Salt-marsh foraminiferal distributions from mainland northern Georgia, USA : an assessment of their viability for sea-level studies.

Abstract

We investigated foraminiferal distributions from two salt-marsh sites at Thunderbolt and Georgetown, in mainland northern Georgia, U.S. Atlantic coast. We analyzed modern epifaunal foraminiferal assemblages across multiple transects consisting of 54 surface samples. Multivariate statistical analysis (Partitioning Around Medoids and Detrended Correspondence Analysis) revealed that dead foraminiferal assemblages are divided into three faunal zones, which are elevation-dependent and site-specific. At Thunderbolt, an intermediate salinity marsh (17‰), high marsh assemblages are dominated by Haplophragmoides spp. with an elevational range of 1.19 to 1.68 m mean tide level (MTL) between Mean Higher High Water (MHHW) to Highest Astronomical Tide (HAT). Low marsh assemblages are dominated by Miliammina fusca and Ammobaculites spp. with an elevational range of – 0.05 to 1.14 m MTL (between MTL and MHHW). At Georgetown, a low salinity marsh (6‰), the assemblages are dominated by Ammoastuta inepta with an elevational range of 0.43 to 1.16 m MTL (between MTL and MHHW). We also enumerated living infaunal foraminiferal populations from six 50-cm sediment cores from the two salt marshes to assess implications for interpretations of sea-level change. Peak concentrations of living foraminiferal populations occur in the upper 1-cm surface sediment in five of the six cores. An exception was observed in high marsh settings of Thunderbolt, where Haplophragmoides spp. and Arenoparrella mexicana were observed living down to 40 cm depth and both the live and dead abundance peaked (32 and 520 specimens per 10 cc respectively) between depths of 15–35 cm in the core. The dominant infaunal species were similar to those observed in modern surface samples, and the total number of infaunal foraminifera was typically less than 15% compared to the total number of dead specimens in the surface samples. Finally, we com­pared the down-core patterns of living and dead foraminiferal abundance that suggest that 90% of the tests were removed within the upper 10 cm of sediment in most cores. This may be due to taphonomic alteration from bioturbation and/or microbial processes. Selective preservation between resistant species such as A. mexicana and fragile species like M. fusca and Ammobaculites spp. can change the subsurface foraminiferal assemblage. This has the potential to cause errors in sea-level reconstructions using foraminiferal assemblage from low marsh sediments. This study highlights the modern vertical distribution of salt-marsh foraminifera in mainland northern Georgia and their potential as modern analogues for fos­sil counterparts in reconstructing sea-level changes. Taphonomic processes may cause the absence of foraminiferal tests or differences between modern and fossil assemblages, which could be problematic when performing RSL reconstructions in low marsh environment.