetid reefs on Camp Key, Round Key, and Nursery Island were all comparable. The maximum elevations of these three reef structures were also analogous (Table 5). The morphologies at Camp and Round Keys are very different, however, representing contrasting environmental conditions at the time of growth. It is inferred that sea level had not limited the growth of the Camp Key reefs, 43 as evidenced by the vertical orientation of Vermetus nigricans individuals. Conversely, those at Round Key appeared to grow with limited space overhead, forming massive radially-oriented platters and domes (Figure 16). The radiocarbon dates from these locations show us that the reefs on Camp Key are older than those of Round Key by about 700 years. This information serves as evidence for a rapid rise in sea level followed by a stillstand or draw-down of sea level during the past 1,500 years.

The fact that V. nigricans obtained the same elevations at different times in the late Holocene, while exhibiting differing growth morphologies, suggests that sea level remained relatively stable from 1315 yBP to modern day. Analysis of other Vermetus species (Table 1) supports the use of V. nigricans as a local sea-level indicator, as the other species of this genus occupy the lower intertidal zone in the studies reviewed.

Oysters are well accepted as indicators of sea-level position (Milliman and Emery

1968, Kennedy et al. 1996, Pirazzoli 1996). The relationship between the relict vermetid reefs of the TTINWR and modern-day oyster reefs was examined with respect to ecological zonation and tolerance to subaerial exposure. Crassostrea virginica is an intertidal oyster with depths of occurrence as deep as -1.135 meters MHW and as shallow as -0.440 meters MHW. V. nigricans has a similar maximum depth. The intergrowth of oysters with vermetids is common in the TTINWR, as seen at the relict surface of Camp

Key and in many cores (1103-1, 1203-8, and 1205-18). More important is the relative difference in both species’ tolerance of desiccation and, therefore, their upper limitations for water depth. The research determines that both vermetids and oysters can tolerate some air exposure during low tide (Shier 1969, Safriel 1975), but relative comparisons between the two populations have not yet been made. The upper limits for vermetids in 44 this study were consistently lower than those for oysters (Table 5), so it is determined that the youngest surfaces of the relict vermetid reefs of the TTINWR reside in the modern- day low intertidal range. These data are consistent with a late Holocene sea level that must have been at its current position or lower during times of vermetid growth.

Vermetid height and age data do not support the existence of a sea-level highstand above present day sea level during the late Holocene.

V. Refining the Local Sea-Level Curve

Prior to this study, the precise height of MHW was ambiguous and ill-defined.

This study has calibrated MHW, defined as the height of lower red mangrove leaves, relative to NAVD88, and subsequently positioned sea level relative to MHW through use of vermetids and oysters as sea-level indicators.

Even though there was no evidence for a highstand within this study, there was evidence suggesting a fluctuating sea-level curve for the area. A sea-level curve was created using radiocarbon dates and elevation data for vermetid occurrences (Figure 37).

Since vermetids represent the low intertidal, the position of the curve must reside above these points. All sea-level curves previously discussed are in agreement regarding a visible overall decrease in rates of SLR through the late Holocene. Some disagreement occurs at the interval between 3,000 to 2,000 yBP. Results from this study indicate that prior to 3,000 yBP, sea level rises at a rate similar to the curves reviewed above.

However, at 2,460 yBP, the vermetid facies rests stratigraphically lower than its predecessors of an earlier time, suggesting that the position of sea-level had dropped.

This particular sample, taken from the initiation of the dense vermetid reef facies in core 45

1205-18, sits within the valley of the curve, signifying a draw-down in sea level. The younger sample, still lower in elevation than the sample from 2,940 yBP, was taken from the termination of the vermetid reef facies in core 1205-18. A peat facies sits directly above this point, further supporting this evidence for a draw-down and stillstand event around 2,500 yBP.

Conclusions

Refining our understanding of Holocene sea-level activity and its relationship with local reef-builders is of importance given the pending SLR of the Anthropocene.

