Primary Cores_Excess 210Pb activity_Age/Date_Mass Accumulation_Organic Matter Accumulation_Inorganic Matter Acumulation_CaCO3 Accumulation

A total of 24 soil cores were collected from four study sites located in two study regions (Tampa Bay and Naples, FL, USA). The two Tampa Bay sites were the E.G. Simmons Conservation Park (EG) and the Fred and Idah Schultz Nature Preserve (SP). The two Naples sites were Windstar on Naples Bay (WS) and Henderson Creek (HC). Within each site there were three 10 x 10 m plots targeting different relative surface elevations (high, medium, and low) in each of two mangrove conditions: natural (N) and created (C). PVC soil cores (10 cm diameter x 50 cm length) were collected from the center of each plot (n=24). Cores were returned to the laboratory and sectioned in intervals of 0-2 cm for the first interval, 1-cm intervals for the 2–10 cm depths, then 2-cm intervals for the remainder of the core. A high-purity germanium detector coupled with a multi-channel analyzer was used to quantify 210Pb and 226Ra activities in freeze-dried soil samples. Detector efficiency and gamma-ray intensity factors, derived from calibration standards, were applied to convert isotope count rates (counts per minute) into activity values. The 46.5 keV gamma peak provided a direct measure of 210Pb activity, while the 351.9 keV gamma peak was used to estimate 226Ra activity via its decay product 214Pb. To ensure secular equilibrium between 226Ra and 214Pb, sealed sample tubes were stored for a minimum of three weeks prior to gamma counting. Total 210Pb activity was corrected by subtracting 226Ra activity to obtain excess 210Pb, which served as the basis for age determination using the Constant Rate of Supply (CRS) model. This model was applied to calculate age and mass accumulation rates. Mass accumulation was then multiplied by the measured organic matter, inorganic matter, and CaCO₃ fractions to determine the accumulation rates of these components over time. For created sites, a slight modification to the CRS model was necessary because these cores do not capture the entire excess 210Pb profile required for the standard CRS model. Therefore, we applied a forced-age CRS model, using the identified creation horizon to constrain and force the model.