Subrecent Intertidal Diatoms from St. Catherines Island, Georgia: Taphonomic Implications

Abstract

The taphonomic overprinting of diatom assemblages has been underemphasized in paleoecological studies. Subrecent (˂700 YBP) cohesive muds, interpreted on the basis of previous macrofloral and macrofaunal evidence as intertidal salt marsh deposits, are exposed on Middle Beach, St. Catherines Island, Georgia. Analysis of nine diatom-bearing sediment samples from a 1.2 meter section yielded a total of 79 species representing 34 planktic and benthic genera. Depth vs percentage plots for selected taxa yielded no clear-cut ecological zonation among the diatom assemblages. Diatoms from all samples display strong diagenetic and taphonomic overprinting. Delicate benthic taxa reported to be living in the marshes on Sapelo Island, to the south, are either absent, rare, or severely altered in the relict marsh muds of St. Catherines Island, most likely due to fragmentation and dissolution. As a result, the most abundant species, Cymatosira belgica, although rare in modern salt marsh environments, has been proportionally enriched. This species accounts for up to 75% of the diatom individuals in the upper third of the section and approximately 50% at greater depths. Fragmentation and dissolution can alter a salt marsh diatom assemblage so drastically that the end product does not even resemble the initial living flora. In the interpretation of temporal sequences of littoral diatoms, taphonomic intensity has to be considered in terms of possible selective removal of certain key taxa. Taphonomic processes can mask the original ecologic relationships that exist in 8 salt marsh flora, and result in proportional over-representation of resistant species (e.g. Cymatosira belgica) confounding paleoecologic and paleogeographic reconstructions.