Further, the interpretation of past coastal geomorphologic change along the Southwest

Florida coastline provides insight into the predicted natural succession of environments and biotic interactions as we experience varying rates of SLR into the future. This study corroborates two hypotheses. First, the presence of the Vermetus nigricans reefs within the TTINWR indeed controls the geomorphology of the mangrove-forested coastal islands. The reef-builder’s existence from at least 3,850 yBP to 550 yBP has also helped facilitate the progradation of this coastline as SLR of the late Holocene decelerated. This, in turn, had considerable influence on the development of the Ten Thousand Island’s estuarine ecology. Second, the vermetid reefs exhibit varying morphologies, and those morphologies each have a distinct control on the style of island geomorphology. The shift between these two types of geomorphologies occurs as a function of accommodation space and the intensity of detrital sedimentation. Vermetus nigricans reefs contain low- density intergrowths of vermetid individuals with vertically-oriented growth when accommodation space was not limiting and when rates of detrital sedimentation were 46 high. These conditions occurred when sea level was rising at a rate equal to or greater than the vermetid reef accretion rate and where proximity to the Cape Romano Shoals provided a significant source of quartz sand. This growth morphology is exemplified by the offshore islands seen in the Camp Key region, characterized by long, thin sandy shoals that run parallel to the salinity and tidal-flow gradients upon which vermetids became established. These islands are products of physical sedimentation; vermetids became established as secondary structures upon the sandy substrate when sedimentologic conditions were hospitable. The other style of island geomorphology is generated by vermetids with dense intergrowths of individuals, laterally directed growth, and under conditions of negligible detrital sedimentation. Accommodation space was also limited by either a sea-level stillstand or draw-down, when rate of vermetid reef accretion exceeded the rate of SLR. These environmental characteristics result in the platter-shaped geometries of the reefal boundstones. The hierarchical geomorphic pattern seen within the islands of the Round Key region of the TTINWR (platter, domes, and lobes) is a consequence of vermetid growth under these conditions.

I. Vermetids of the TTINWR as Local Sea-level Indicators

The eastern oyster (Crassostrea virginica) is a vital modern-day reef builder for the coastal estuaries in Florida (Savarese et al. 2004). Their colonization is dependent on sea-level position at the time of juvenile recruitment, and reef height is often used to position the historic change in sea level. Because vermetid reefs are also influential in the development of coastal geomorphology and because the gastropods’ viability requires shallow subtidal or low intertidal depths, the relationship between their vertical 47 distribution with respect to sea level and in comparison with the distribution of oyster reef heights was investigated.

The youngest vermetids found at the surface lived within low intertidal depths in the TTINWR. The modern oyster reefs in this estuary, as well as in Estero Bay, exist at maximum elevations consistently higher than vermetid reefs. This evidence, combined with the documented environmental occurrences reported in the literature, confirms the local vermetid species, Vermetus nigricans, is a low-intertidal dweller and therefore a legitimized sea-level indicator. This confirmation was necessary before V. nigricans could be used to develop and refine sea-level curves for Southwest Florida. The sea-level curve shown in Figure 36 includes all dated vermetid samples from this study.

The sea-level curve constructed from this study’s vermetid samples shows a relatively steep rate of SLR up to 3,000 yBP. However, dated vermetids at 2,460 yBP sit below the older sample at 2,940 yBP. This suggests a draw-down in sea level sometime after 3,000 yBP, as seen by the dip in the curve (Figure 36). At 2,270 yBP, sea level was at a position one half-meter lower than at 3,000 yBP, further confirming this evident drop in sea level. As with draw-downs in the Balsillie and Donoghue (2004) curve (Figures 2 and 3), this draw-down is punctuated by a rapid rise event. The rate of SLR using the vermetid data from 1,500 yBP to present is significantly slower relative to the rate shown by the prior events. This slower rate is observed in other local curves as well (Toscano and Macintyre 2003, Savarese and Hoeflein 2012) (Figures 1 and 4), supporting the mangrove progradation of the Southwest Florida coastline. There is no evidence for a sea-level highstand event in this curve or from any data collected as part of this study.

