A 5700-year-old beach-ridge set at Cape Canaveral,Florida,and its implication for Holocene sea-level history in the southeastern USA

LiDAR mapping of beach-ridges

Compilation of a Cape Canaveral beach-ridge plain LiDAR digital elevation map was accomplished using the 2007 LiDAR coverage available for download at http://digir.fiu.edu/LIDAR/LIDARNew.php and processed using the Global Mapper© GIS software package. The LiDAR data was processed to display bare ground elevations only and eliminated other potential elevations such as structures and vegetation from consideration. The vertical resolution of the LiDAR data is 0.15 m. A custom color gradient was developed in order to display subtle elevation changes over the relatively flat terrain as a Digital Elevation Model (DEM). For the full suite of LiDAR images and base data see Doran et al. (2014). A highly detailed topographic map was produced from the LiDAR images, which permitted the beach ridges to be organized into 16 beach-ridge sets (Figure 2a, b). Additionally, three detailed crest-perpendicular topographic profiles constructed from the LiDAR data. Profile AA′ was positioned to cross the swales in more proximal locations relative to ancient, eastern tips (Figure 2b) and profile DD′ to cross the swales in more distal positions (Figure 2b). A third crest-perpendicular topographic profile PA-PA′ was positioned to cross the crests of Beach-ridge Set 1 starting at the location of core CCT22 and extending to the dune fringe immediately west of the False Cape beach-ridge set (Figure 3a). As far as ridge and swale elevations are concerned, these profiles can be considered purely random. These topographic profiles contain examples of the following geomorphologies: (1) individual ridges separated by distinct swales, (2) amalgamated ridge and swale sets with elevations of the intervening swales noticeably higher than the obvious “bounding” swales, (3) swales of much lower, but nearly concordant, elevations which occur in all of the beach-ridge sets, and (4) isolated ridges that are markedly higher than the vast majority of ridges. A contour map of Beach-ridge Set 1 was constructed from the DEM using a contour interval of 0.3 m with zero being Mean Sea Level (MSL); this map shows the detailed topography of ridges and, more importantly, of the intervening swales (Supplemental Figure 1).

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Figure 3.

(a) This LiDAR image shows the region of Beach-ridge Set 1, the oldest subaerial, coastal deposit preserved at the Cape Canaveral beach-ridge plain. The white numbers 1 and 2 identify the beach-ridge sets, the borders of which are shown by the solid white lines. The very much younger False Cape Beach-ridge Set is separated by a dune fringe (DF) that transgresses over Beach-ridge Sets 1 and 2. The northern segment of topographic profile AA′ is shown by a white dashed line as is the Beach-ridge Set 1 crest perpendicular topographic profile PA-PA′ (Figure 8). Sand samples for OSL dating were collected in Core CCT22 which is located at the landward margin of Beach-ridge Set 1. Sand samples for OSL dating were also collected from pits dug into the crests of the two oldest beach ridges in Set 1, sites CCAFS1 and CCAFS2. (b) The stratigraphic column shows the various facies units described in core CCT22. No sedimentary structures were observed except for the upper and lower planar contacts of the blue, plastic clay. The four OSL samples taken in this core are located with the short black arrows. The double-headed arrow marks the interval where a disconformity is hypothesized to exist. The evidence for this disconformity is the 1500-year age difference between OSL samples collected from −283 and −411 cm; there is no physical surface.

OSL dating

OSL samples were primarily collected from three vertical cores each targeting three geographically distinct beach ridge sets along the Cape Canaveral beach-ridge plain. Core CCT22, which targeted the subsurface sand facies ~20 m west of the oldest beach ridge of Set 1, was collected by push-coring with a 5 cm diameter opaque black plastic tube (Figure 3). Two samples (CCAFS1 and CCAFS2) were also collected from the crests of the two oldest beach ridges of Set 1 by horizontally hammering 8 cm diameter aluminum tubes into the basal walls of ~1 m deep and wide, hand-dug pits (Figure 3a). CCT14 at Beach-ridge Set 8 was collected using similar push-coring techniques. At Beach-ridge Set 14, core CCPS02 was collected through the overlying freshwater by VibraCoring with an 8 cm diameter aluminum pipe (Figure 2b). The core lithologies were described in the field using clear tubes that were obtained by push coring adjacent to the opaque black tubes that were used for OSL dating.

The cores used for OSL dating were transported to the AGE Laboratory at McMaster University (Hamilton, Canada). In a darkroom laboratory under dim orange light, the cores were opened, and sandy intervals were sampled for OSL preparation and analysis. Following oven drying (~60°C), the samples were first treated with 10% HCl and 30% H₂O₂ to remove carbonates and organic material, respectively. Standard dry sieve methods were used to obtain the 212–150 µm grain size fraction. Heavy minerals were then removed from the 212–150 µm grain size fraction for each sample using Lithium polytungstate (2.70 g/cm³). The remaining quartz grains were treated with 40% HF for 40 min to etch the alpha-irradiated surface of the grains (Aitken, 1998). The samples were then immediately treated with a 10% HCl solution for 15 min and then re-sieved with 150 µm sieve mesh.

Untreated sub-samples (~30 g) of the samples removed for dating were used to determine the radioelemental concentrations of U, Th, and K via neutron activation analysis and delayed neutron counting (conducted at the McMaster Nuclear Reactor) for dose rate calculation. Gravimetric moisture content was measured for all samples. Dose rate calculations incorporated dose rate conversion factors from Liritzis et al. (2013) and Guérin et al. (2011), estimated moisture content over the burial period, and cosmic dose contribution following Prescott and Hutton (1994). All annual dose rates include an internal contribution of 10.5 μGy/a (Rink and Odom, 1991). Dose rate calculations were carried out using the DRAC v. 1.2 (Durcan et al., 2015).

Multigrain aliquots were prepared on 9.8 mm-diameter stainless steel discs. Grains were loaded onto the stainless steel discs by applying Silkospray™ through a 0.5 mm-diameter mask (~15 grains). Considering only 5–10% of quartz grains contain a measurable OSL signal (Duller et al., 2000), we expect the 0.5 mm aliquots to closely approximate single grain measurements. Luminescence measurements on samples from cores CCT22, CCT14, and CCPS02 were conducted using a Risø OSL/TL-DA-15 reader. Photons were detected through an EMI 9635QA photomultiplier tube fitted with a 7 mm-thick Hoya U-340 filter. Measurements were carried out using blue diodes (470 ± 30 nm) operating at 90% power (~30 mW/cm²). Laboratory irradiations were performed using a calibrated ⁹⁰Sr/⁹⁰Y radioactive source attached to the Risø luminescence reader (dose rates of ~0.08 Gy/s). Measurements on samples CCAFS1 and CCAFS2 were made using the same experimental set-up but on an automated Risø TL-DA-20 luminescence reader housed at the Desert Research Institute in Reno, Nevada, USA with a ⁹⁰Sr/⁹⁰Y beta radioactive source delivering ~0.13 Gy/s.

