SALT-MARSH FORAMINIFERAL DISTRIBUTIONS FROM MAINLAND NORTHERN GEORGIA, USA: AN ASSESSMENT OF THEIR VIABILITY FOR SEA-LEVEL STUDIES

High-resolution microfossil-based reconstructions of relative sea level (RSL) from salt-marsh environments requires an understanding of their modern distribution. We investigated epifaunal and infaunal foraminiferal distributions from two salt-marsh sites with differing salinity regimes: Thunderbolt (salinity 17‰); and Georgetown (salinity 6‰) in mainland northern Georgia, U.S. Atlantic coast to assess their relationship with tidal level and implications for interpretations of RSL change.

Modern epifaunal foraminiferal distributions across multiple transects were analyzed by multivariate statistical analysis and revealed dead foraminiferal assemblages as modern analogues for fossil counterparts in reconstructing RSL changes. The modern assemblages were divided into three faunal zones, which were elevation-dependent and site-specific: (1) At Thunderbolt, an assemblage dominated by Haplophragmoides spp. was found in a high marsh environment between Mean Higher High Water (MHHW) to Highest Astronomical Tide (HAT), with an elevational range of 1.19 to 1.68 m mean tide level (MTL); (2) At Thunderbolt, an assemblage dominated by Miliammina fusca and Ammobaculites spp. was found at low marsh between MTL and MHHW with an elevational range of - 0.05 to 1.14 m MTL ; and (3) At Georgetown, the assemblage was dominated by Ammoastuta inepta between MTL and MHHW with an elevational range of 0.43 to 1.16 m MTL.

Living infaunal foraminifera in six 50-cm sediment cores from the two salt marshes showed potential taphonomic processes that may cause the absence of foraminiferal tests or differences between modern and fossil assemblages, which could be problematic for RSL reconstructions, especially in low marsh environments. Maximum concentrations occurred in the upper 1-cm surface sediment in five of the six cores. The dominant infaunal species were similar to those in modern surface samples, and the total number of infaunal specimens was typically less than 15% compared to the total number of dead specimens in the surface samples. However, down-core patterns of foraminiferal abundance suggested that 90% of the tests were removed within the upper 10 cm of sediment in most cores. In low marsh sediments selective preservation between resistant and fragile foraminiferal species can change the subsurface assemblage.