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emoved without an additional input of energy (Aitken, 1998). If the defect is large enough, electrons can be held for long periods of geologic time. Therefore, over time, defect sites accumulate electrons. The magnitude of the accumulation is therefore a function of time and the production rate of the trapped electrons. This then forms the basis of a geochronologic method. The trapped electrons are released from the defects by exposure to light or heat. Not all electron traps have an equal probability of being emptied (Huntley, 1985). Such differences could be a result of attenuation of light while passing through a mineral grain or a result of differences in the physical properties of the traps (Huntley, 1985). The de-trapping process produces light, or luminescence, representing the release of photons as the electrons return to the ground state. The amount of light emitted is proportional to the number of trapped electrons, and thus the radiation dose accumulated since the sediment was last exposed to light (Huntley and Lian, 1999). If the de-trapping mechanism is light energy, the method is referred to as optically stimulated luminescence (OSL). At the time of sedimentation, the luminescence signal that was acquired in the past is reset to zero. This zeroing is a response to the sunlight exposure that occurs during erosion and transport. After deposition, exposure to ambient radiation then repopulates the traps (Huntley and Lian, 1999). This repopulation continues until the sediment is heated in the lab, or exposed to sunlight during another sedimentation cycle. OSL was developed in 1985 (Huntley et al., 1985) as a method of determining when sediments were last exposed to sunlight. The method involves exciting the trapped electrons in the sediment by shining laser light on grains isolated from a sample and measuring the wavelength and intensity of light emitted in response. To calculate OSL ages, the paleodose is divided by the annual dose rate. The dose rate is the rate at which energy is absorbed by the mineral grain from the incoming flux of radiation (Aitken, 1998). Its components are environmental beta and cosmic radiation, external and internal beta radiation from K and Rb in the crystal lattice and internal alpha radiation from U and Th embedded in the grains (Mejdahl and Christiansen, 1994). This is shown schematically in Figure 3.4. There are also minor contributions to the dose rate from gamma radiation. The dose rate is determined by measuring the radioactivity of the surrounding sediment. The paleodose is the amount of radiation received since the traps were last emptied during a bleaching, or zeroing, event (Smith et al., 1986). The laboratory equivalent of the paleodose is the equivalent dose (DE) which is the laboratory dose of radiation that is needed to induce a 35
luminescence signal equivalent to that acquired by the sample after the most recent bleaching event (Aitken, 1998). The DE is evaluated using either the additive dose method or the regenerative dose method. For the additive dose method, the sample aliquots being measured are divided into several groups. One group is used to measure the natural OSL. The remaining groups are given different radiation doses delivered by an artificial source before being measured. Dose is added with each measurement cycle to build a response curve (Folz and Mercier, 1999). In the regeneration method, all aliquots are bleached to near zero and then given a laboratory dose. The natural aliquots are not given a radiation dose. The natural OSL is then compared to the OSL resulting from aliquots that received a dose. Advantages of Luminescence Dating for Coastal Sediment Deposits Radiocarbon dating has typically been used to date coastal deposits in environments similar to that of the present study. For example, Stapor and Tanner (1977) analyzed thirteen sediment samples from coastal deposits from the Apalachicola region and from nearshore sands seaward of pre-Holocene barriers using radiocarbon dating. They obtained ages ranging between 22,000 and 40,000 yr B.P. A study by Missimer (1973) examined dune ridges on Sanibel Island, a barrier island located 160 km south of Tampa, Florida. The island has seven to twelve sets of subparallel ridges, which have been used to estimate the growth of the island. The age relationships between the ridge sets were determined using radiocarbon dates obtained from aragonitic mollusc shells. The dates ranged between 547 and 4,310 yr B.P. All ridges were found to have been deposited in the past 4,300 years and Missimer (1973) estimated that it takes 14-18 years for each ridge to fully develop. As an alternative to these methods, several studies have applied OSL dating to coastal deposits. Wintle et al. (1998) collected fifteen samples of dune sand and three samples from a core taken in the beach face of a sand spit that protrudes into Dingle Bay in southwest Ireland. Feldspar grains were dated using a single aliquot protocol for infrared stimulated luminescence. No ages over 600 years were obtained. The youngest age obtained was 150 years. These ages are consistent with the belief that the majority of dunes in Ireland formed within the last 6,000 years (Carter, 1986). Jungner et al. (2001) collected eight sediment samples from a parabolic dune in Cape Kiwanda, which is part of a complex of four dunes formed by onshore winter storm 36
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THE FLORIDA STATE UNIVERSITY COLLEG
Page 3 and 4: ACKNOWLEDGEMENTS I would like to th
Page 5 and 6: 4. RESULTS Laboratory Analyses ....