48

II. Significance of Vermetid Reef Development

During climatic change, a rapid rise in sea level can be destructive to coastal land, through groundwater contamination, saltwater intrusion into aquifers, and physical damage caused by marine transgression. The TTINWR currently serves as a protective barrier from the Gulf of Mexico; that protective barrier is a function of the history of reef and island development. Despite late Holocene sea-level rise, the coastline has continued to prograde and to provide shoreline protection. As the rate of SLR remains low, oyster and vermetid reefs, and mangrove peats, can accrete and keep pace with or exceed SLR rates. Under these conditions, the coastal elevation and its protective capabilities are maintained. However, if future SLR rates exceed these accretion rates, the coastline will degrade, altering the coastal geomorphology and predisposing the mainland to transgression.

This study documents 3,000+ years of vermetid reef development, resulting in approximately 1,200 meters of seaward progradation. This coastal expansion currently supports a huge area of brackish water wetlands and mangrove-forested habitat. This demonstrates the natural system’s potential for sustainability in response to climate change and SLR. 49

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Table 1. Table displaying the various tidal depth zonation observed among a variety of vermetid species from other subtropical locations. By definition, high intertidal residents have a high tolerance to desiccation. Medial intertidal residents can only tolerate moderate to low amounts of exposure. Low intertidal species can tolerate very little exposure to air. Subtidal species have no tolerance for desiccation and must constantly remain submerged for optimal health. References describing tidal ranges for each species below are presented in parenthesis.

(Hughes (Safriel (Laborel (Hadfield (Hadfield (Safriel

(Schiaparelli et macrophragma

s (Hadfield et al.

Littoral Range Genus Vermetid alii Vermetus 1972) enderi Vermetud 2006) al. triquetrus Vermetus 1975) gregaria Dendropoma 1972) al. et maximum Dendropoma Lewisand1974) novastoa Dendropoma 1986) petraeum Dendropoma 1975) keenae Petaloconchus 1972) al. et Petaloconchu (Laborel1986) High Intertidal Medial Intertidal X X Low Intertidal X X X X X X X Subtidal X X X X X X X

54

Table 2. Twelve cores and corresponding locations, GPS coordinates, and decompacted lengths.

Core Core Length Number General Locale GPS Location (cm) 1103-1 Camp Key 25° 53’ 24.1” N x 81° 36’ 31.1" W 450 1103-2 Camp Key 25° 53’ 35.1” N x 81° 36’ 32.9” W 505 1106-3 Round Key 25° 50’ 19.9” N x 81° 31’ 44.6” W 416 1203-8 Nursery Island 26° 06' 29" N x 81° 46' 30" W 322 1203-9 Nursery Island 25° 51' 23" N x 81° 31' 02" W 306 1203-10 Nursery Island 25° 52' 12" N x 81° 31’ 07" W 310 1205-18 Turtle Key 25° 52’ 55.6” N x 81° 35’ 11.5” W 291 1205-19 Turtle Key 25° 53’ 12.6” N x 81° 35’ 11.3” W 291 1205-20 Turtle Key 25° 53’ 29.1” N x 81° 35’ 06.8” W 237 1205-22 White Horse Key 25° 52’ 09.6” N x 81° 34’ 54.1” W 338 1205-23 White Horse Key 25° 52’ 33.3” N x 81° 33’ 46.5” W 196 1205-24 White Horse Key 25° 52’ 55.9” N x 81° 33’ 39.8” W 270

55

Table 3. Mean, median, minimum, and maximum density reported for surface boundstones from each of the three sites tested. Standard deviation from the mean also reported for all three sites.

Surface Boundstone Densities (percent cover) Site Mean Median Min Max SD Camp Key Eastern Shoal 31.3 25.5 16.5 68.5 16.8 Camp Key Main Island 48.9 44.5 26 70.5 15.1 Round Key 85.6 90.5 64 98.5 12.0

56

Table 4. Radiocarbon samples and ages with location and elevation relative to sea level as defined by NAVD88 elevation datum. Each sample’s height is also provided in meters below MHW. Camp Key elevations are marked with an asterisk because those elevations are taken of the reef crest and represent the maximum height achieved by the vermetid reefs on the parallel structure near Camp Key. Samples 611, 18-85, 18-121, 8-77, and 8-194 came from below the surface within cores and were directly measured in relation to distance from lowest mangrove leaves. One sigma calibrated age ranges (68% probability) and two sigma calibrated age ranges (95% probability) are provided for each sample. Calibrated age in years before present (yBP) is the midpoint of the 1 sigma range.

