Reconstructing Common Era relative sea-level change on the Gulf Coast of Florida

Abstract

A B S T R A C TTo address a paucity of Common Era data in the Gulf of Mexico, we reconstructed~1.1 m of relative sea-level (RSL) rise over the past~2000 years at Little Manatee River (Gulf Coast of Florida, USA). We applied a regionalscale foraminiferal transfer function to fossil assemblages preserved in a core of salt-marsh peat and organic silt that was dated using radiocarbon and recognition of pollution, 137 Cs and pollen chronohorizons. Our proxy reconstruction was combined with tide-gauge data from four nearby sites spanning 1913-2014 CE. Application of an Errors-in-Variables Integrated Gaussian Process (EIV-IGP) model to the combined proxy and instrumental dataset demonstrates that RSL fell from~350 to 100 BCE, before rising continuously to present. This initial RSL fall was likely the result of local-scale processes (e.g., silting up of a tidal flat or shallow sub-tidal shoal) as saltmarsh development at the site began. Since~0 CE, we consider the reconstruction to be representative of regional-scale RSL trends. We removed a linear rate of 0.3 mm/yr from the RSL record using the EIV-IGP model to estimate climate-driven sea-level trends and to facilitate comparison among sites. This analysis demonstrates that since~0 CE sea level did not deviate significantly from zero until accelerating continuously from~1500 CE to present. Sea level was rising at 1.33 mm/yr in 1900 CE and accelerated until 2014 CE when a rate of 2.02 mm/yr was attained, which is the fastest, century-scale trend in the~2000-year record. Comparison to existing reconstructions from the Gulf coast of Louisiana and the Atlantic coast of northern Florida reveal similar sea-level histories at all three sites. We explored the influence of compaction and fluvial processes on our reconstruction and concluded that compaction was likely insignificant. Fluvial processes were also likely insignificant, but further proxy evidence is needed to fully test this hypothesis. Our results indicate that no significant Common Era sea-level changes took place on the Gulf and southeastern Atlantic U.S. coasts until the onset of modern sea-level rise in the late 19th century.

Salt-Marsh Foraminiferal Distributions from Mainland Northern Georgia, USA: An Assessment of Their Viability for Sea-Level Studies

“…As has previously been identified from more temperate locations, high marsh zones (TB-1) occupy more narrowly defined vertical zones than low marsh assemblages and, therefore, are the most precise indicators of former RSL (Gehrels et al, 1994;Scott et al, 2001). The foraminiferal assemblages are similar to those found along the U.S. Atlantic and Gulf coasts such as in North Carolina (Kemp et al, 2009), South Carolina (Collins et al, 1995) and Florida (Gerlach et al, 2017). Similar assemblages are also reported from the Mississippi Delta (Phleger, 1955) and from lower latitudes such as mangroves in Guaratuba, Brazil (Scott et al, 1990).…”

“…Ammobaculites spp. have been found in other low marsh environments (e.g., Kemp et al, 2009;Kemp et al, 2013;Gerlach et al, 2017). In Virginia, Spencer (2000) used Ammobaculites spp.…”

“…Salt-marsh studies from North Carolina and New Jersey also show that modern assemblages of A. inepta usually occupy low-salinity environments within tidal elevations close to MHW and MHHW (Kemp et al, 2009;Kemp et al, 2013). In Florida salt marshes, A. inepta together with A. mexicana and Haplophragmoides wilberti dominated tidal elevations from below MHW to above MHHW in brackish to normal salinity settings (Gerlach et al, 2017), and mixed with M. fusca and Ammobaculites spp. at lower elevations (Hawkes et al, 2016).…”

“…This loss is caused by the oxidation of the organic cement facilitated by high temperatures and sediment oxygenation (increased by bioturbation from fiddler crabs) (Berkeley et al, 2007). However, this dramatic loss of agglutinated tests was not encountered in marshes from the southeastern U.S. in similar temperature zones such as South Carolina (Collins et al, 1995) and Florida (Gerlach et al, 2017). Further study is required to understand the complex reasons behind the taphonomic loss of agglutinated foraminifera in Georgia salt marshes.…”

“…Along the U.S. Atlantic coast, modern salt-marsh foraminiferal training sets have primarily been established in regions north of Cape Hatteras, North Carolina (Figure 1) (e.g., Scott and Medioli, 1980b;Gehrels et al, 2000;Edward et al, 2004;Kemp et al, 2009Kemp et al, , 2012a with only a few studies to the south (Kemp et al, 2014;Hawkes et al, 2016;Gerlach et al, 2017). Goldstein and Watkins (1998) investigated the salt-marsh foraminiferal distributions from St. Catherine's Island, Georgia and demonstrated while assemblages exhibit relationships with elevation, they are regionally distinct.…”