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CHAPTER 6 CONCLUSIONS This investigation has resulted in a better understanding of the late Quaternary history of the northern Gulf of Mexico, its response to sea-level change and the impact of small-scale changes in sea level on coastal evolution. This investigation sought to test the six hypotheses outlined in chapter 1. Each hypothesis was confirmed over the course of this study and is briefly summarized below. The first hypothesis was that the direct luminescence dating of quartz sand grains in beach ridges can provide a reliable history of barrier beach ridge plain evolution. This has been confirmed by the OSL depositional ages obtained during this investigation, which range from zero to approximately 4,100 years. The ridge with the oldest age is the northernmost site and the ridge with the youngest age is the southernmost site, as was expected. The second hypothesis tested during this investigation was that beach ridge progradation rates and beach ridge heights are influenced primarily by the rate and direction of sea level change. The initial source of the sediment making up the St. Vincent Island strandplain was the nearby Apalachicola River. The ridges that comprise the island’s strandplain were formed from this sediment at rates that varied over the course of the island’s 4,000+year history and appear to be correlated with the direction of sea-level change. At times when sea level was falling or stable, the island’s progradation rates were high. When sea level was rising, progradation rates were low. This could be related to changes in accommodation space and sediment supply associated with a fall in sea level or related to increased erosion as a result of sea level rise. This investigation has confirmed the close relationship between barrier evolution and small-scale changes in the rate and direction of sea-level change. The third hypothesis was that abrupt changes in sea level can have a significant effect on the development of coastal landforms over a geologically brief span of time. The fact that progradation rates changed over what can be considered a geologically short time span, suggests that even brief and relatively small (+/- 1 meter) changes in sea level can have a significant effect on the development of coastal landforms over a geologically brief span of time. 125
The fourth hypothesis was that sea-level history in the northeastern Gulf of Mexico reflects global eustatic history. The sea level curve produced by Siddall et al. (2003) and the Gulf of Mexico sea level history of Balsillie and Donoghue (2004) are very similar with some minor deviations. This suggests that sea level history in the northeastern Gulf of Mexico is a good mirror of global sea level, and that barrier evolution in the northeastern Gulf of Mexico is typical of barriers in general. The fifth hypothesis was that beach ridges may hold evidence of sea-level highstands during the mid- to late Holocene. Of the eight beach ridges dated on the island, this investigation has shown that the majority formed during sea level highstands. These eight ridges represent a small fraction of the 100+ beach ridges covering the surface of St. Vincent Island. Of the ridges dated, those that formed during sea level highstands could lend support to the hypothesis that beach ridges may hold evidence of sea level high stands during the mid to late Holocene. However, this hypothesis requires further investigation. The sixth hypothesis was that beach ridges are built by swash processes, as opposed to storms or aeolian processes. Ground-penetrating radar (GPR) is a non-invasive technique that has proven to be a useful tool in the analysis of the internal structure of barrier island strandplains. It can provide a high-resolution image of the internal character of beach ridges and when used in conjunction with dating techniques can be applied to the study of the evolution of coastal geomorphic features. Based on the GPR studies of the internal structure of the beach ridges on St. Vincent Island, whose subsurface expression consists of a series of seaward-dipping cross-bedded units, the majority of ridges on the island were built by swash processes, as opposed to storm events. The granulometric data from this investigation also indicates that the bulk of the beach ridge sediments are not aeolian. These findings support the hypothesis that beach ridges are built primarily by swash processes rather than storms. Based on the granulometric data collected over the course of this investigation, some additional conclusions, related to the depositional origin of the sediment making up the St. Vincent Island beach ridge plain, can be made. The granulometric analyses indicate that the source of the sand that has formed the beach ridges of St. Vincent Island has changed little over the long history of the island. The original source of the sand is riverine and its most likely source is the nearby Apalachicola River mouth. The granulometric analyses of the ridges also provide several lines of evidence that energy levels were lower when strandplain development 126
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THE FLORIDA STATE UNIVERSITY COLLEG
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ACKNOWLEDGEMENTS I would like to th
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4. RESULTS Laboratory Analyses ....
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LIST OF FIGURES 1.1 Location map of
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preheating is carried out on a heat
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4.17 Site SVI 023. a) Trench. b) Sa
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4.38 Location relative to the Apala
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A.37 Granplot analysis of sample 05
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A.83 Granplot analysis of sample 05
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ABSTRACT The goal of this investiga
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4) To further develop Optically Sti
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the rate of erosion or deposition a
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esult of the buildup of offshore ba
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Recent studies of barriers using Gr
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and, therefore, sea level) (Dorsey,
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Holocene Sea Level Over 900 Holocen
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first is that sea level rose steadi
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Figure 1.2. Beach ridge sets on St.
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-100 Figure 1.4. Pleistocene glacia
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Figure 1.6. The Apalachicola River
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Figure 1.8. Global sea-level histor
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K and L have crest elevations of ap
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Geoarchaeology The earliest inhabit
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Oyster Pond Figure 2.1. Infrared or
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Figure 2.3. St. Vincent Island shor
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Pickalene Middens Paradise Point Si
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At each vibracore or trench locatio
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luminescence signal equivalent to t
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Application of OSL to Coastal Studi
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atio between the first and the fift
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Topographic Surveying Topographic s
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time elapsed between when this ener
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a) b) Figure 3.1. a) Collecting a v
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Increasing energy T CONDUCTION BAND
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Figure 3.5. Sample removed from lar
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Figure 3.7. Location of samples col
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Figure 3.9. Basis of airborne LIDAR
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Sample Sites CHAPTER 4 RESULTS A to
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SVI005 This site is located just we
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collapsed the trench. Therefore, a
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SVI025 This site is located within
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m and 3.7 m respectively. A cross s
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Vincent Island has changed little o
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(sorting). Standard deviation decre
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4,100 and 3,500 years ago. The corr
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Table 4.1 Continued Site Samples Be
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Page 103 and 104: Figure 4.5. Site SVI 003. Trench ex
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Page 107 and 108: a) b) 050505-04C 050505-04A Figure
Page 109 and 110: Figure 4.13. Site SVI 015. Vibracor
Page 111 and 112: a) b) 011006-15 011006-16 Figure 4.
Page 113 and 114: a) b) 011106-08 011106-07 011106-10
Page 115 and 116: Figure 4.21. Dutch gouge-auger core
Page 117 and 118: Figure 4.23. Line A-A’ represents
Page 119 and 120: Elevation (m) NAVD 88 5.00 4.50 4.0
Page 121 and 122: 1200 +/- 100 yr 102 Figure 4.27. GP
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Page 125 and 126: 6 Set B Set C Set E 5 Set A Set F S
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Page 129 and 130: ENVIRONMENTS OF DEPOSITION -- SKEWN
Page 131 and 132: mean grain size (phi) 3 2.5 2 1.5 1
Page 133 and 134: SVI 015 SVI 002 SVI 003 114 SVI 004
Page 135 and 136: CHAPTER 5 DISCUSSION The beach ridg
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Page 147 and 148: APPENDIX A INDIVIDUAL SIEVE ANALYSI
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Berger, G., 1988, Dating quaternary
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Douglas, B., Kearney, M. and Leathe
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Jol, H.M., Peterson, C.D., Vanderbu
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Northwest Florida Water Management
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Stapor, F.W. and Tanner, W.F., 1977
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BIOGRAPHICAL SKETCH Beth Margaret F
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