U.S. Department of the Interior
U.S. Geological Survey
OFR 2007-1203

Synthesized Age Information

In the following discussion the cores are organized geographically, beginning with northern and central Biscayne Bay cores, moving south into the southern Biscayne system, and then into Florida Bay. Figure 1 shows the location of the individual cores. Data for cores with no new or revised age information are included in the tables and the appendices for comparison, but are not discussed below.

Biscayne Bay, Rickenbacker Bank

One core was collected from northern Biscayne Bay just south of the Rickenbacker Causeway (SEI0299 Rick1). Total ²¹⁰Pb reaches background levels at 22-24 cm (Figures 7 and 10). No pollen analyses were conducted on this core. Five radiocarbon dates were obtained from the core with the upper three samples from 0-2 cm, 32-34 cm, and 66-68 cm yielding ages ranging from 468-658 yrBP (Figure 10). The lower samples at 98-100 cm and 134-136 cm yield slightly older dates of 810-957 yrBP and 990-1186 yrBP. Increased LOI values between 40 and 100 cm indicate greater organic content in that interval, which may indicate the presence of a grass bed and accompanying changes in grain size and sedimentation rate. The similar dates over widely spaced samples indicate this site has been subject to sediment disruption and no age model was developed for the core.

Figure 10. Age data for Rickenbacher core (SEI299 Rick1). No age model was developed for this core. Location of core is shown in Figure 1 and described in Table 1. Data shown can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core. [larger image]

Biscayne Bay, Featherbed Banks

Three cores were collected and analyzed from Featherbed Bank: SEI297 FB1 was collected in 1997 on the southwestern side of the bank, and GLW402 FBA and FBB were collected side by side in 2002 on the western edge of the bank. ²¹⁰Pb and pollen biostratigraphy indicate that the upper 70-80 cm of GLW402 FBA were deposited during the 20^th century (Table 2; Figure 2A), yielding average 20^th century sedimentation rates of ~ 0.7 cm yr^-1. Increased LOI values in the upper 20 cm of core GLW402 FBA correspond to a unit of soft mud with Thalassia and shells (Figure 11). Below that depth, sediments consist of firmer muds and clays with scattered shells and plant material and lower organic content. Radiocarbon dates were obtained on the marine gastropod Turbo castanea from three depths in core GLW402 FBB: 109 cm, 137 cm, and 187 cm (Table 3). The ¹³C values of each shell (-0.8 to 1.4 ‰ (per mil)) are consistent with growth in a marine to estuarine environment, and the three dates are in stratigraphic order. Examination of x-radiographs and stratigraphy of the paired cores (FBA and FBB) indicate comparable deposition patterns in both cores (Wingard and others, 2003), justifying construction of a single age model for both cores. A basal age of ~1800 AD is indicated for core GLW402 FBB (Figure 11) and the average pre-20^th century sedimentation rate is ~1.4 cm yr^-1.

Figure 11. Age model developed for 2002 Featherbed Bank cores (GLW402 FBA and FBB). Two cores (FBA and FBB) collected side by side from the western edge of the bank contributed to the development of this age model. Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be fond in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core (data from FBA). Lithologic column is FBB, with differences for FBA indicated. Complete lithologic description can be found in Wingard and others (2003). [larger image]

In core SEI297 FB1, ²¹⁰Pb reaches background levels at 58-60 cm (Table 2, Figure 2B), indicating a similar sedimentation rate (0.6 cm yr^-1) to core GLW402FBA. No pollen data were generated for this core. Core SEI297 FB1 consists entirely of sandy mud with shell fragments and vegetation debris. Scattered peaks in organic content probably correspond to intervals enriched in plant debris (Figure 12). Four radiocarbon dates were obtained on unidentified shells collected at 61 cm, 101 cm, 141 cm, and 221 cm depth (Table 3). Although carbon isotope analyses were not conducted on these shells, the presence of marine-estuarine mollusk, ostracode, and benthic foraminifer assemblages indicates that the assumption of a marine isotope signature is appropriate for determination of conventional radiocarbon ages. Because of the large 2 age range inherent to the youngest radiocarbon date at 61 cm, it was omitted from age model calculation, which is based on the lowest ²¹⁰Pb datum and the dates at 101 cm, 141 cm, and 221 cm. The resulting regression line includes the 2 range of the upper date. A basal age of 1452-1651 AD (346-545 yrBP) is indicated for core SEI297FB1 (Figure 12), and the average pre-20^th century sedimentation rate is ~0.5 cm yr^-1, significantly lower than at GLW402 FBB.