57

Table 5. Table showing precise elevations of vermetid reefs from the reef profiles on the Eastern Shoal of Camp Key, Round Key, and Nursery Island.

Reef Surface Relative to Mean High Water

Site Minimum Elevation (m) Maximum Elevation (m) Reef Thickness (cm) Camp Key -1.276 -0.657 61.9 Round Key -1.298 -0.705 59.3 Nursery Island -1.256 -0.697 55.9

58

Table 6. This table shows maximum elevations of relict vermetid reefs and modern-day productive oyster reefs. The mean upper bound of oyster productivity is 0.464 meters below mean high water.

Maximum Trimble Elevation Relative Sample Location Elevation (m) to MHW (m) Vermetid Reef Camp Key -0.357 -0.657 Vermetid Reef Round Key -0.405 -0.705 Vermetid Reef Nursery Island -0.405 -0.705 Modern Oyster Reef Horseshoe Keys -0.140 -0.440 Modern Oyster Reef Lindland Reef -0.184 -0.484 Modern Oyster Reef Blackwater Bay -0.168 -0.468 59

Figure 1. From Toscano and Macintyre (2003), exhibiting changes in the position of relative sea level throughout the Atlantic during the Holocene. Radiometrically dated mangrove peats and Acropora palmata are plotted according to their position above or below sea level. Dates are reported in calibrated years before present. 60

Figure 2. A high-resolution sea-level curve for the Gulf of Mexico spanning the past 7,000 years. Horizontal bars denote sea-level highstands. From Balsillie and Donoghue (2004). 61

Figure 3. Broad range Holocene sea-level curve from Balsillie and Donoghue (2004) spanning the past 22,000 years. The high-resolution shows instances of quick rises and draw-downs in sea level during the Holocene transgressive period. 62

Figure 4. From Savarese and Hoeflein (2012), a sea-level curve developed for Southwest Florida using local sea- level indicators during the mid- to late Holocene. 63

Figure 5. Two regions of Southwest Florida considered in this study. From Savarese et al. (2004). Estero Bay was used as a reference site at times throughout this study. Previous stratigraphic analysis has been done here by Savarese et al. (2004), Wohlpart (2007), and Hoye (2008).

64

Figure 6. A: Map of mangrove islands within the Ten Thousand Islands National Wildlife Refuge (outlined in black). B: Enlarged section of the inland islands. C: Enlarged section of the outer islands. 65

Figure 7. Models showing coral reef development during two different modes of accommodation space. A: Reef accretion is vertical; corals are keeping up with SLR as long as ample accommodation space overhead is available. B: Accommodation space is limited while SLR is slow or has stalled; reef accretion is directed seaward. From Kennedy and Woodroffe (2002). 66

Figure 8. Detailed maps of studied sites in A: the Ten Thousand Islands and B: Estero Bay. 67

Figure 9. Image of a vermetid boundstone from Round Key that was sampled for density. 68

1106-3 Center of Vermetid Platter Formation, Round Key Ten Thousand Islands National Wildlife Refuge Described on: 6/19/11 Core Length: 416cm 0cm last vermetid Facies A (0-86cm) Dense Vermetid Boundstone Fine Clay Sediment 28cm, 1030+/- 30 yBP No sand present after between 40-70cm Vermetid Tubes Vertically Oriented

A 50cm

first vermetid 86cm Facies B (86-161cm) 100cm Very fine sand, no clay Small broken shells abundant B Moderate taphonomic preservation Packages of light sand 105cm-C contact Broken shells oriented in layers

161cm Facies C (161-416cm) Gradual change from Clayey sand to Sandy Clay C Clay component increases down facies Shells are mostly whole, more prominent 300cm-end 200cm Poor taphonomic preservation

