Evolution of a freshwater barrier-island marsh in coastal Georgia, USA

Beach Pond is a freshwater pond and marsh, located approximately 50–100 m from the beach scarp in the mid-southern portion of St. Catherines Island, Liberty County, Georgia (31°37′N latitude, 81°09′W longitude). A 4.5-m sediment core was obtained from the pond in an effort to reconstruct the paleoecology of the site. The sediments (sand, clay, and peat) are Holocene age (<10,000 years). Gross sedimentological characteristics of the core suggest cyclic depositional trends. The modern vegetation of Beach Pond is dominated byPluchea and other composites,Typha, Cyperaceae, and Poaceae. The palynology of the core reveals dynamic changes in depositional environments and plant communities during sediment accumulation. Sediments from the lower portion of the core were derived from nearshore marine environments and probably represent accumulation in a shallow lagoon; these are characterized by the abundant pollen ofPinus and a large percentage of brokenPinus pollen grains. A piece of wood recovered from the uppermost lagoonal sediments yielded a radiocarbon date (AMS) of 1210±40 BP. These sediments are overlain by tidal-flat-derived sediments, which are overlain by a thin peat layer derived from an interdunal swale community dominated byMyrica. A return to brackish marsh conditions then occurred, as indicated by the presence ofLimonium, Cheno-Am type (e.g.,Salicornia), and abundant Poaccae pollen. The modern freshwater pond plant community became established as the salinity decreased; this is indicated by the abundance of freshwater plant taxa (i.e.,Azolla, Typha, Cyperaceae). Sea-level fluctuation, erosion, storm overwash, and anthropogenic factors (i.e., road building, water-well drilling) have all influenced the development of past and modern plant communities by altering the hydrology and salinity of the site.