Figure 12. Age model developed for 1997 Featherbed Bank core (SEI297 FB1). Data for the age model are from a single core collected from the southwestern side of the bank. No pollen data were available for this core. The uppermost radiocarbon date was not included in construction of the age model because lead-210 evidence indicates the sample is approximately 100 years old, whereas the 2 sigma range of the radiocarbon date is 90-321 yrBP. However, the two sigma range does intersect the resulting age model. Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be found in Table 3 and the appendix. LOI, loss on ignition, indicates organic material in core. Complete lithologic description can be found in Ishman (1997). [larger image]

Biscayne Bay, No Name Bank

Two cores collected side by side at No Name Bank in 2002 (GLW402 NNA and NNB) were combined to form a single age model (Figure 13), based on stratigraphic and x-radiograph evidence of comparable deposition rates (Wingard and others, 2003). ²¹⁰Pb and pollen biostratigraphy indicate that the upper 60 cm of core GLW402 NNA were deposited during the 20^th century (Table 2, Figure 2C), yielding average 20^th century sedimentation rates of ~0.6 cm yr^-1. Increased LOI values and organic content in the upper 20 cm of core GLW402 NNA, correspond to an interval with abundant Thalassia (Fig. 13). Below 35-40 cm, sediments consist of clays and sandy clays with scattered shells and plant material.

Figure 13. Age model developed for No Name Bank cores (GLW402 NNA and NNB). Two cores (NNA and NNB) collected side by side in 2002 contributed to the development of this age model. The uppermost radiocarbon date was not included in construction of the age model because lead-210 evidence indicates the sample is approximately 100 years old, whereas the 2 sigma range of the radiocarbon date is 108-329 yrBP. However, the two sigma range does intersect the resulting age model. Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core (data from NNA). Lithologic column is NNB, with differences for NNA indicated. Complete lithologic description can be found in Wingard and others (2003). [larger image]

Radiocarbon dates were obtained on two specimens of the gastropod Turbo castanea (65 cm and 89 cm) and one specimen of the bivalve Chione cancellata (139 cm) from core GLW402 NNB (Table 3). Because the uppermost sample has a very broad 2 age range (108-329 yrBP), it was excluded from construction of the age model, which is based on the basal ²¹⁰Pb date at 60 cm and the lower two radiocarbon dates of 114-336 yrBP at 89 cm and 272-453 yrBP at 139 cm (Figure 13). The 2 ranges of the excluded radiocarbon dates lie on the resulting regression line, and the basal radiocarbon date indicates deposition of sediments within the core since about 1600 AD, with an average pre-20^th century sedimentation rate of ~ 0.3 cm yr^-1. The ¹³C range on the mollusk shells in core B was -0.7 to -0.1 ‰, consistent with deposition in a marine to estuarine environment.

Biscayne Bay, Black Point

Two cores were collected in the Black Point area. The core from near the mouth of Black Creek Canal (SEI0297 BP1) was analyzed for ²¹⁰Pb to a depth of 20 cm and yielded no significant change with depth, so no additional analyses were conducted. The core collected just north of Black Point (core GLW603 BPNA) is characterized by variable lithology and organic content (Figure 14) and contains evidence for disruption in the upper portion of the core (above 44 cm). Layers of poorly sorted sands correspond to fluctuations in the total and excess ²¹⁰Pb (Figure 2D). A sharp decrease in mangrove pollen in the upper 4 cm of the core is interpreted as a signal from the destruction of mangroves in the area during Hurricane Andrew in 1992 (Wingard and others, 2004). Organic content is highest in the upper 15 cm and remains fairly high (average 12%) from 15 - 60 cm. Below 60 cm the interbedded sands and clayey sands have the lowest LOI values for the core indicating decreased organics in this segment. The entire core contains large numbers of worn and fragmented shells, with the largest numbers in the zone from 38-62 cm, perhaps due to the location of the core on a bar at the mouth of small tidal inlet where shell debris tends to accumulate.