250cm

300cm

350cm

400cm

416cm

Figure 10. Stratigraphic column of core 1106-3 showing the location of the radiometric date (in red) 28 cm below the reef surface within the reef boundstone. This diagram and all others to follow represent cores that have been decompacted using a homogenous decompaction model (see text for methods). 69

Figure 11. Image showing locations of the radiometric dates obtained from surface boundstones on Camp Key. 70

Figure 12. Twelve sediment cores taken within the Ten Thousand Islands National Wildlife Refuge in 2011 and 2012. 71

Figure 13. Sites where the mean high water calibration elevations were taken. Oyster reefs were measured at A: Camp Key and B: Lindland reef. The elevations of lowest mangrove leaves were recorded in C: Blackwater Bay. D: Oyster reef heights and lowest mangrove leaves were also measured on Horseshoe Keys in Estero Bay.

72

Figure 14. Differences in island geomorphology are evident from aerial photography. A: Camp Key to the left and the eastern shoal to its right have twin linear extensions of sediment buildup following the tidal gradient. B: Round Key and the islands around it exhibit a circular geomorphology. 73

Figure 15. A: The relict vermetid reef crests upward and out of the sediment. B: Vertically oriented vermetid individuals exhibiting preferential upward growth. C: Vermetids competed for space with the eastern oyster on Camp Key and the shoal to the east. 74

Figure 16. Circular growth morphology of vermetids at Round Key. A: Platters are common across the reef surface, ranging from 0.5 to 1 meter in diameter. B: Many platters next to one another form a circular dome generally around 10 meters in diameter C: Domes also grow within close proximity to one another. D: Domes make up lobes that are commonly seen in aerial photography, ranging from 50 to 100 meters in diameter. The yellow line represents a 50 meter scale for image D only. 75

Figure 17. Locations from which the Camp Key Main Island and Eastern Shoal surface boundstone samples were collected in the field. 76

Figure 18. A: Locator map showing the three locations of profiles taken with the Trimble RTK8 unit. B: The eastern shoal adjacent to Camp Key was 15 meters long. C: The profile transect of the dome in front of Round Key was 15 meters long. D: The profile of another dome just inshore of Round Key was measured for 8 meters. 77

Figure 19. The profiles recorded at the Camp Key, Round Key, and Nursery Island are reported in meters below MHW. All profiles reach a similar maximum height, but Camp Key has significantly more relief when compared to Round Key and Nursery Island. 78

1103-1 Southwestern Tip of Vermetid Reef, Camp Key Ten Thousand Islands National Wildlife Refuge Described on: 3/30/11 Core Length: 450cm 0cm Facies A (0-43cm) Very Shelly Gravels Broken Vermetid Tubes A Fragmented Oysters Some Small Root Fibers

B 43cm Facies B (43-56cm) Degraded Roots 56cm Poorly Develped Peat C Facies C (56-84cm) Oyster Fragments Poorly Sorted Mud 84cm, Shell Gravel Last Vermetid D Facies D (84-118cm) Vermetid Boundstone Fine Mud and Shell Gravel 118cm 121cm-First Vermetid Facies E (118-231cm) Fine Silt with Increasing Gravel Component Oyster Valves with Great Preservation up to 140cm Increasing Shell Gravel Through Facies E 150cm Shell Hash Layer at End of Facies

200cm

231cm Facies F (231-335cm) F Very Fine Mud 250cm Some Shell Gravels Shell Hash 209-225 Coarse Mud with No Shell First appearance of oyster shell

300cm Shell Hash and Whole Shells 265-304 Chione sp.

335cm Facies G (335-386cm) G Degraded Organic and Peaty Matter Sand/Mud laminations Intact Root Materials Near H contact

386cm

H Facies H (386-427cm) Sand with Large Organic Material Poorly Developed Peat 379-387

427cm Facies I (427-450cm) I Muddy Sand Becomes Sand with Laminations Large Degraded Root Persists Throughout 450cm

Figure 20. Stratigraphic column of core 1103-1, taken from the crest of the relict vermetid reef surface, located on the distal shoal seaward of Camp Key. 79