Equivalent dose (D_E) measurements followed a standard SAR protocol with an embedded IR depletion test (Duller, 2003; Murray and Wintle, 2000, 2003). A total stimulation time of 100 s was used for measurements; the OSL signal was integrated from the first 0.4 s of the decay curve and the subtracted background integrated from the last the last 4 s. Dose recovery tests (Murray and Wintle, 2003) applying different preheat temperatures (ranging from 160°C to 260°C) were performed on 12 aliquots of CCT22-3 in order to test determine the optimal preheat temperature for OSL measurement. Laboratory bleaching for dose recovery tests was carried out using blue light stimulation for 400 s. Thermal transfer tests were also carried out over the same range of preheat temperatures to assess the possibility of charge transfer from light insensitive shallow traps to light sensitive OSL traps following preheating (Rhodes, 2000). A preheat temperature of 200°C and a cut-heat of 160°C was chosen for use in subsequent D_E measurements.

All measurements were required to pass the following criteria for further analysis: <10% test dose error, <10% recycling ratio error, <10% recuperation, and a signal greater than 3σ above background. An additional acceptance criterion, a fast ratio of >20, was also applied to each aliquot (Duller, 2012; Durcan and Duller, 2011). All individual D_E values incorporated an instrumental error of 1.5%. D_E values used for age calculation were modeled using the central age model (CAM) of Galbraith et al. (1999) with a σ_b of 0.1 added in quadrature to reflect the intrinsic overdispersion in D_E’s determined through dose recovery tests. OSL ages were determined by dividing the resultant D_E by the total environmental dose rate of the surrounding sediments (Aitken, 1998; Durcan et al., 2015). Errors on age estimates were added in quadrature and incorporate errors related to dose rate and modeled D_E calculations.

Radiocarbon dating

Gastropod and disarticulated pelecypod shells were collected for radiocarbon dating by the author Stapor in the late 1970s, before the advent of OSL dating, from beach-rock exposed in wave-cut cliffs at Launch Complex (LC) 34 (10 samples) and LC19 (11 samples) both of which are located within Beach-ridge Set 5 (Figure 2a). Sample pretreatment and analysis was conducted at the South Carolina Marine Resources Research Institute; liquid scintillation beta counting was employed to determine the uncorrected C14 age. Ages were corrected for carbon isotope fractionation using an assumed δ¹³C of 0‰ reflecting the average of marine shells (Keith and Weber, 1964). The corrected radiocarbon ages were converted to calendar ages using the Marine13 calibration curve (Reimer et al., 2013) with no localized reservoir correction. Beta Analytic Radiocarbon Laboratory of Miami, Florida carried out the calibration (Ron Hatfield, written communication). By convention, radiocarbon ages are reported in cal years BP, where 0 BP is the year 1950.

OSL performance and ages

Samples consistently showed good reproducibility of the given dose throughout the range of preheat temperatures used in dose recovery experiments. Thermal transfer tests also showed no evidence for thermal transfer over the range of preheat temperatures tested (Figure 4a). Signals were bright and generally dominated by the fast component (Figure 4b, <5% of all measured aliquots were rejected on the basis of the fast ratio test). Growth curves displayed exponential growth and samples had natural signals well below signal saturation (Figure 4b). Sensitivity changes showed little change over the course of the SAR cycle. Sensitivity decreased but remained within ~20% of the first test dose response. Recuperation in our samples had a mean of ~1.4%, further supporting the appropriateness of the applied SAR protocol to these samples.

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Figure 4.

(a) Results of a dose recovery (upper) and thermal transfer test (lower) for sample CCT22-4. The cut-heat was fixed at 160°C for all measurements. (b) A typical quartz decay curves for a 0.5 mm (~15 grains) aliquot of sample CCT22-4. The inset shows a SAR dose response curve for the same aliquot. (c) Representative equivalent dose distribution for sample CCAFS1 displayed as a probability density plot. (d) Radial plot showing that 87% of data fall within 2σ of the CAM equivalent dose (shaded region).

Analytical data for luminescence dating are summarized in Table 1. The low percentage of accepted aliquots across all samples was largely the result of insensitive grains– a direct consequence of carrying out measurements of small aliquots. D_E distributions were normally distributed and displayed low overdispersion (<20%) which is typical of coastal sediments (e.g. Ballarini et al., 2003). A representative D_E distribution for sample CCAFS1 is displayed as both a probability density plot and radial plot in Figure 4c and d, respectively. D_E distributions for all other samples can be found in Supplemental Figure 2. Samples collected from core CCT22 displayed higher overdispersion compared to the other samples, potentially owing to incomplete bleaching of sediments within the subtidal conditions in which the core CCT22 sediments were deposited. Subtidal sediments may have a greater probability of incomplete bleaching relative to foreshore or intertidal deposits due to the attenuation of solar radiation by water absorption and turbidity (Berger, 1990). An additional external source of D_E scatter may be related to microdosimetry, a probable feature of coastal Florida’s low dose rate sediments (Mayya et al., 2006), or bioturbation (e.g. Bateman et al., 2007; Rink et al., 2013).

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Table 1.

A summary of quartz OSL equivalent doses, dose rates, and calculated ages.

Sample name	Depth below the surface (cm)	U (ppm) a	Th (ppm) a	K (%) a	Moisture content (%)	Cosmic dose rate (µGy/a) b	Total dose rate (µGy/a) c	Aliquots accepted (measured)	Equivalent dose (Gy)	Over-dispersion (%)	Age (a)
CCT22-1	172	0.93 ± 0.1	0.63 ± 0.05	0.0077 ± 0.001	6.7	168 ± 17	430 ± 25	13 (48)	1.56 ± 0.07	9.3 ± 2.6	3630 ± 270
CCT22-2	283	0.85 ± 0.1	0.27 ± 0.03	0.014 ± 0.002	3.7	147 ± 15	380 ± 24	12 (48)	1.33 ± 0.06	9.6 ± 2.1	3500 ± 270
CCT22-3	411	0.56 ± 0.1	0.28 ± 0.03	0.012 ± 0.002	5.7	127 ± 13	290 ± 21	17 (48)	1.58 ± 0.07	11.1 ± 4.4	5450 ± 470
CCT22-4	534	0.51 ± 0.1	0.36 ± 0.03	0.013 ± 0.002	6.0	111 ± 11	267 ± 20	11 (48)	1.50 ± 0.09	14.8 ± 5.5	5590 ± 540
CCAFS1	80	0.84 ± 0.1	2.25 ± 0.23	0.017 ± 0.002	5.5	188 ± 19	547 ± 30	23 (96)	3.07 ± 0.08	1.0 ± 0.6	5620 ± 345
CCAFS2	90	0.28 ± 0.1	0.35 ± 0.04	0.033 ± 0.003	5.5	185 ± 19	311 ± 25	25 (96)	1.88 ± 0.05	1.6 ± 1.2	6050 ± 510
CCT14	382	0.97 ± 0.1	1.03 ± 0.07	0.25 ± 0.007	3.2	131 ± 13	672 ± 32	18 (48)	0.83 ± 0.04	4.8 ± 2.6	1240 ± 80
CCPS02	144	1.7 ± 0.1	2.42 ± 0.05	0.55 ± 0.016	30.0	174 ± 17	1008 ± 36	16 (48)	0.26 ± 0.02	3.7 ± 2.8	260 ± 22