Figure 14. Age data for Black Point North core (GLW603 BPNA). No age model was developed for this core. Location of core is shown in Figure 1 and described in Table 1. Data shown can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core. Complete lithologic description is available in Wingard and others (2004). [larger image]

The first occurrence of Casuarina pollen is at 20-24 cm, and ²¹⁰Pb reaches background levels between 30 cm and 40 cm depth (Table 2, Figure 2D). A radiocarbon date on a gastropod shell (Prunum sp.) collected at 19 cm depth yielded a conventional radiocarbon age of 120.93 +/- 0.44 pMC (percent modern carbon; Table 3), indicating growth after production of excess ¹⁴C by thermonuclear testing in the 1950's and 1960's (Lowe and Walker, 1997). Collectively, this evidence indicates deposition of 20^th century sediments in the upper 30-40 cm of core GLW603 BPNA, and average sedimentation rates of ~0.4 cm yr^-1.

Radiocarbon dates were obtained on four other samples from GLW603 BPNA: a bivalve Anomalocardia auberiana at 38-40 cm, an unidentified oyster at 62-64 cm, and a wood sample and Anomalocardia specimen both collected at 74-76 cm (Figure 14). Although the samples were collected over a range of nearly 40 cm, all yielded ages of in the range of 355-571 yrBP. ¹³C values for the shells ranged from -4.4 to -5.0 ‰ (Table 3), indicating freshwater influence and the possible need for an additional reservoir correction. Given the distribution of ages within this interval, construction of a pre-20^th century age model is not merited. However, the correspondence of wood and shell dates at 74-76 cm is suggestive that the lower part of the core was deposited ~500 yrBP.

Biscayne Bay, Card Bank

Cores have been collected from two locations at Card Bank: on the southeastern side of the bank in 1997 (SEI297 CB1), and on the northwestern side of the bank in 2002 (GLW402 CBA and CBB). These two sites differ lithologically. Core SEI297 CB1 coarsens upward, with muds predominating in the lower 135 cm of the core, transitioning to shelly sandy mud and shelly sand in the upper 15 cm (Figure 15). Cores GLW402 CBA and CBB were collected side-by-side as replicate cores and consist primarily of muds with scattered shells (Figure 16). Increased organic content in the upper 50-60 cm corresponds to a unit of soft muds with Thalassia and shells.

Figure 15. Age data for 1997 Card Bank core (SEI297 CB1). No pollen data are available for this core and no age model has been developed. Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be found in Table 2, Table 3, and appendix. Complete lithologic description can be found in Ishman (1997). [larger image]

Pollen data were not generated from core SEI297 CB1, and initial ²¹⁰Pb analyses were done only to a depth of 28 cm, yielding no significant change in lead activity with depth. This is interpreted as indicating very rapid deposition, and analyses were not completed. The beginning of 20^th century deposition cannot be identified in this core. Five ¹⁴C dates were obtained from this core at depths ranging from 28 to 132 cm, three on Bittiolum varium and two on shells that were not identified prior to analysis (Figure 15). The two uppermost ¹⁴C samples yielded were collected from zones with high concentrations of worn or fragmented specimens. The sediment characteristics and relatively old ages of these two samples indicate that they may have come from lag deposit and/or potential disruption of the sedimentary sequence. Based on the lowermost shell date, we estimate an age of 900-600 yrBP for the base of the core. The ¹³C range (1.4 to -2.6 ‰) is consistent with deposition in a marine to estuarine environment.

Age information for both 2002 cores is presented in Figure 16 because of the proximity of collection; however, careful examination of sediment descriptions (Wingard and others, 2003) indicates there may be a partial offset between the A and B cores (Figure 16). Although ²¹⁰Pb values appear to reach background levels around 45 cm depth in core GLW402 CBA, there is considerable disruption in the upper 10 cm of the core (Figure 2E). The first occurrence of Casuarina is between 20 cm and 30 cm depth (Table 2). These data indicate that the upper 30-46 cm of core GLW402 CBA were deposited during the 20^th century, providing average sedimentation rates of ~0.3 - 0.5 cm yr^-1 during that period. Three radiocarbon dates were obtained from this pair of cores (Table 3): one a specimen of Chione cancellata (a bivalve) at 123 cm in core GLW402 CBA and two Bittiolum varium (gastropods) from 41 cm and 105 cm in core GLW402 CBB. The uppermost ¹⁴C sample at 40-42 cm in the B core yielded an age of 174-390 yrBP. This is older than samples from the same depth in core GLW402 CBA and supports the stratigraphic offset between the two cores; use of a common age model for the two is not justified in this case.