1103-2 Mangrove island inshore of 1103-1, Camp Key Ten Thousand Islands National Wildlife Refuge Described on: 3/31/11 Core Length: 505cm 0cm Facies A (0-129cm) Vermetid gravel rich A Larger fragmented vermetid clumps throughout Entire facies gravel rich Quartz sand 50cm No organics in facies

100cm

Facies B (129-309cm) 150cm Muddy sand B Entire facies shell rich Many broken marine bivalves and gastropods Some whole shells down-facies

200cm

250cm

300cm Facies C (309-355cm) Shell rich at B contact C Degraded organics throughout facies

350cm Facies D (355-417cm) Shelly layer at C contact Organic rich D Shell presence lost after 370 cm Sediment fining down-facies

400cm

Facies E (417-505cm) Sandy clay Charred organics Iron oxidation of sediment E 450cm

505cm

Figure 21. Stratigraphic column of core 1103-2 taken landward of 1103-1 on the southern shore of Camp Key. 80

1106-3 Center of Vermetid Platter Formation, Round Key Ten Thousand Islands National Wildlife Refuge Described on: 6/19/11 Core Length: 416cm 0cm Facies A (0-86cm) Dense Vermetid Boundstone Fine Clay Sediment 28cm, 1030+/- 30 yBP No sand present after between 40-70cm Vermetid Tubes Vertically Oriented

A 50cm

first vermetid 86cm Facies B (86-161cm) 100cm Very fine sand, no clay Small broken shells abundant B Moderate taphonomic preservation Packages of light sand 105cm-C contact Broken shells oriented in layers

161cm Facies C (161-416cm) Gradual change from Clayey sand to Sandy Clay C Clay component increases down facies Shells are mostly whole, more prominent 300cm-end 200cm Poor taphonomic preservation

250cm

300cm

350cm

400cm

416cm

Figure 22. Stratigraphic column of 1106-3, the most seaward core along the Round Key transect. Core was taken from the center of a vermetid platter. Numbers in red denote radiocarbon age. 81

1203-8 Lobate Mangrove Island Landward of Round Key Ten Thousand Islands National Wildlife Refuge Described on: 3/13/12 Core Length: 322cm 0cm Facies A (0-23cm) A Fine quartz sand Organics prominent 23cm Shell fragments Facies B1 (23-52cm) B1 Sandy mud, weak peat development Fragmented oysters 52cm Shell gravels Facies B2 (52-90cm) Muddy clay, dense peat B2 No shell gravels Many root casts 90cm Well preserved oyster full and half shells 2810+/-30 yBP, last vermetid Facies C (90-226cm) Silty clay Dense vermetid boundstone Oyster intergrowth more prominent down facies C Vermetid shell fragments interspersed through facies No organics past C contact boundary Oyster and vermetid shells well preserved 150cm

200cm

226cm 3870+/-70 yBP, first vermetid Facies D (226-306cm) D Clay transitions to muddy clay down facies 250cm Loss of whole shells Shell fragments

Facies E (306-322cm) 306cm Organic-rich mud E Well developed peat 322cm Small and large root fibers and casts Figure 23. Stratigraphic column of core 1203-8 on Nursery Island, just landward of 1106-3 along the Round Key transect. Numbers in red denote radiocarbon age. 82

1203-9 Seaward edge of lobate mangrove island, landward of 1203-8 Ten Thousand Islands National Wildlife Refuge D escribed on: 3/14/12 Core Length: 306cm 0cm Facies A (0-130cm) Muddy Clay Oyster shells scattered throughout organics present from 0-35 cm light in color Shell gravels common, many vermetid A Whole shells present (bivalves and 50cm gastropods <1cm in size)

100cm Facies B (130-306cm) Sandy clay No organics present No oysters present Some whole bivalves and gastropods Most bivalves disarticulated 150cm B Vermetid fragment @ 210cm Individual vermetid @ 255cm

200cm

250cm

306cm

Figure 24. Stratigraphic column of core 1203-9, landward of 1203-8 along the Round Key transect. 83

1203-10 Seaward edge of mangrove island along tidal channel, landward of 1203-9 Ten Thousand Islands National Wildlife Refuge Described on: 3/15/12 Core Length: 310cm 0cm Facies A (0-75cm) Sandy mud Organic rich, root/stick pieces Articulated and disarticulated oysters prominent A well-preserved until 20cm to contact at B Sand content increases toward facies B 50cm

