Term of Award

Spring 1998

Degree Name

Master of Science in Biology

Document Type and Release Option

Thesis (restricted to Georgia Southern)

Department

Department of Biology

Committee Chair

Frederick J. Rich

Committee Member 1

John E. Averett

Committee Member 2

Gale A. Bishop

Committee Member 3

Donald J. Drapalik

Abstract

St. Catherines Island (Liberty County) and Skidaway Island (Chatham County) are barrier islands on the Georgia coast. Sediments of both islands are of Late Pleistocene and Holocene age; understanding their relationship to each other and to other inland and marine sediments is critical to assessing geological and biological changes that have occurred in the region. Sediment cores from the island were analyzed to determine ages, environments of deposition, relationships to other coastal and inland localities, and to assess vegetational change in the region.

Four sediment cores were obtained from St. Catherines Island using a vibracorer; they comprise a transect in the mid-southern portion of the island. Localities are known as Cracker Tom Bridge, Cracker Tom Hammock, Cracker Tom Rosetta, and Beach Pond. The sediment consists of peat, shells, sand, and clay. Peat from 5.02-5.12m at Cracker Tom Bridge was radiocarbon dated at 47,620 B P. Above this peat lies an irregular erosional surface which is in turn overlain by marine mollusk shells and charcoal. The charcoal was dated at 6,020 B.P. and an Americardia shell was dated at 4060 B.P., indicating a depositional hiatus of at least 41,600 years following the deposition of the peat. The subsequent marine transgression resulted in the accumulation of the sediments which make up the remainder of the strata in the cores.

Strata were exposed on Skidaway Island during the excavation of ponds at what is known as the Jones Girls Site. Two large excavations exposed layers of clay, shell, and two distinctly different sand units. Two pieces of coral from the shell bed were individually uranium-series dated at approximately 80,000 B.P. Twelve peat clasts removed from a channel lag deposit were palynologically analyzed and individual palynofloras were compared. A plant fragment from within one peat clast was radiocarbon dated at 36,000 B P.

Pleistocene sediments from both islands were deposited during low sea level and were derived from inland plant communities; the 47,620 year old Cracker Tom Bridge peat consists dominantly of Woodwardia spores and other hydrophytic taxa, and the 36,000 year old peat clasts represent accumulation in a shrub and fern dominated bog. Regional vegetation represented in the Pleistocene sediments includes Pinus, Quercus, Carya, and Poaceae. Pollen of boreal taxa is infrequent, though an indication of the presence of northern temperate forest in the region is suggested in a few samples by very low percentages of Picea, Tilia, Fagus, and Tsuga.

Holocene age sediments are derived from near shore marine and salt marsh/tidal flat environments. Dynamic palynology and geology characterize these deposits, and the relationship between vegetational change and geologically driven processes is especially evident in the Beach Pond core, where fluctuating hydrologic conditions have altered plant community composition for at least the last 1200 years. Palynological and radiocarbon evidence suggests that the modern local plant community was established sometime after 3,000 years ago at the Cracker Tom Locality, and progradation of the island extended to the site of Beach Pond sometime after 1200 B P.

Results of the palynological analysis of these island sediments attest to the stability of southeastern floral elements and plant communities throughout the Late Pleistocene in coastal Georgia. Even as sea level fell, making the islands a part of the mainland, and north-temperate and boreal species migrated southward, the familiar southeastern plant species remained on the coastal plain.

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