a

U, Th, and K values determined by neutron activation analysis and delayed neutron counting at the Nuclear Reactor facility of McMaster University.

b

Cosmic dose rate value calculated using an overburden density of 1.8 g/cm³ and accounting for depth of sample, altitude, and geographic location. The cosmic dose rate assumes instant sediment accumulation above the sample position (Prescott and Hutton, 1994).

c

All annual dose rates include an internal alpha and beta dose in quartz of 10.5 μGy/a (Rink and Odom, 1991). Dose rates were determined using the conversion factors of (Liritzis et al., 2013) assuming secular equilibrium in the ²³⁸U and ²³²Th decay chains, and using the grain size attenuation factors of Guérin et al. (2011).

The fundamental control on the reliability of OSL ages is whether they support the sedimentologic and stratigraphic interpretations based on geologic evidence. In the case of beach-ridge plain studies they must agree with the relative stratigraphic age relationships of the various beach ridges and sets. In order to meet this requirement, it is imperative that a suite of OSL ages be determined. The calculated OSL ages describe a coherent Holocene-age sequence with decreasing age toward the southeast of the beach-ridge plain (Table 1). Samples collected from the CCT22 core at Beach-ridge Set 1 have stratigraphically consistent ages ranging from 3630 ± 270 years to 5590 ± 540 years. The two OSL samples collected from the crests of the geographically oldest beach ridges, CCAFS1 (5620 ± 345 years) and CCAFS2 (6050 ± 510 years) (~25–150 m southeast of CCT22, respectively, Figure 3a), have ages that are statistically indistinguishable from the ages of the lower two samples collected from CCT22 (Table 1). Consistent with beach-ridge geography, OSL ages decrease toward the southeast from 1240 ± 80 years at Beach-ridge Set 8 (CCT14) to 260 ± 22 years at Beach-ridge Set 14 (CCPS02) (Table 1).

Radiocarbon ages

Calibrated radiocarbon ages, and their respective uncorrected values, made on individual marine mollusk shells from LC34 and LC19 are shown in Table 2. Ages at each site describe a ~20,000-year spectrum ranging from 2100 to 25,000 cal years BP indicating that reworking of shells from older, nearby deposits and their redeposition into younger deposits had occurred (Table 2). The youngest age at LC34 is 2100 ± 100 and 2300 ± 175 cal years BP at LC19. Stapor et al. (1991) argue that a cluster of overlapping ages at the younger end of a spectrum of shells can be used to more accurately estimate the actual, rather than just a maximum, depositional age of a deposit containing reworked shells. The larger the number of individual ages in a cluster, the greater the probability that the clusters average age estimates the actual depositional age. There is no such cluster at either the LC34 or the LC19 spectrums and thus their youngest ages of 2100 and 2300 cal years BP, respectively, are only maximum estimates and thus deposition at these two sites can be no older than these respective ages and may well be much younger. It should be noted that although a combined total of 21 individual shells were radiocarbon dated at these 2 sites, only a maximum estimate of depositional age for Beach-ridge Set 5 could be determined. We consider all of the shell radiocarbon ages of Brooks (1972) to be only maximum estimates of depositional ages as well. This maximum age estimate of Beach-ridge Set 5 is within the ~2000 year-interval of the adjacent OSL ages of Rink and Forrest (2005) determined at Beach-ridge Set 4 (CC6: 4020 ± 170, Figure 2a) and Beach-ridge Set 7 (CC2: 1880 ± 90, Figure 2a). The ~20,000 year-spectrum of ages determined on these 21 samples dramatically illustrates that redeposition of older shells into much younger deposits is a major complicating factor in the ability of shells to indicate depositional age.

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Table 2.

Radiocarbon ages from Launch Complex 34 and 19, Beach-ridge Set 5. The calibrated ages with the white background could have been reworked from known, earlier Cape Canaveral deposits. The ages with the dark gray background most likely are reworked from the nearby Pleistocene deposits of Merritt Island; their ages are most likely the result of Holocene contamination of ~100,000-year old material. The ages with the light gray background are most likely from shells reworked from Holocene sediments located a short distance offshore which were deposited at earlier and lower sea levels. There are no clusters of overlapping ages at the young ends of these two age distributions and therefore the youngest respective ages should be considered maximum estimates of the depositional ages.

Location	MRRI* a sample number	Mollusk species	Uncorrected 14C Age a (years BP)	Calibrated 14C Age b (cal years BP)
Launch Complex 34 Beach-Ridge Set 5	165	Busycon carica	2030 ± 80	2094 ± 100
	162	Anadara sp.	3920 ± 130	4466 ± 173
	163	Mercenaria sp.	4900 ± 120	5685 ± 130
	169	Busycon carica	5090 ± 130	5887 ± 144
	166	Busycon carica	6110 ± 180	7012 ± 196
	168	Busycon carica	17,220 ± 600	20,867 ± 724
	170	Busycon carica	18,260 ± 380	22,016 ± 463
	164	Busycon carica	19,740 ± 400	23,791 ± 508
	167	Anadara sp.	19,980 ± 710	24,075 ± 791
	161	Busycon carica	20,620 ± 480	24,790 ± 528
Launch Complex 19 Beach-Ridge Set 5	372	Anadara brasiliana	2180 ± 130	2308 ± 175
	371	Anadara ovalis	3050 ± 120	3318 ± 154
	370	Anadara ovalis	3960 ± 160	4494 ± 212
	131	Busycon carica	4210 ± 80	4820 ± 120
	143	Anadara sp.	4560 ± 170	5243 ± 199
	134	Anadara ovalis	5030 ± 290	5793 ± 334
	138	Dinocardium robustum	6990 ± 370	7916 ± 386
	144	Anadara sp.	7357 ± 220	8259 ± 248
	130	Anadara brasiliana	19,250 ± 970	23,232 ± 1142
	132	Busycon carica	20,080 ± 640	24,191 ± 730
	145	Busycon carica	21,080 ± 900	25,268 ± 983

a

South Carolina Marine Resources Research Institute.

b

Beta Analytic Radiocarbon Laboratory, Miami, Florida.

