Reconstructing Late Holocene Relative Sea-level Changes on the Gulf Coast of Florida

Abstract:

Little is known about late Holocene relative sea-level (RSL) along the Gulf Coast of Florida. A RSL reconstruction from this region is needed to fill a spatial gap in sea-level records which can be used to support coastal management, contribute geologic data for Earth-Ice models estimating late Holocene land-level change and serve as the basis for which future projections of sea-level rise must be superimposed. Further, this dataset is crucial to understanding the presence/absence and non-synchronous timing of small sea-level oscillations (e.g. rise at ~ 1000 A.D.; fall at ~ 1400 A.D.) during the past 2000 years on the Atlantic and Gulf Coasts of the United States that may be linked to climate anomalies. We present the results of a high-resolution RSL reconstruction based on the sediment record of two salt marshes on the eastern margin of the Gulf of Mexico. Two ~1.3m cores primarily composed of Juncus roemeranius peat reveal RSL changes over the past ~2000 years in the southern end of Tampa Bay and in Charlotte Harbor, Florida. Two study sites were used to isolate localized factors affecting RSL at either location.

Lithostratigraphic analysis at both sites identifies a transition from sandy-silt layers into salt-marsh peat at the bottom of each core. The two records show continuous accumulation of salt-marsh peat with Juncus roemeranius macrofossils and intermittent sand horizons likely reflecting inundation events. We used vertically zoned assemblages of modern foraminifera to assign the indicative meaning. The high marsh is dominated by Ammoastuta inepta, Haplophragmoides wilberti, and Arenoparella mexicana, with low marsh and tidal flats identified by Ammobaculites spp. and Miliammina fusca. Chronologies for these study sites were established using AMS radiocarbon dating of in-situ plant macrofossils, Cs¹³⁷, Pb²¹⁰ and pollen and pollution chronohorizons. Our regional RSL curve will add additional data for constraining the mechanisms causing RSL change.