Figure 16. Age data for 2002 Card Bank cores (GLW402 CBA and CBB). Data from two cores (CBA and CBB) collected side by side in 2002 from the northwestern side of the bank are shown here. No age model was developed for these cores. Location of cores are shown in Figure 1 and described in Table 1. Data shown can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core (data from CBA). Lithologic column is CBB, with differences for CBA indicated. Complete lithologic description can be found in Wingard and others (2003). [larger image]

Biscayne Bay, Middle Key Basin

A single core (GLW603 MKA) was collected from the Middle Key Basin, just south of Card Sound Bridge. This core has a complex lithology, ranging from carbonate mud with abundant shells and plant material at the top (0-44 cm), to peaty muds with abundant terrestrial plant debris (44-92 cm), to carbonate muds and limestone rubble from the underlying bedrock at the base of the core (92-114.5 cm) (Figure 17). LOI analyses were only completed to a depth of 62 cm; below that level the sediment was so rich in organics that there was a possibility of volatizing the ²¹⁰Po needed for the ²¹⁰Pb analyses. ²¹⁰Pb reaches background levels at approximately 26-28 cm, and the first appearance of Casuarina pollen is at 20-22 cm (Table 2, Figure 2F). The upper 28 cm of sediments were deposited during the 20^th century, yielding average sedimentation rates of ~0.3 cm yr^-1.

Figure 17. Age data available for Middle Key Basin core (GLW603 MKA). No pre-20th century age model was developed for this core. Location of core is shown in Figure 1 and described in Table 1. Data shown can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core. Complete lithologic description available in Wingard and others (2004). [larger image]

Four ¹⁴C ages were obtained on two samples of wood (41 cm and 79 cm) and two freshwater gastropods (Physa sp.) (25 cm and 41 cm). All four dates are >700 yrBP, and they are not in stratigraphic order (Table 3, Figure 17). The uppermost date of 749 - 844 yrBP at 25 cm is inconsistent with pollen biostratigraphy and ²¹⁰Pb data. Faunal assemblages also change significantly within this interval, as does the overall abundance of faunal remains. Although this has previously been interpreted as an environmental change (Wingard and others, 2003), these data indicate that it may also represent a depositional hiatus.

Because of similar dates on plant material from widely spaced samples and uncertainty on the existence of a depositional hiatus between 20 cm and 25 cm depth, no age model was constructed for the lower meter of the core. The lowest samples are likely to be more than 3,000 years old and deposited in a fresh-water regime, but no more refined age model is possible for pre-20^th century sediments based on these data.

Biscayne Bay, Manatee Bay

One core was collected near the northeastern edge of Manatee Bay in 1996 (SEI1196 MB1). This core includes a basal peat overlain by peaty marl, and muds, with abundant plant material in the upper 30 cm (Figure 18). Although LOI data were not obtained for the entire core, values >10% in the upper 36 cm are consistent with the presence of abundant plant fragments in the same interval. ²¹⁰Pb analyses were only conducted in the upper 36 cm, and they did not approach background levels. Casuarina first occurs at 65 cm (Table 2), and a radiocarbon date of 112.6 +/- 0.4 pMC was obtained on a gastropod shell (Bittiolum varium) collected at 44-46 cm, signifying deposition after 1950 AD. Together, these data indicate that at least the upper 66 cm were deposited during the 20^th century, indicating a minimum 20^th century sedimentation rate of 0.7 cm yr^-1.