Although these radiocarbon results were unable to determine the actual depositional age of Beach-ridge Set 5, they provide significant information about the sources that may have supplied the shells deposited in Set 5. The ages less than 6000 cal years BP (Table 2) most likely have come from the erosion of the previously deposited Beach-ridge Sets 1–4. The ages 7000- and 8000-cal years BP, highlighted in light gray in Table 2, most likely have come from the erosion of an as yet unidentified, earlier Holocene deposit that formed at a lower sea level. The ages 20,000 cal years BP and older, highlighted in darker gray in Table 2, are possibly the result of an undetectable <3% contamination by modern carbon and/or <5% undetected recrystallization in shells that are essentially “dead” with respect to C14 (Chappell and Polach, 1972; Olsson, 1968; Olsson et al., 1968). Furthermore, 20,000 years ago during the Wisconsin glacial lowstand, sea level was some 120 m below present day (Berger, 2013). The survival of marine shells transported across some 100 km of nearshore shelf to be deposited into Holocene beach ridges formed arguably within no more than ~5 m of present-day MSL is extremely problematic. A simpler explanation for these >20,000 cal years BP shells is that they have been eroded from the geographically and topographically nearby late Pleistocene Anastasia Formation dated by OSL to near 100,000 years (Burdette et al., 2009) that comprises Merritt Island (Cape Chester, Figure 1), and were contaminated with modern carbon at some time during their transit. Thus, the older radiocarbon ages of marine mollusk shells that comprise these two spectrums can help to identify potential nearby sources of the material redeposited into Beach-ridge Set 5.

A revised chronometric framework for the Cape Canaveral beach-ridge plain

A combination of the OSL ages of Rink and Forrest (2005) and the OSL and radiocarbon ages presented in this paper has enabled the creation of a much more complete chronometric framework for the Cape Canaveral beach-ridge plain (Figure 5). The oldest preserved deposits are some 5700 years old (Beach-ridge Set 1) and the youngest (Beach-ridge Set 16) are less than 150 years old. Most of the existing Cape Canaveral beach-ridge plain (75% of total area) was deposited in no more than 1400 years and is represented by: (1) Beach-ridge Sets 5 and 6 (24% of total area) deposited over no more than 200 years between 2100 and 1900 years, and (2) Beach-ridge Sets 8–16 (51% of total area) deposited over 1000 years between 1150 and present-day. The remaining 4300 years are presented by (1) Beach-ridge Sets 1–4, and (2) the erosional truncations or disconformities that define the boundaries between the various beach-ridge sets, especially those that bound Beach-ridge Set 4.

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Figure 5.

Chronometric framework diagram for the Cape Canaveral beach-ridge plain based on: (1) OSL ages (black stars) from Rink and Forrest (2005), (2) OSL ages (white stars) from this paper, and (3) radiocarbon ages (white pentagons) from this paper. The Cape Canaveral beach-ridge plain records a 5700-year period of alternating deposition and erosion. The majority of the existing Cape Canaveral beach-ridge plain (75% of total area) was deposited over no more than 1400 years of the 5700-year long period and is represented by: (1) Beach-ridge Sets 5 and 6 (24% of total area) deposited over no more than 200 years between 2100 and 1900 years, and (2) Beach-ridge Sets 8–16 (51% of total area) deposited over 1000 years between 1150 and present-day. The remaining 4300 years are represented by (1) Beach-ridge Sets 1–4, and (2) the erosional truncations or disconformities that define the boundaries between the various beach-ridge sets.

The OSL depositional age of 1880 ± 90 years for the oldest beach ridge present in the adjacent, younger Beach-ridge Set 7 (CC2 of Rink and Forrest, 2005) provides a younger bound to the deposition of Beach-ridge Sets 5 and 6. This younger bound strongly suggests that the part of Beach-ridge Set 5 between LC34 and the youngest beach ridge truncated by the erosion boundary between Sets 6 and 7 was deposited over no more than some 300 years, the difference between the 1σ maximum depositional age estimate of 2100 ± 100 cal years BP at LC34 and the 1σ minimum actual depositional OSL age of 1880 ± 90 years for Set 7.

The OSL age of 1240 ± 80 years from Core CCT14, taken in one of the oldest beach ridges in Set 8, is a maximum estimate for the deposition of Beach-ridge Sets 8–15. The OSL ages of 260 ± 22 years for the sand base of the major swale in Set 14 (Core CCPS02) and 150 ± 25 years for one of the youngest beach ridges in Set 15 (CC7 of Rink and Forrest, 2005) confirm that deposition of Beach-ridge Sets 8–15 occurred over no more than 1000 years.

Cape Canaveral beach-ridge geography and geomorphology

The LiDAR-based map produced in this study emphasizes that the Cape Canaveral beach ridges are oriented in a northeast-southwest direction and that the progressive accretion of beach ridges occurred in a south-easterly direction (Figure 2b). The Cape Canaveral beach ridges are (1) composed primarily of quartz sand with minor amounts of shell fragments, (2) organized into sets that contain multiple parallel to subparallel, mostly regularly-spaced, continuous, linear to gently curved, wall-like ridges with variations in crest and swale elevations along strike typically less than 1 m, and (3) have crests that are essentially parallel to the southeast-facing shoreline. The 16 mapped beach ridge sets are separated from each other by erosional truncations, onlapping of older by younger ridges, and abrupt southwest shifts of southeastward prograding ridges (Figure 2a). The occurrence of the beach ridges organized into sets separated by erosional truncations indicates fluctuations in the local sand supply between abundance and shortage with the former resulting in deposition and shoreline progradation and the latter in erosion and shoreline retreat.

