THE APPLICATION OF NATURALLY OCCURRING RADIONUCLIDES TO THE STUDY OF ESTUARINE SEDIMENTARY PROCESSES: EXAMPLES FROM GEORGIA AND SOUTH CAROLINA

Estuaries are important geologic repositories containing the signatures of the latest sea level rise, large natural events and anthropogenic inputs to the coastal zone. For the past three decades, long-lived, naturally occurring radionuclides (210Pb and 137Cs) have been applied to the study of estuarine sediment accumulation on 100-y timescales. Within the past decade, new approaches with short-lived radionuclides (7Be and 234Th) have been developed that allow us to examine sedimentary processes on much shorter timescales (days to months).

Because particles are dynamically remobilized and redistributed within the estuarine environment, sediment distribution and accumulation patterns exhibit spatial and temporal heterogeneity on a variety of scales that can be highlighted by the application of radiochemical tools with a range of half-lives. Work during the past decade in the fluvial and saltmarsh estuaries of the Georgia and South Carolina coasts documents the short- and long-term signatures of sedimentary processes in the estuarine stratigraphic record. In South Carolina, both Winyah Bay and the Ashepoo River in the ACE Basin National Estuarine Research Reserve exhibit extremely rapid fine-grained sediment deposition at rates over 10 cm/month as documented by the distribution of the 234Th and 7Be, although 210Pb rates are orders of magnitude lower. These deposits in the ACE Basin are removed each year and widely distributed into the extensive salt marshes. The residual deposits, a black, low-porosity mud overlain by a few centimeters of sand, represents the stratigraphic signature of this massive deposition. In Winyah Bay, harbor dredging annually creates accommodation space that is quickly filled by muddy deposits within at most a few months, necessitating annual maintenance dredging. In the Satilla River estuary, sediment distribution and accumulation patterns exhibit variability on tidal to decadal timescales and on broad spatial scales as well. Redistribution of mobile sediments as evidenced by high-concentration mud suspensions (up to 5 g/L) occurs with each tide but sediment accumulation as documented with Pb-210 accumulation rates is discontinuous on decadal timescales.