Figure 18. Age data for Manatee Bay core (SEI1196 MB1). No age model was developed for this core. Note: the break in x-axis scale. Location of core is shown in Figure 1 and described in Table 1. Data shown can be found in Table 2, Table 3, and appendix. LOI, loss on ignition, indicates organic material in core. Complete lithologic description is in Ishman (1997). [larger image]

Five other radiocarbon dates were obtained on shells: gastropods (Bittiolum) collected at 75 cm, 87 cm, and 95 cm, and two unidentified mollusks at 75 cm and 117 cm (Table 3, Figure 18). The Bittiolum shells had more negative ¹³C ratios (-3.2 to -5.2 ‰), indicating freshwater influence. Because ¹³C was not measured on the two unidentified shells, and because values for other shells indicated freshwater influence, calibration of dates on unidentified shells using the standard marine correction of 400 years is tenuous at best. The remaining Bittiolum dates are not in stratigraphic order, so no age model was constructed. The combination of a much older date of 4719-5091 yrBP at 117 cm depth, an abrupt change in the fauna and flora at 106 to 110 cm, and a change to peat lithology at 118 cm (Ishman and others, 1998) is suggestive of a depositional hiatus in this interval. A single radiocarbon date of 3213-3425 yrBP was obtained from peat at the base of a second Manatee Bay core (SEI1196MB2), located near the mouth of the Glades Canal.

Florida Bay, Little Madeira Bay / Taylor Creek mouth

Four cores were collected within Little Madeira Bay (Table 1, Figure 1), one from the mouth of Taylor Creek. ²¹⁰Pb profiles are shown for four Little Madeira Bay cores in Figure 4. Pollen analysis was conducted on one core (FB594 24), and Casuarina first appears at a depth of 40-42 cm, and ²¹⁰Pb approaches background at ~40 cm (Table 2, Figure 3A). These data indicate that 20^th century deposition occurred in at least the upper 50 cm of the core yielding a minimum 20^th century sedimentation rate of 0.5 cm yr^-1 for core FB594 24. An estuarine bivalve shell (Anomalocardia) collected from above the basal peat at 76-78 cm was used to obtain a single ¹⁴C date for core FB594 24 (Table 3, Figure 19), with a 2 age range of 1440-1681 yrBP. However, the ¹³C value of -6.90 ‰ indicates deposition in an environment with significant freshwater influence, therefore calibration of the dates using the standard marine correction of 400 years is questionable.

Figure 19. Age data for Taylor Creek core (FB594 T24). No pre-20th century age model was developed. Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core. [larger image]

Florida Bay, Russell Bank

Nine cores were collected on Russell Bank (Table 1, Figure 1), and ²¹⁰Pb analyses were conducted on all cores (Table 2, Figure 5 shows 7 of cores). The resulting data indicate high 20^th century sedimentation rates, ranging from 0.9 to 1.2 cm yr^-1 (Holmes and others, 2001, tab. 2). These cores are predominantly carbonate muds with varying amounts of shell and organics from marine grasses, as seen in the percent of fine grain material and LOI in Figure 20 for core FB295 19A. Pollen analyses were conducted on core FB295 19A, and Casuarina first occurs at a depth of 92-94 cm, which is consistent with ²¹⁰Pb reaching background values at 120 cm in the same core (Table 2, Figure 3B). Core FB295 19A was collected next to core FB295 19B, which was sampled for micropaleontological analyses and radiocarbon dates. Assuming stratigraphic equivalence between the two cores, data from the two cores were combined to develop a single age model (Figure 20). A single ¹⁴C date was obtained on the gastropod Bittiolum collected at 136-138 cm depth (Table 3). The ¹³C value of -0.10 ‰ indicates deposition in a marine to estuarine environment, and the ¹⁴C 2 age range is 45-196 yrBP. These data indicate deposition of the entire sequence during the last 200 years, and the upper 100-120 cm in the 20^th century.

Figure 20. Age model developed for Russell Bank cores (FB295 19A and 19B). Two cores (19A and 19B) collected side by side in 1995 from the south side of the bank contributed to the development of this age model. Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core. [larger image]