The geometry of the Cape Canaveral swales changes dramatically along depositional strike from the proximal area of the southeast tip toward the more distal areas to the southwest (Figure 6). At the southeast tip many swales have a crescent-shape which follows the convex pattern of the beach ridges, with a NW-SE dimension perpendicular to strike of up to several hundred meters (Figure 6). Both ridges and swales have been significantly truncated by long-term erosion occurring along the NE-facing shore. These crescent-shape swales contain freshwater lakes that are commonly floored with a several decimeter-thick layer of very fine-grained organic debris and minor amounts of siliciclastic clay overlying fine- to medium-grained quartz sand containing marine shells. The base of the crescent-shape, lake-filled swale in Beach-Ridge Set 14 (CCPS02, Figure 6) occurs at an elevation of −0.6 m MSL which is within 3 cm of the Mean Lower Low Water (MLLW) at the Cocoa Beach Atlantic Pier tide gage (Figure 7); this concordance is not surprising given that the marine-shell bearing sand at the base of this freshwater lake is only 260 ± 22 years old (CCPS02, this paper). This pattern of convex seaward beach ridges and 100 m wide swales suggests a relatively high rate of progradation with beach-ridge formation occurring on the tops of emergent nearshore bars perhaps similar to the mechanism proposed by Hine (1979). This crescent-shape swale pattern changes southwestward to the classic linear, narrow geometry within less than 1 km of the southeast tip (Figure 6).

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Figure 6.

The southeast tip of Cape Canaveral showing both the crescent-shape and linear, narrow swales. Sw designates individual linear, narrow swales within a particular beach-ridge set; the other numbers identify crescent-shape swales within respective beach-ridge sets. AA′ and DD′ are topographic profiles. CCPS02 locates the core which penetrated to the fine- to medium-grained sand containing marine shells at the base of crescent-shape swale 14; sample CCPS02 (−144 cm) yielded an OSL age of 260 ± 22 years. Core CCT14 locates the initiation of beach ridge deposition at Beach-ridge Set 8; sample CCT14 (−382 cm) yielded an OSL age of 1240 ± 80 years.

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Figure 7.

A matrix showing the elevations of the lowest swales present in the various Cape Canaveral beach-ridge sets. P is the lowest elevation of the proximal portion and D the distal portion of the linear, narrow swales. MHHW is Mean Higher High Water and MLLW is Mean Lower Low Water. The columns for Beach-ridge Set 1 and Set 14 are highlighted in gray. The numbers on the dashed lines are the distances in kilometers separating the respective P- and D-values.

The vast majority of swales separating the Cape Canaveral beach ridges have the narrow, linear geomorphology common to beach-ridge plains worldwide. The ridge crest elevations indeed have considerable variability, probably due to differences in the amount of aeolian dune caps or decoration, however, the elevations of the lowest swales (all of the narrow, linear geometry) in the various beach-ridge sets are, unexpectedly, markedly similar. Proximal (P, from profile AA′ of Figure 8 and Supplemental Figure 3) and more distal values (D, from profile DD′ of Supplemental Figure 4) of the lowest-elevation, linear, narrow swales are shown in Figure 7 along with the present-day tidal datums of MHHW and MLLW as determined at the Cocoa Beach Atlantic Pier tide gage (USDC, 2020). Except for Beach-ridge Set 11, P-values are lower than their associated D-values, however, differences ⩽0.15 m are at or below the level of LiDAR resolution and may not be significant (Figure 7). About 90% of the P-values of the 11 beach-ridge sets that have both P- and D- values for their respective lowest swales are within 0.5 m of present-day MHHW.

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Figure 8.

(a) LiDAR image of the northernmost part of Cape Canaveral that contains Beach-ridge Sets (BRS) 1–5. The darker areas are of lower elevations (swales) and the lighter areas are higher elevations (crests). FC locates the False Cape beach-ridge set. DF is the dune fringe landward of the False Cape beach ridges. Topographic profiles are designated, PA-PA′, AA′ Segments 1 and 2. Individual swales are identified with 1 (BRS 1), 2 (BRS 2), 3 (BRS 3), and 4 (BRS 4). Notice that swales 1c and 1d are present in section PA-PA′ but do not extend southwest to Segment 1 of Section AA′. LC34 is Launch Complex 34. (b) Topographic profiles PA-PA′ and AA′ Segments 1–2 have been constructed from LiDAR data with a 0.15 m vertical resolution; the vertical exaggeration of these sections is 115×. Because profiles AA′ Segments 1–2 are not perpendicular to the ridge crests the widths of both crests and swales are somewhat exaggerated. BRS is Beach-ridge Set, Sw is a particular swale, Dd? is possible dune decoration. Numbers are OSL ages in years; the number with (RC) is the youngest radiocarbon age (in cal years BP) of a spectrum of ages made on single shells collected in a sea cliff at Launch Complex 34 (LC34). Notice that the lowest swales in BRS 2–5 are essentially of equal elevation, about 1 m MSL, which is equal to the elevations of the majority of the swales in BRS 1. Only BRS 1 swales 1b and 1c are lower at an elevation of present-day MHHW.

There are four major potential sources that probably supplied sediment during the deposition of the Cape Canaveral beach-ridge plain: (1) southeast littoral transport from regions further to the north, (2) erosion of the NE-facing shoreline, (3) net onshore transport from the adjacent Southeast Shoal, and (4) direct onshore transport from the adjacent nearshore shelf. The littoral-drift model of Stapor and May (1983), based on present-day bathymetry and wave parameters of deep-water height and period, indicates that littoral transport along the NE Florida coast is highly compartmentalized with numerous cells separated by drift divides and convergences. One such divide is located ~20 km north of False Cape with a predicted southward littoral transport of no more than 30,000 m³/year coming from local erosion of this 20 km stretch of coast. The predicted existence of a drift divide along this NE-facing shoreline allows for the net, long-term southern transport of the majority of the material eroded from the Pleistocene Cape Chester beach-ridge plain of Merritt Island, the southern half of which is now missing (Figure 1). Net onshore transport from Southeast Shoal as well as direct onshore transport from the immediate nearshore cannot be evaluated at present.

Beach-ridge Set 1 geomorphology and stratigraphic interpretation

Beach-ridge Set 1 has one of the smaller areas (0.47 km²) of all the 16 identified set remnants. However, it contains the oldest, and topographically lowest, beach ridges preserved in the Cape Canaveral beach-ridge plain (Figures 3 and 8). The constituent ridges have crest elevations of ~1.5 m MSL, are linear, and gently converge to the south (Figure 8). The swales consist of fine- to medium-grained quartz sand, the same material as the ridge crests, and have elevations that range from 0.5 to 1.1 m MSL (profile AA′, Segments 1 and 2 and profile PA-PA’ of Figure 8). The low value is ~0.1 m below the MHHW elevation recorded at the nearby Cocoa Beach Atlantic Pier tide gage.