Page 7 and 8: LIST OF FIGURES 1.1 Location map of
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Page 11 and 12: 4.17 Site SVI 023. a) Trench. b) Sa
Page 13 and 14: 4.38 Location relative to the Apala
Page 15 and 16: A.37 Granplot analysis of sample 05
Page 17 and 18: A.83 Granplot analysis of sample 05
Page 19 and 20: ABSTRACT The goal of this investiga
Page 21 and 22: 4) To further develop Optically Sti
Page 23 and 24: the rate of erosion or deposition a
Page 25 and 26: esult of the buildup of offshore ba
Page 27 and 28: Recent studies of barriers using Gr
Page 29 and 30: and, therefore, sea level) (Dorsey,
Page 31 and 32: Holocene Sea Level Over 900 Holocen
Page 33 and 34: first is that sea level rose steadi
Page 35 and 36: Figure 1.2. Beach ridge sets on St.
Page 37 and 38: -100 Figure 1.4. Pleistocene glacia
Page 39 and 40: Figure 1.6. The Apalachicola River
Page 41 and 42: Figure 1.8. Global sea-level histor
Page 43 and 44: K and L have crest elevations of ap
Page 45 and 46: Geoarchaeology The earliest inhabit
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Page 49 and 50: Figure 2.3. St. Vincent Island shor
Page 51 and 52: Pickalene Middens Paradise Point Si
Page 53: At each vibracore or trench locatio
Page 57 and 58: Application of OSL to Coastal Studi
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Page 61 and 62: Topographic Surveying Topographic s
Page 63 and 64: time elapsed between when this ener
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Page 67 and 68: Increasing energy T CONDUCTION BAND
Page 69 and 70: Figure 3.5. Sample removed from lar
Page 71 and 72: Figure 3.7. Location of samples col
Page 73 and 74: Figure 3.9. Basis of airborne LIDAR
Page 75 and 76: Sample Sites CHAPTER 4 RESULTS A to
Page 77 and 78: SVI005 This site is located just we
Page 79 and 80: collapsed the trench. Therefore, a
Page 81 and 82: SVI025 This site is located within
Page 83 and 84: m and 3.7 m respectively. A cross s
Page 85 and 86: Vincent Island has changed little o
Page 87 and 88: (sorting). Standard deviation decre
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Page 91 and 92: Table 4.1 Continued Site Samples Be
Page 93 and 94: Table 4.3. Measured elevations of S
Page 95 and 96: Table 4.5. Application of Tanner’
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a) b) 050505-01E 050505-01K 050505-
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a) b) 050505-04C 050505-04A Figure
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Figure 4.13. Site SVI 015. Vibracor
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a) b) 011006-15 011006-16 Figure 4.
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a) b) 011106-08 011106-07 011106-10
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Figure 4.21. Dutch gouge-auger core
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Figure 4.23. Line A-A’ represents
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Elevation (m) NAVD 88 5.00 4.50 4.0
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1200 +/- 100 yr 102 Figure 4.27. GP
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0.65 Set C 0.6 0.55 0.5 Set A 0.45
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6 Set B Set C Set E 5 Set A Set F S
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0.7 Set C 0.6 Set A 0.5 Set E Set F
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ENVIRONMENTS OF DEPOSITION -- SKEWN
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mean grain size (phi) 3 2.5 2 1.5 1
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SVI 015 SVI 002 SVI 003 114 SVI 004
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CHAPTER 5 DISCUSSION The beach ridg
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vibracore or trench, both of which
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Skewness can be used to identify th
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island. Another is a C-14 date of 2
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sea level. It was at this point tha
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The fourth hypothesis was that sea-
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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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