Abstract Introduction Acknowledgements Material and Methods - Core Sites and Collection - Faunal/Floral Analyses - Ostracode Shell Chem. Analyses > Prelim Age Model Results Discussion Significant Findings, Implications And Future Work References Appendices PDF version

Preliminary Age Model for Cores

The preliminary age model for each core site (A cores; Figures 5-7) was established using three methods of dating: 1) lead-210 analyses of the sediments (Figures 8-10); 2) carbon 14 analysis of a shell from the lower portion of the cores; and 3) first occurrence of Casuarina pollen (Australian Pine). The lead-210 analysis was used to establish the chronology of the upper portions of the core since lead-210 provides reliable age models for samples up to approximately 100 years old (see Holmes and others, 2001, for detailed explanation of the methodology). Future analyses will investigate the causes of different background lead-210 and radon-226 from one bank to another within Biscayne Bay and the potential effects these differences have on age models.

Table 4: Data on carbon-14 analyses performed at USGS radiocarbon laboratory. Years before present (yrBP) = years before 1950. Core ID Depth (cm) Sample Conventional C14 Age (yrBP) ± 1 Calibrated yrBP 2 calibrated age range Years before collection (2002) * Corrected age range * GLW402-CBA 122-124 mollusk Chione 830 40 470 515-413 522 567-465 GLW402-FBB 186-188 mollusk Turbo 540 35 167 268-0 219 320-52 ^ GLW402-NNB 138-140 mollusk Chione 670 35 297 405-263 349 457-315 * Data plotted on Figures 5-7. ^ Reported result indicates a calibrated age of 0 BP (corrected age of 52 years before 2002) and is beyond the limit of radiocarbon dating.

Carbon-14 analyses on individual shells from the lower portion of the cores established a tie-point to the oldest lead-210 date (Table 4). Because we have only dated the basal portion of each core, these are preliminary age models only. Conventional radiocarbon ages on marine shells must be corrected to account for reservoir effect of ocean circulation of carbon. In this report, ages were calibrated using the standard marine reservoir correction of 400 years using the Calib 43 program (Stuiver and others, 1998). We refer to calibrated ages in terms of calendar years (AD); the conventional calibrated radiocarbon dates (in years before 1950 (yr BP)) and the two sigma errors are also provided in Table 4. Because local atmospheric and oceanic processes also may affect the radiocarbon ages of marine shells, future research will include dating of mollusks collected live in the early 20th century before atomic testing elevated carbon-14 values above 1950 AD levels. This will establish the appropriate reservoir correction for Biscayne Bay mollusks and facilitate correlation with other regions.

Figure 5. (right) Age vs. depth, No Name Bank cores. yrBP indicates years before 2002 AD. The age model for 1900-2002 is based on lead-210 from GLW402-NNA; the age model from 1900 AD to the base of the core is a linear interpolation between the lowest lead-210 date and a radiocarbon date at 139 cm in GLW402-NNB. [larger version] Figure 6. (right) Age vs. depth, Featherbed Bank cores. yrBP indicates years before 2002 AD. The age model for 1900-2002 is based on lead-210 from GLW402-FBA; the age model from 1900 AD to the base of the core is a linear interpolation between the lowest lead-210 date and a radiocarbon date at 187 cm in GLW402-FBB. [larger version] Figure 7. (right) Age vs. depth, Card Bank core (GLW402-CBA). yrBP indicates years before 2002 AD. The age model for 1900-2002 is based on lead-210; the age model from 1900 AD to the base of the core is a linear interpolation between the lowest lead-210 date and a radiocarbon date at 123 cm. [larger version]

The first occurrence of Casuarina pollen (Australian Pine), an exotic introduced into south Florida in the late 1800s (Langeland, 1990), provides a good stratigraphic marker for the beginning of the 20^th century in the cores. Consistency between Casuarina pollen records and lead-210 age models provides confidence in the position of this important horizon in the cores. A lack of correspondence between Casuarina and lead-210 age models provides a warning that the sediments may be disturbed.

Figure 8. (right) Lead-210 data for No Name Bank core (GLW402-NNA). A. Projected year at depth. B. Loss on ignition (% dry weight). C. Excess lead-210 activity (dpm/g). [larger version] Figure 9. (right) Lead-210 data for Featherbed Bank core (GLW402-FBA). A. Projected year at depth. B. Loss on ignition (% dry weight). C. Excess lead-210 activity (dpm/g).[larger version] Figure 10. (right) Lead-210 data for Card Bank core (GLW402-CBA). A. Projected year at depth. B. Loss on ignition (% dry weight). C. Excess lead-210 activity (dpm/g).[larger version]