75cm Facies B (75-310cm) Muddy sand No oysters present 100cm B Contact marked by shell rich layer 2nd shell layer present at 95cm Facies rich in shell gravels Sand content decreases until finally lost at the end of facies Whole bivalve and gastropods present throughout

150cm

200cm

250cm

310cm

Figure 25. Stratigraphic column of core 1203-10, the most inland of all cores in the Round Key transect. 84

1205-18 Lobe on east side of Turtle Key, most seaward of transect Ten Thousand Islands National Wildlife Refuge Described on: 5/29/12 Core Length: 291cm 0cm A1 Facies A (0-11cm) 11cm Fine quartz sand and organics Vermetid gravels A2 Facies A2 (11-93cm) Fine mud and shell gravel Loosely developed peat and root casts 50cm Fragmented oysters throughout peaty material Root casts lost at 70 cm Vermetid cluster present at 72 cm

2225 +/- 65 yBP 93cm Facies B (93-119cm) B Very fine clay loam Vermetids loosely contained in matrix Other bivlaves present in loose vermetid facies 119cm C Facies C (119-291cm) Fine, consolidated clay 2475 +/- 125 yBP Fragmented shells and gravels throughout facies Small root fibers present until 200 cm 150cm Vermetids lost at 165 cm Oysters abundant (disarticulated and fragmented)

200cm

250cm

291cm

Figure 26. A stratigraphic column of core 1205-18 located on Turtle Key, as the most seaward of the Turtle Key transect. Numbers in red denote radiocarbon age. 85

1205-19 Turtle Key, landward of 1205-18 along transect Ten Thousand Islands National Wildlife Refuge Described on: 5/31/12 Core Length: 291cm 0cm Facies A (0-156cm) Fine clay loam A Dense oysters and other marine shells Organics present Transition to muddy clay down-facies

50cm

100cm

B 156cm Facies B (156-218cm) Fine clay Organics rich at contact Oyster prescence lost Shell hash bed 122cm Vermetid fragments found at 155cm

218cm Facies C1 (218-257cm) C1 Clayey sand Many fragmented shell gravels 50% matrix Razor clams, chione, and other bivalves articulated

257cm Facies C2 (257-291cm) C2 Clay with very little fine sand Shell poor Vermetid cluster at 197cm Shell gravel bed at 203cm 291cm

Figure 27. Stratigraphic column of core 1205-19, landward of 1205-18. 86

1205-20 Landward of Turtle Key, inland of 1205-19 along transect Ten Thousand Islands National Wildlife Refuge Described on: 6/1/12 Core Length: 237cm 0cm Facies A (0-90cm) Fine mud Rich organic material (fine pieces) A Disarticulated oysters prevalent Shell gravels prominent from 25-30cm, lost and reappear from 40-50cm 50cm 28cm-down, root fibers oriented horizontally No sand found in facies

90cm B Facies B (90-213cm) Sandy clay from contact to 110cm Many small gastropod and bivalve shells Organics up to 110cm, sond content lost Larger whole bivalves from 110-120cm Fine clay/mud until 140 Sand content becomes prominent again Fine shell gravels increasing again towards C contact 150cm

213cm Facies C (213-237cm) Contact marked by loss of sand and mottling C Well-developed peat and mud Large mangrove root casts 237cm

Figure 28. Stratigraphic column of the most landward core on the Turtle Key transect, 1205-20. 87

1205-22 Island behind White Horse Key, most seaward on transect Ten Thousand Islands National Wildlife Refuge Described on: 6/4/12 Core Length: 338cm 0cm Facies A (0-72cm) Muddy clay loam High organics at surface Sand and shell gravel more prominent at 28cm A 33cm, gravels and sand content increases and organics lost Well-preserved oysters persist throughout facies

72cm

B Facies B (72-163cm) At contact, gradual shift from clayey sand to silty clay 100cm Well preserved bivalves and gastropods throughout Shell gravels Vermetid gravels present through lower half of B Loss of organics