Florida Bay, Bob Allen mudbank

Core FB294 6A and five other cores were collected on Bob Allen mudbank (Table 1, Figure 1). Sediments were lithologically similar to Russell Bank, with predominantly carbonate muds and interspersed layers of shells and Thalassia debris. Core FB294 6A (Figure 21) illustrates some of these shifts in LOI and grain size. The coarser sediment and slightly elevated LOI indicates the presence of a grass bed in the upper 10-15 cm, and from 40-55 cm. The fine grain size and low LOI values between 15 and 40 cm and between 55 and 150 cm are consistent with carbonate mud. The molluscan faunal data indicate the presence of grasses near the core site in the interval above 55 cm, and below 130 cm, with almost barren sediments between 55 and 130 cm (Brewster-Wingard and Ishman, 1998). ²¹⁰Pb data from these cores indicate fairly high sedimentation rates (0.7 - 1.6 cm yr^-1 during the 20^th century (Figure 6) (Holmes and others, 2001, tab. 2). Pollen analyses also were conducted on core FB294 6A, and Casuarina pollen first occurs at 51 cm. ²¹⁰Pb reaches background levels at 82 cm in FB294 6A (Figure 3C), indicating average sedimentation rates of ~0.8 cm yr^-1 at the core site, and deposition of the upper 82 cm in the 20^th century. One radiocarbon date was obtained on a gastropod shell (Bittiolum) at 146-148 cm depth in core FB295 6C (Figure 21) and it indicates deposition of the entire 1.7 meter sequence in the last 200-300 years. A ¹³C value of -0.80 ‰ is consistent with deposition in a marine/open estuarine environment.

Figure 21. Age model developed for Bob Allen mudbank core (FB294 6A). Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core. [larger image]

Florida Bay, Whipray Basin

Three cores were collected in Whipray Basin, and ²¹⁰Pb reaches background levels between 28 and 44 cm in two of these (Figure 3D and Figure 7). Holmes and others (2001, tab. 2) calculated average 20^th century sedimentation rates for these sites at 0.37 to 0.43 cm yr^-1. Micropaleontological analyses and a radiocarbon date were obtained from core FB697 25B, and no pollen data were generated from any cores from Whipray Basin. Relatively high organic content (12-15% LOI; Figure 22) in the upper 20 cm of core FB697 25B is consistent with abundance of plant fragments throughout the interval. Grain size values are variable throughout the core, indicating periodic changes in sedimentation at the site.

Figure 22. Age model developed for Whipray Basin core (FB697 25B). No pollen data were available for this core. Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be found in Table 2 and the appendix. LOI, loss on ignition, indicates organic material in core. [larger image]

²¹⁰Pb reaches background rates at approximately 44 cm in core FB697 25B, although it fluctuates a fair amount, indicating some disturbance of the sediments (Table 2, Figure 3D), which agrees with the fluctuations in grain size. One radiocarbon date was obtained on a Prunum shell (gastropod) from 64-66 cm (Table 3), which yielded a 2 age range of 232-451 yrBP. The age model developed from these two data points puts the start of 20^th century deposition between 35 and 45 cm and the calculated age for the bottom of the core at ~ 500 yrBP (Figure 22). The average sedimentation rate for the pre-20^th century interval in the core ranges between 0.08 and 0.22 cm yr^-1. Although this age model is based on only two lines of evidence, the data are relatively consistent and provide a general model for deposition over the last 100 to 500 years in Whipray Basin. The ¹³C value of -0.50 ‰ is consistent with deposition in an estuarine environment.

Florida Bay, Rankin Basin

The age model for Rankin Basin is developed from a single core collected in 2001 (GLW601 RL1) (Figure 23). This core was located at the site of a documented sea grass die-off that occurred in 1987-1988 (Zieman and others, 1999). The ²¹⁰Pb profile in the upper 20 cm of the core indicates that this segment reflects post die-off deposition (Figure 3E). Death of the seagrass would mobilize sediments on the top of the bank, which would migrate and potentially deposit on the side of the bank where the core was taken, explaining the reverse profile of the ²¹⁰Pb. Elevated LOI values in the upper 28 cm of the core are consistent with the presence of organics from a grass bed, and the organic debris left by the die-off of a grass bed. In situ deposition is represented by the start of the normal decay profile at 20 cm. For this reason, the ²¹⁰Pb data in the age model start at 20 cm and are set at 1987. There is excellent agreement between the first appearance of Casuarina and the zone at which ²¹⁰Pb reaches background levels (38-40 cm and 36-38 cm, respectively) for the core (Table 2). Twentieth century deposition began in this core between 36 and 40 cm and the average sedimentation rate for the 20^th century is 0.4 cm yr^-1.

