| Summary |
Environmental conditions existing within Pamlico Sound over the last century were analyzed using foraminiferal data, stable isotope (C and N) data, C:N ratios, percent OC, and radionuclide data. Environmental conditions were evaluated at three time slices; (1) the modem environment as determined by surficial (0-2cm) sediments, (2) the intervals representing approximately 40 years BP, as determined by ¹³⁷Cs activity, and (3) the intervals representing approximately 120 years BP, as determined by ²¹⁰Pb activity. Cluster analysis distinguished four foraminiferal assemblages at the surface (0- 1cm); (1) Estuarine Biofacies A, (2) Estuarine Biofacies B, (3) Marsh Biofacies, and (4) Marine Biofacies. Estuarine Biofacies A is distinguished from Estuarine Biofacies B by the greater relative abundance of the agglutinated species Ammotium salsum and Ammobaculites crassus in the former and the greater relative abundance of Elphidium excavatum in the latter. The Marsh Biofacies is characterized by typical marsh foraminifera such as Tiphotrocha comprimata, Trochammina inflata, Miliammina fusca, and Haplophragmoides wilberti. The Marine Biofacies is comprised completely of calcareous foraminifera (e.g., Elphidium excavatum, Hanzawaia strattoni, Cibicides lobatulus, Elphidium subarcticum, Quinqueloculina seminula, and Elphidium galvestonense) and is restricted to tidal inlets. The down-core foraminiferal data indicate that there has been a steadily increasing marine influence within Pamlico Sound over the last century; supportive evidence is provided by down-core C:N ratios. Approximately 120 years BP, Pamlico Sound was dominated by Estuarine Biofacies A, which is indicative of brackish conditions. The particular abundance of hurricanes at this time may explain the brackish nature of the foraminiferal assemblage. Up-core, Estuarine Biofacies B becomes the more prominent assemblage within Pamlico Sound; this is indicative of increased salinity over time. C:N ratios steadily decrease up-core throughout the sound indicating an increase in marine influence over the last century. [delta]¹⁵N signatures began to steadily enrich at approximately 40 years BP. The resultant [delta]¹⁵N signature is a combination of nitrogen cycling within the water column and the interaction with surficial sediment, as well as the organic matter source material. An increase in the amount of shrimp trawling during the early 1960s is a possible explanation for the enriched signatures. The average range for [delta]¹³C signatures (-25 to -22) is typical of terrestrial sediment input and has not significantly changed over time. This suggests that the source area for the sediments being deposited within Pamlico Sound has not changed. |