163cm Facies C (163-338cm) Contact marked by sand lenses and thin laminations C Fine silty clay Shell gravel lost at contact 200cm Shell gravels reappear at 160cm, lost from 240-260 Mottling and darkening down-facies From 170 cm down-core sediment becomes more sandy 250 to end, all sand lost Shell gravel abundant 260 to end of core Root casts present at end of facies

250cm

338cm

Figure 29. Stratigraphic column of core 1205-22. 88

1205-23 Landward of White Horse Key, landward of 1205-22 on transect Ten Thousand Islands National Wildlife Refuge Described on: 6/4/12 Core Length: 196cm 0cm Facies A (0-19cm) A Silty clay 19cm Many small root fibers Fragmented oysters Organics present with small roots

50cm Facies B1 (19-135cm) Oysters lost at contact B1 Silty clay Many mangrove roots oriented vertically Prominent horizontal root fibers to 88cm Increasing shell gravel down-facies Disarticulated oysters extend from 110cm to B2 Sandy clay loam gradually increasing to contact at B2 100cm

135cm Facies B2 (135-196cm) Sandy clay Oyster prescence lost B2 Dense shelly fragments Vermetid fragment present Small gastropods and bivalves found within shell gravel 170cm to end of core, thick, red roots oriented vertically Degraded vermetid clump at 155cm

196cm Figure 30. Stratigraphic column of core 1205-23. 89

1205-24 Backside of mangrove island, inland of 1205-23 on transect Ten Thousand Islands National Wildlife Refuge Described on: 6/5/12 Core Length: 270cm 0cm Facies A (0-22cm) A Sandy Clay High gravel compoent 22cm Whole and fragmented vermetids Facies B1 (22-109cm) B1 Gravel and sand lost 50cm Silty clay Small horizontal root fibers prominent Whole and disarticulated oysters throughout Root fibers increase towards B2

Facies B2 (109-114cm) 109cm Shell hash layer with many vermetid gravels B2 114cm Facies B3 (114-156cm) Many fragmented oysters concave down Clay loam with little gravel component Organics increasing towards C B3 Vermetid gravels lost at end of facies 156cm Facies C (156-251cm) C Well developed peat Interlaminated sands at top of facies Silty, clayey mud Root casts and sheaths common through facies 200cm

D 251cm Facies D (251-270cm) Contact marked by mottling with quartz sand Peat poorly developed and lost at end of core 270cm

Figure 31. Stratigraphic column of core 1205-24. 90

Figure 32. A: Aerial view of the coring transect taken at Round Key. B: Fence diagram representing the environmental transitions from A to A” moving seaward to landward on the Round Key transect. Numbers in red denote radiocarbon age. 91

Figure 33. Map suggesting the range of vermetid reef influence on geomorphology. The dashed line represents the region where boundstones were no longer observed in the cores from the Round Key transect. The dotted line represents the inland limit of vermetid gravels present in the cores. The junction that represents the shift from geomorphic control by vermetids to oysters sits somewhere between the two lines shown. 92

Figure 34. A: Turtle Key coring transect moving seaward to landward. B: Sedimentary fence diagram of all three cores. Numbers in red denote radiocarbon age. 93

Figure 35. Boundary indicating the transition between vermetid gastropods and the eastern oyster as the principal reef-builder. The white dashed line denotes the region of significant V. nigricans boundstone development. The dotted line indicates the boundary at which vermetid gravels are present in the cores, but there is no evidence for significant boundstone development. Beyond the dotted line is the region of the TTINWR in which all vermetid presence is lost and C. virginica becomes the reef-builder for those islands. 94

Figure 36. A: Transect illustrating the locations of the most offshore cores. B: Fence diagram illustrating the stratigraphic positions of vermetid reef facies in these three cores. The vermetid boundstone was either formed upon an oyster reef or subtidal marine sands. Numbers in red denote radiocarbon age. 95

Figure 37. Hand-drawn sea-level curve developed using all dated vermetid materials. The stratigraphic heights of dated materials were obtained using a homogenous decompaction algorithm (See Methods). Vermetids were treated as a low intertidal indicator.