RECONSTRUCTING COMMON ERA RELATIVE SEA-LEVEL CHANGES ON THE GULF COAST OF FLORIDA

Late Holocene relative sea-level (RSL) reconstructions provide information on how sea level has responded to past climate variation and serve as a baseline for contextualizing projected sea-level rise over the coming decades. To address a spatial gap in late Holocene RSL data on the eastern margin of the Gulf of Mexico, we present the results of a high-resolution (decadal to centennial age errors; cm to dm vertical errors) sea-level reconstruction based on the sediment record of a southwestern Florida salt marsh at the Little Manatee River, in eastern Tampa Bay. The chronology for our selected ~1.25m core was developed using AMS radiocarbon dating of in-situ plant macrofossils, Pb²¹⁰, Cs¹³⁷ and pollution and pollen chronohorizons. We use the combined chronological markers to construct an age-depth model using Bchron that reveals RSL changes over the past ~2100 years.

Lithostratigraphic analysis reveals salt-marsh peat began to accumulate on top of an organic sandy-silt that serves as the basis for the salt-marsh platform throughout the study site. The sub-surface peat contains abundant Juncus roemeranius macrofossils except for a 0.30 m interval located at a depth of 0.45 - 0.75 m. Vertically zoned assemblages of foraminifera are employed as sea-level indicators to reconstruct RSL. Foraminiferal analysis reveals distinct changes in fauna throughout the core. The bottom 0.25 m of recovered sample is dominated by Ammobaculites spp. and Miliammina fusca. An assemblage composed of Arenoparella mexicana, Ammoastuta inepta and Haplophragmoides wilberti is the major component of the remaining 1.00 m of core, except for a 0.20 m section occurring at a depth of 0.45 - 0.65 m. In this interval, the low marsh and tidal flat species Ammobaculites spp. increase to ~20% abundance with an associated decline in high-marsh species indicating RSL rise outpaced sediment accumulation during this time.

Reconstructed RSL from this study will be evaluated against late Holocene sea-level records from sites farther north on the U.S. Atlantic Coast (Northern Florida, North Carolina, New Jersey, Connecticut) and the U.S. Gulf Coast (Louisiana) to identify potential mechanisms causing presence/absence and non-synchronous sea-level oscillations during the Common Era.