ASSESSING COASTAL ESTUARINE SEDIMENT EROSION THROUGH BASIN SEDIMENTATION: CAPE LOOKOUT BIGHT, NC

Coastal zones are degrading globally as a result of sea-level rise, storm frequency/intensity and human influence. However, the relative impact of these factors is not well understood. Elucidating the rate, source and variability of sedimentation in a coastal basin provides the first step to determining where and when preceding erosion is occurring, providing evidence for how each of these factors may influence coastal erosion rates.

Cape Lookout Bight (CLB), North Carolina has been shown to be a highly accreting coastal basin, acting as a sediment trap for the southern Outer Banks. A core taken in CLB, with corresponding isotopic analysis of excess ²¹⁰Pb done by Chanton et al. (1983) showed high rates of sediment accumulation in the basin, and an accumulation of excess ²¹⁰Pb approximately 30 times its estimated atmospheric supply. This demonstrated that CLB traps sediments supplied from a large region within the back-barrier and adjacent watersheds. Storm inputs from hurricanes were also identified in the core through the presence of predominant sand concentrations.

Building on this initial work, we were able to extract a 4.6-m long sediment core of fine sediment and isolated thin sand beds (paleotempestites) in the same location in CLB. We sampled the core at 1-cm intervals (10 samples/annum) for grain-size, radiometric and stable isotope analyses. This much smaller interval (compared to earlier work by Chanton et al., 1983) allowed us to establish a higher resolution excess ²¹⁰Pb profile down core. In comparing our preliminary results to that of Chanton et al. (1983), it is apparent that an increased rate of sedimentation occurs at a depth in both cores that corresponds to a date of ~1980. In addition to this increased sedimentation rate, our higher resolution profile of ²¹⁰Pb shows a great deal of variability between samples, indicating fluctations in the rate of sediment supplied to the basin. These initial results do reveal a change has occurred in the rate of sedimentation in the basin, as well as the variability of the rate of sedimentation throughout the core. Changes in the supply of sediment seasonally, variations in storm climate, sea-level rise and/or variations in sediment source are all potential causes for this apparent variation, and will be investigated as part of this research.