Because the beach ridges preserved in Beach-ridge Set 1 converge to the south, the younger swales are truncated southward by even younger beach ridges: Swales 1c and 1d shown in the crest-perpendicular profile PA-PA′ are not present in profile AA′ (Segment 1, Figure 8). The +0.55 m MSL elevation of Swale 1c of profile PA-PA′ is the lowest elevation present in Beach-ridge Set 1 and the lowest elevation of all the linear, narrow swales recorded on the LiDAR-based topographic profiles AA′-Segments 1 and 2 (Figure 8). Swale 1b, the next lowest in Beach-ridge Set 1, is at an elevation equal to present-day MHHW, and the two remaining Set 1 swales, 1a and 1d, are 0.4 m above MHHW (Figure 8). These swales are composed of fine to medium sand; silt, and organic and/or siliciclastic clay-size material is absent, unlike the crescent-shape swales located near the eastern tip. These very similar swale elevations occur in Beach-ridge Sets 1–3 that were deposited over a 1700-year interval (Figure 8). The ridge-crest to swale-bottom relief ranges from 0.75 to 1.25 m in Beach-ridge Sets 1 and 2 while in Set 3 the range is 1–1.5 m (Figure 8). The topographic inset map of the portion of Beach-ridge Set 1, covers the area that includes core CCT22, section PA-PA′, OSL sample CCAFS1, and OSL sample CCAFS2, clearly shows that these low swale elevations extend throughout the respective swales intercepted by the various topographic profiles (Figure 3a for topographic map location and Supplemental Figure 1 for contour map).

Core CCT22 is located some 20 m west of the oldest ridge in Beach-ridge Set 1. The sediments penetrated in this core comprise a sequence of fine- to medium grained, quartz sand with a variable content of moderately sorted shell fragments; these shell fragments are primarily of marine pelecypods and gastropods (Figure 3b). There were no visible sedimentary structures in core CCT22 except for the bedding plane which separates the overlying shelly sand from the basal clay. The marine shell content and the juxtaposition immediately adjacent to a beach-ridge plain strongly argue that, at the very least, a significant fraction of the sediments present in the lower half of core CCT22 were deposited in a sub-tidal setting, most probably on a shoreface.

Core CCT22 provides evidence for the age of the earliest Holocene sediments preserved on the Cape Canaveral beach-ridge plain. The sequence of OSL ages show a clear trend of decreasing age moving upward in the core, ranging from 5590 ± 540 years at the base (CCT22-4) and 3630 ± 270 years for the uppermost sample (CCT22-1; Table 1 and Figure 3b). However, the basal 1.25 m (CCT22-3: 5450 ± 470 years and CCT22-4: 5590 ± 540 years) is of a similar age and the section between −178 cm (CCT22-1: 3630 ± 270 years) and −283 cm (CCT22-2: 3500 ± 270 years) is also of a similar age. These two sections are different by some 1500–2000 years, suggesting either a continuous, but variable, depositional rate or the existence of at least two discrete depositional episodes separated by a disconformity between samples CCT22-2 and CCT22-3. Because there are no sedimentary structures present that might suggest an erosion surface, a more exact location for this disconformity is not possible without additional OSL dating (Figure 9).

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Figure 9.

A stratigraphic section and topographic profile of the ridges comprising Beach Ridge Set 1, the geographically oldest ridge set on the Canaveral Peninsula. Superimposed on this figure are (1) the OSL sample locations for the ages reported in this study, and (2) a generalized lithologic description of core CCT22. The position of Mean Higher High Water (MHHW) is from the Cocoa Beach Atlantic Pier; MSL at this location is 0.575 m NAVD88 (USDC, 2020). Observe that the swales are either at present-day MHHW or several decimeters above. The vertical portion of Disconformity 1 is required by the 1500–2000 year age difference between core samples CCT22-1 (−172 cm, 3630 ± 270 years) and CCT22-2 (−283 cm, 3500 ± 270 years) and sample CCAFS1 (5620 ± 345 years) from the top of the geographically adjacent, and topographically elevated, oldest beach-ridge of Beach-ridge Set 1. Disconformity 1 continues downward separating the younger material in core CCT22 from the older deposits of Beach-ridge Set 1. Because the ages of core samples CCT22-3 (−411 cm, 5450 ± 470 years) and CCT22-4 (−534 cm, 5620 ± 345 years) are statistically indistinguishable from the age of sample CCAFS1, and CCAFS2 (6050 ± 510 years) as well, these two deposits can be considered co-eval and Disconformity 1 can reasonably be projected into the core somewhere between sample CCT22-2 (3500 ± 270 years) and CCT22-3 (5450 ± 470 years). Because there are no sedimentary structures present that might suggest an erosion surface, a more exact location for of Disconformity 1 is not possible without additional OSL dating. Disconformity 2 is the projection of the truncation boundary that separates Beach-ridge Sets 1 and 2 (Figure 3); disconformity 3 occurs between the overlying dune fringe and the underlying deposits of Beach-ridge Set 2. The 5500–6000 year-old MSL position of Khan et al. (2017) and Hawkes et al. (2016) are shown with the gray and dark gray 2σ bands, respectively. Using this sea-level position, the entire Beach-ridge Set 1 topography would have had to result from 5 to 6 m high wave run-up. This vertical component of wave run-up should require offshore wave heights of 7–8.5 m (Guza and Thornton, 1982) or a significant breaker height of 5–7 m (Roberts et al., 2010); both of these estimates are at least five to six times the present-day, average, significant wave-height determined by Corson et al. (1981) and three to four times the storm wave-height estimate (95th percentile of measured waves) of Thompson (1977).

OSL samples CCAFS1 and 2, collected from ~1 m below the crests of the adjacent oldest and second-oldest beach ridges comprising Beach-ridge Set 1, have yielded ages of 5620 ± 345 and 6050 ± 510 years respectively, and are statistically indistinguishable from the ages of the basal 1.25 m of core CCT22 (CCT22-3: 5450 ± 470 years and CCT22-4: 5590 ± 540 years). Together the four samples yield an average age of 5680 ± 240 years (hereafter rounded to 5700 years). The 5500–6000 year-old, beach-ridge crest samples are 3.5–4 m above the uppermost section (−283 to −178 cm) of core CCT22 which is 2000 years younger, a geometric relationship that requires the existence of a significant disconformity between this uppermost section of core CCT22 and Beach-ridge Set 1 ridge-crest sample CCAFS1. We interpret the two beach-ridge crest samples, CCAFS1 and CCAFS2, and the two subtidal samples from the adjacent core CCT22 from 4 to 5 m depth to represent the initial supra-/intertidal and subtidal deposits respectively of a beach-ridge plain that prograded into this area some 5500–6000 years ago (Figure 9).