Figure 23. Age model developed for Rankin Basin core (GLW601 RL1). Lead-210 data in the upper 20 cm of the core are excluded from the age model, and the lead-210 curve is assumed to start normal decay at 1987 (see Figure 3E and text), preceeding seagrass die-off in the area. Location of core is shown in Figure 1 and described in Table 1. Data contributing to the model can be found in Table 2, Table 3, and the appendix. LOI, loss on ignition, indicates organic material in core. [larger image]

The uppermost radiocarbon date on the gastropod Prunum sp. was obtained at 41 cm depth, directly below the point at which ²¹⁰Pb reaches background levels (Table 3, Figure 23). The 2 age range for the sample is 51-284 yrBP, which is consistent with age models based on ²¹⁰Pb and pollen biostratigraphy. Two additional radiocarbon dates were obtained, one at 73 cm on a Prunum shell and another at 129 cm on a Brachidontes shell. The ¹³C values on these shells range from -0.80 to 1.42 ‰ and are consistent with deposition in an estuarine to marine setting. The age model based on these dates indicates that the 140 cm sequence was deposited during the last 1600 years, and the pre-20^th century average sedimentation rate was 0.3 cm yr^-1.

Additional Age Data from Florida Bay Sites

²¹⁰Pb data are included from additional sites in Florida Bay (most previously reported in Holmes and others, 2001) to indicate variations in sedimentation rates within the Bay, and for comparison to sites analyzed for paleoecologic data. At sites from the western portions of Florida Bay (Rankin Bight, Crocodile Point, Johnson Key, and Rabbit Key) ²¹⁰Pb generally reaches background rates at relatively shallow depths and the average 20^th century sedimentation rates range from 0.3 to 0.4 cm yr^-1 (Holmes and others, 2001, tab. 2). These rates are consistent with the patterns discussed above for Whipray and Rankin Basin cores. The exception for the western bay cores is Lake Ingraham, located on Cape Sable. ²¹⁰Pb does not reach background levels at this site (Figure 8), and the calculated rate of sedimentation is 3.6 cm yr^-1 (Holmes and others, 2001, tab. 2).

Cores from the eastern portion of Florida Bay show more variable sedimentation rates. Pass Key has experienced rapid deposition in the 20^th century, with rates ranging from 0.6 to 5.8 cm yr^-1 (Holmes and others, 2001, tab. 2) (Figure 9). Porjoe Key and Park Key mudbank cores have average 20^th century sedimentation rates of 0.5 and 0.8 cm yr^-1 (Holmes and others, 2001, tab. 2) (Figure 7). Trout Creek core, in the northern transitional zone has a sedimentation rate of 0.4 cm yr^-1 for the 20^th century, lower than the rates in Little Madeira Bay that range from 0.5 to 0.9 cm yr^-1 (Figure 4).

Summary of Results of Age Analyses for Biscayne Bay and Florida Bay

The synthesis of age data from Biscayne Bay and Florida Bay sediment cores and the development of the age models presented here provide a framework to interpret proxy records contained within the cores. In most cores, the start of 20^th century deposition can be identified with a fair degree of accuracy. This time period is significant in the south Florida restoration efforts, because the most significant human alteration of the Everglades ecosystem occurred in the 20^th century. It is more difficult to develop confident pre-20^th century age models for cores collected in Biscayne and Florida Bays for several reasons. First, many of the radiocarbon dates are so young that the 2 age ranges include the 20^th century. For these samples that were deposited below the point at which ²¹⁰Pb reaches background levels, we can only say that the samples were deposited between 1900 and the older end of the 2 range. However, this does provide the ability to contrast pre- and post-20^th century assemblages, which is important for restoration questions. Second, the range of ¹³C values among dated shells raises questions on calibration of dates and reservoir effects. In light of such uncertainty, we have not constructed age models for all the cores; again, it still is possible to distinguish pre- and post-20^th century sediments.

With these caveats, it is possible to draw several broad conclusions on sedimentation patterns in Florida and Biscayne Bays. In general, the models from the mid-bay mudbanks in Biscayne Bay and Florida Bay show more rapid average rates of sedimentation, fewer signs of disruption, and more internal consistency of sediments. The nearshore core age models indicate slower average rates of sedimentation, more disruption in the sedimentary sequences, and indications of more “old carbon” effects. Cores in close proximity to each other (for example Featherbed Banks and No Name Bank in Biscayne, and Russell Banks and Bob Allen mudbank in Florida Bay) generally show similar depositional patterns, lending support to the consistency of the age models.