DOWNCORE VARIATION OF SEDIMENT GRAIN SIZE IN A COASTAL GEORGIA MARSH

A three year study of the impact of development on the marshes of Georgia was continued in 2003 as part of an NSF funded research experience for undergraduates (ACRES). Grain size analyses were performed on vibracores taken from both the high marsh (31° 58.529 N , 81° 01.847 W) and low marsh (31° 58.530 N , 81° 01.913 W) on Skidaway Island, GA. Cores were sampled at 5cm depth intervals, treated with H₂O₂ to remove organics, and washed in sodium hexametaphosphate to prevent flocculation. Sediments were wet sieved through a 63 µm sieve to differentiate fine and coarse grained material. They were analyzed using a Ro-Tap (coarse grained material, >63µm) and Sedigraph (fine grained material, <63µm). The lithology of the low marsh core (3.77 m) consists of a series of fine as well as coarse grained lenses and shell hash layers downcore. The low marsh core shows fine clay material throughout the first meter and progresses through a sandy clay interval to a layer of sand and shell fragments beginning at 1.7 m. A cohesive clay is present from a depth of 2.1 m to a sharp contact with a sand interval at approximately 3.0 m. The high marsh core (2.2 m) is dominated by brown sandy clays with roots and burrows to a depth of 1.3 m. There is a sharp contact at 1.3 m with a blue clay formation that extends to the base of the core at 2.22 m. The low marsh core shows pronounced lithofacies changes that can be attributed to either a migrating tidal channel or relative change in sea level, possibly caused by compaction and/or subsidence. ¹⁴C of the shell and wood material and radiochemistry (²¹⁰Pb and ¹³⁷Cs) of the sediment should aid in interpretation.