The marine-shell-rich sand composition of the ~3500 year deposits in the upper 3 m of core CCT22 argues for deposition in a littoral or marine-influenced environment, however, the continuity of the seaward-adjacent ridges comprising Beach-ridge Set 1 precludes any westward-directed sediment transport across them. We hypothesize that these ~3500-year deposits represent distal portions of flood-tidal deltas that were formed when a barrier was breached at some nearby position farther to the north. This hypothesized paleogeography was similar to that of the present-day False Cape beach-ridge set and the adjacent, narrow Canaveral National Seashore barrier (Figure 1) that has extensive washover and relic flood-tidal delta deposits extending west into the Mosquito Lagoon (Mehta and Brooks, 1973). This hypothesized breaching occurred subsequent to the deposition of Beach-ridge Set 3 and prior to or sometime during the early period of the deposition of Beach-ridge Set 4, given the 4020 ± 170 year OSL age (CC6 in Figure 2a, Rink and Forrest, 2005) on the boundary between Sets 3 and 4. We suggest that the digitate deposits that extend westward from the False Cape Beach-ridge Set into the northern portions of the Banana River (Figure 1) are remnants of other ancient flood-tidal deltas that originally formed landward of pre-existing, similar, narrow barriers. The boundary between Beach-ridge Sets 1 and 2 as well as the contact between the “dune fringe” and the underlying Beach-ridge Set 2 are also disconformities (Figure 9).

The OSL ages reported here for the initial deposition of Beach-ridge Set 1 are ~1000 years younger than the 6610 ± 60 years BP radiocarbon age of Beach-ridge Set 2 or 3 reported by Brooks (1972) and ~1500 years younger than the oldest radiocarbon shell age (7067 ± 80 years BP) reported for Beach-ridge Set 3 or 6 (Figure 2a). This is not surprising if one considers the ages reported by Brooks (1972) to have been determined from reworked clasts which need not estimate the time of deposition (Stapor et al., 1991). However, these OSL ages for Beach-ridge Set 1 are as expected older than the 3740 ± 200 years OSL age reported by Rink and Forrest (2005) for the younger Beach-ridge Set 3 (Figure 2a).

Sea-level interpretation from beach ridges

It should be expected that there is a relationship between beach ridges and sea level, although any such calibration has been, and still can be, difficult to estimate (Otvos, 2000). The boundary between swash zone deposits and any overlying aeolian deposits has been used to estimate the uppermost level of constructive wave action and thus can be regarded as an indicator of sea level (Otvos, 2000). However, it can be difficult to obtain natural or man-made exposures that show the critical sedimentary structures that may be able to identify this interface; commonly used core tubes are not wide enough to adequately identify the critical sedimentary structures. Ground penetrating radar (GPR) can under the right circumstances image these critical structures, especially the beach/subtidal interface (Tamura et al., 2008). However, if the GPR survey uses a sand road bull-dozed nearly flat across or along the ridges, the uppermost part of the radargram may not properly image the upper meter of the deposit as well as not image much, if any, of the ridge itself (Rodriguez and Meyer, 2006). Van Heteren et al. (2000) were able to readily identify the swash-formed, gravel-rich sand from the immediately overlying, dune-deposited sand and interpreted this boundary to be a sea-level indicator, however, the ability of grain-size characteristics of fine to medium sand to differentiate aeolian from swash deposits is a matter of some dispute (see Otvos, 2000; Tanner, 1995 for opposing views).

Unfortunately, there are no natural cliff exposures of Beach-ridge Set 1 and the banks of the canal that crosses this set are so overgrown and slumped that any internal structures present are not visible. In addition, there is no available GPR survey. Thus, estimating the MSL during deposition must be based solely on the geomorphology of the ridges and swales. The ridge crests are linear, continuous and show no obvious effects of wind action, such as the markedly irregular downwind margin of the dune fringe that formed prior to the deposition of the False Cape beach-ridge set (Figures 3 and 8). However, aeolian dune sand capping or decorating the Set 1 ridges cannot at present be ruled out. The elevation of the intervening sand-floored swales can provide a better estimate of sea level in that a swale represents back-beach or supratidal deposition that experienced minimal wave run-up, compared to that responsible for beach-ridge construction, and can be expected to have a minimal amount of dune decoration because of the potential for reworking by subsequent, infrequent, higher-water events. One should consider the lowest swale elevations to be the best estimators of MHHW, because any wave runup will skew elevations upward and thus the lowest swale elevation will be the most conservative estimator. It should be emphasized that this strategy should only be used in swales composed of sand-size material because this material has to be moved by traction transport and is essentially not compressible. It is striking that the 16 beach-ridge sets present in the Cape Canaveral beach-ridge plain, which span some 5700 years, have only a 0.5 m range in the elevation of their lowest, linear, narrow swales (Figure 7) and, furthermore, the vast majority of these lowest swales are within 0.3 m of present-day MHHW.

Sea-level interpretation of the 5700-year beach-ridge Set 1

Based on the elevations alone of the sand-floored swales it is not unreasonable to hypothesize that Beach-ridge Set 1 was deposited at a MHHW position within decimeters of present-day MHHW and hence MSL. If one uses the 5700-year sea-level position of Hawkes et al. (2016) and Khan et al. (2017) then the entire Beach-ridge Set 1 topography would have had to result from a wave run-up that had a vertical component 5–6 m high. This vertical component would necessitate a significant deep-water wave-height of 7–8.5 m (Guza and Thornton, 1982) or a significant breaker-height of 5–7 m (Roberts et al., 2010); both of these estimates are at least five to six times the present-day, average, significant wave-height determined by Corson et al. (1981) and three to four times the storm wave-height estimate (95th percentile of measured waves) of Thompson (1977), in other words, a dramatically different wave climate than what exists today along the Cape Canaveral coast. Furthermore, the minimum elevations of the linear, narrow swales presented in Figure 7 further suggest that this hypothesis can be extended to cover the deposition of all of the beach-ridge sets comprising Holocene Cape Canaveral. This hypothesis does not rule out the occurrence of lower-than-present sea-level events over the past 5700 years, it only argues that any such lower events were not recorded in the preserved beach-ridge deposits of Cape Canaveral. This 5700-year sea-level position hypothesized to be equal to or within decimeters of present-day for the Cape Canaveral Beach-ridge Set 1 is a significant addition to the Gulf of Mexico and southeastern United States sea-level data collected from onshore (subaerial) sites (see Figure 5A in Balsillie and Donoghue, 2004 for a compilation).

Discussion of Holocene sea-level data from the southeastern United States

The vast majority of Holocene sea-level data from the southeastern United States come from the interpretation of supra- to intertidal, organic-rich material recovered in cores which sample subsurface Holocene deposits that directly overlie Pleistocene material. This has resulted in a depositional model of coastal onlap interpreted to have been caused by a continuous, monotonically rising sea-level resulting from the gradual collapse of a glacial forebulge (Clark et al., 1978; Roy and Peltier, 2015; Walcott, 1972). Simply put, no matter how expertly one can determine a depositional environment nor how accurately one can determine a depositional age, if one is looking in the wrong place one will never identify a deposit formed at a sea-level position equal to, or higher than, present day. That this probable bias against the recognition of “highstand” events may well be inherent to the onlapping, intertidal paradigm, is a fundamental issue, and should not be dismissed as trivial.

