RAPID DEPOSITION OF FINE-GRAINED SEDIMENTS IN THE ACE BASIN NATIONAL ESTUARINE RESERVE

ALEXANDER, C.R.,* ELFERS, T.C., Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411; MILLIGAN, T.G., Habitat Ecology Section, Bedford Institute of Oceanography, Dartmouth, N.S. B2Y 4A2; KINEKE, G.C., Dept. of Geology and Geophysics, Boston College, Chestnut Hill, MA 02167

Presented at 1999 SE GSA Meeting, Athens, GA

Previous research in the ACE Basin National Estuarine Research Reserve has suggested rapid accumulation of fine-grained, metal-rich sediment in the lower Ashepoo River. Box cores, kasten cores, bottom grabs and acoustic surveys were collected in May 1998 to better document the spatial and temporal dimensions of these deposits. Textural analyses show that the most extensive fine-grained deposit is restricted to the lower Ashepoo River where a deposit approximately 1 km long and 1m thick is observed. Within this deposit, grain-sizes are between 7-8 phi, whereas outside this region, sizes are significantly coarser, ranging from 1-3 phi. Downcore profiles of grain size exhibit variability, suggesting changes in bed stress and may represent spring/neap or seasonal discharge conditions. The distributions of short-lived radioisotopes in the sediment column (Th-234, half-life 24 d and Be-7, half-life 53 d) demonstrate that these sediments are rapidly deposited on annual time scales. Accumulation rates determined using Pb-210 (half-life 22 y) suggest that very little of the sediment accumulates on longer (100-y) timescales. The characteristics of this deposit are important in understanding estuarine sediment dynamics and the redistribution of anthropogenically enriched material in the ACE Basin, and is the focus of on-going research.