Characterizing Coastal Subenvironments with Modern Foraminiferal Assemblages: Bear Island and Bogue Banks, North Carolina

Abstract

North Carolina's coastline consists of an extensive barrier island system, with inlets acting as pathways for the exchange of sediment and water from estuarine and open ocean environments. Reconstructing past coastal changes in active systems, like those seen in North Carolina, is commonly achieved through the use of a variety of techniques including seismic surveys, sedimentological characterization (e.g., grain-size, lithology, sedimentary structures), and micropaleontological analyses. Foraminifera are common and useful biostratigraphic markers and paleoenvironmental indicators. However, the documentation and utility of foraminifera in coastal systems, can be further developed in nearshore and barrier island related environments (e.g., shoreface, inlet, beach, foreshore). This study aimed to characterize modern foraminiferal assemblages for barrier island-related subenvironments defined a priori, in order to enhance their use as paleoenvironmental indicators in these active coastal environments. Foraminiferal assemblages in Holocene sediments from vibracores were compared with the established modern dataset to test their ability to differentiate subenvironments in the stratigraphic record. Bear Island (Hammocks Beach State Park) was chosen for its lack of human interference and its proximity of previously collected core material from Bogue Inlet and offshore of Bogue Banks, North Carolina. Twenty-six subenvironments from shoreface, inlet, and beach environments were identified and observed on Bear Island and in Bear Inlet. Three replicate surface samples (0-1 cm) were taken via ponar or hand grab sample from each subenvironment, for a total of 78 samples. Sediments were predominantly fine- to medium-grained quartzose and carbonate-rich (e.g., mollusk fragments, echinoid spines) sands. Eighty foraminiferal taxa were identified dominated by Elphidium excavatum, Quinqueloculina lamarckiana, Ammonia parkinsoniana, Eponides repandus, Quinqueloculina seminula, and Ammonia tepida. Discriminant analysis was used to test the hypothesis that modern foraminiferal assemblages of coastal siliciclastic subenvironments can be distinguished statistically. Four supergroups were defined: A) shoreface; B) ebb-tide delta; C) flood-tide delta and inlet channels; and D) foreshore, dune, overwash, spit, longshore bar, and trough. The modern dataset was then used to interpret environments of 68 down-core foraminiferal samples in 16 vibracores taken from the modern shelf, shoreface, ebb-tide delta, and inlet environments from the Bogue Banks region. A majority of paleoenvironmental classifications of down-core deposits using discriminant analysis were of low probability. However, application of the results of this study to interpretation of cores from previous studies in coastal North Carolina provided more detailed paleoenvironmental interpretations that complemented lithofacies interpretations.