A critical review of submarine-collected, coral, sea-level data from subtidal coral reefs in south Florida by Stathakopoulos et al. (2020) has resulted in a significant reduction in the number of previously used data points. They evaluated the visual taphonomic characteristics of 303 existing coral data points and selected 134 as being suitable sea-level indicators. Of these 134 suitable samples only 6 samples of the commonly used Acropora palmata (shallow water, monospecific framework deposits, 5 m ecologic range, Lighty et al., 1982) define the SLR covering the past 6000 years (Figure 10). Rise and fall events less than this 5 m range may be difficult to identify, although certain “finger-like” morphologic features have been used to identify and/or interpret reef-crest samples from deeper water samples (Lighty et al., 1982). The observation that in-situ corals only comprise from 15% to 40% of total reef mass (Blanchon and Perry, 2004; Hubbard et al., 1998) presents a further limiting factor as does the challenge of identifying such in-situ corals in cores (Hubbard et al., 1998). Khan et al. (2017) presents a compilation of Caribbean coral data. It is unlikely that subtidal coral deposits from a previous sea-level position equal to that of today can be readily recognized given that such a deposit would serve as an excellent substrate for subsequent colonization by encrusting corals and, furthermore, any such disconformable interface could be difficult to identify in a core of only several inches in width.

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Figure 10.

The Cape Canaveral Beach-ridge Set 1 position versus the sea-level curves of (1) Hawkes et al. (2016) for northeast Florida and Khan et al. (2017) for southeast Florida, both based on subsurface data and (2) the Balsillie and Donoghue (2004) curve, a 7-point floating average, based on onshore data from the northern Gulf of Mexico and south Florida. Four OSL ages with a mean of 5680 ± 240 years provide the chronometric estimate of the depositional age of the Cape Canaveral Beach-ridge Set 1. Calibrated radiocarbon ages provide the chronometric control for the Hawkes et al. (2016), Khan et al. (2017), and the Stathakopoulos et al. (2020) sea-level data; OSL ages provide the chronometric control for the Cape Canaveral Beach-ridge Set 1. The small, solid, black squares show the mangrove peat data points; error estimates for those data in the 4000–6000-year range are shown by the white crosses. The Hawkes et al. (2016) basal estuarine foraminifera data points and error estimates are shown by the dashed line rectangles. The white and black circles are the Acropora palmata data points from Stathakopoulos et al. (2020); their associated 2σ error estimates are shown by the black vertical lines. The 2σ regression envelopes for the Hawkes et al. (2016) and Khan et al. (2017) are shown by the two shaded, sloping bands. This figure is a modification of Figure 4 in Stathakopoulos et al. (2020).

The best, perhaps the only, place to look for deposits of earlier sea-level highstands is up at present-day sea level, the material referents of which can be (1) skeletons of intertidal and/or subtidal organisms that encrust a well-lithified substrate as in Brazil (Angulo et al., 1999; Van Andel and Laborel, 1964) and (2) the inter- and supratidal clastic deposits of beach-ridge plains as in Australia (Hesp, 1999, 2006); geomorphic referents, such as elevated notches and any associated secondary erosional terraces, cannot at present be dated.

Pre-Holocene, well-lithified rocks on which skeletonized, intertidal marine organisms can attach are essentially absent along coasts of the southeastern United States. The Silver Bluff notch cut into lithified, Pleistocene Miami Oolite in Coral Gables, Florida (Cooke, 1945), and the scarps and terraces cut into humate-impregnated, Pleistocene sand scattered along the Franklin Co., Florida, Gulf of Mexico coast (Figure 9 of Stapor, 1975) are secondary, erosional geomorphic features 1–2 m above present-day MSL whose chronometric time of origin, at present, cannot be determined. Thus, beach-ridge plains provide arguably the best places to look for material referents of Holocene sea-level positions equal to, or even higher than, present-day. However, prior to the development of OSL dating, the vast majority of beach ridges in the southeastern United States could not be dated because of their fundamental lack of shells. Wide, non-deltaic, beach-ridge plains that could record sea-level highstands are not common in the southeastern United States. There are only four major candidates: (1) the Little Point Clear and Lake Shelby areas, Baldwin Co., Alabama, have 5500-year records, see Blum et al. (2003) and Rodriguez and Meyer (2006) for opposing sea-level interpretations; (2) St. Vincent Island, Florida, has a 4000-year record (Forrest, 2007; López and Rink, 2007, 2008); (3) Sanibel Island, Florida, has a 3000-year record (Stapor et al., 1991) and (4) Cape Canaveral, Florida, now with a 5700-year record. Stapor et al. (1991) were able to date Sanibel Island beach ridges because this beach-ridge plain alone has abundant shells; the other three previously mentioned beach-ridge plains were dated by means of OSL. This interpreted MSL position of at least equal to present-day for the ~5700-year Cape Canaveral Beach-ridge Set 1 is ~4 m above the arguably coeval positions in both the Hawkes et al. (2016) and the Khan et al. (2017) curves, both based on subsurface data (Figure 10). It is 3.5 m above the coeval Acropora palmata position in southeast Florida (Figure 10, Stathakopoulos et al., 2020). However, it fits on the Balsillie and Donoghue (2004) curve based on onshore or subaerial data from the northern Gulf of Mexico and South Florida (Figure 10). Given the documented recovery of sea-level to it present-day position some 5000–6000 years ago along tectonically stable coasts well removed from centers of continental glaciation, evidence of this recovery should be expected to be found along the Florida Gulf of Mexico and Atlantic coasts, both of which meet the stable tectonic and distant geographic location requirements. The deposition and preservation of beach-ridge plains requires the presence of sand, which, however, is, and apparently has been, in very short supply along the vast majority of the Florida coastline and nearshore shelf, given the paucity of laterally extensive, kilometers-wide, barrier islands and/or beach-ridge plains. These four previously mentioned beach-ridge plains from the Gulf of Mexico coast and the Atlantic coast of Florida strongly argue for the existence of a mid-Holocene, sea-level “highstand” equal to that of present-day which is, however, not indicated by any of the SLRs based on subsurface data. The sea-level index points from beach-ridge plains scattered along the Florida Gulf of Mexico and Atlantic coasts argue for the serious consideration of a regional SLR of the past 5000–6000 years similar to the fluctuating model of Fairbridge (1961).