Paper No. 8
Presentation Time: 10:50 AM
COLONIZATION OF A HURRICANE OVERWASH FAN BY FORAMINIFERA, OCRACOKE ISLAND, NORTH CAROLINA
Hurricane Isabel made landfall on September 18th, 2003, depositing several large, 0.5 to 1 m thick overwash fans across the back-barrier marsh of Ocracoke Island, North Carolina. For the next seven years, we surveyed the elevation annually, measured sand thickness, and collected samples for lithologic and foraminiferal analysis across an overwash fan that extended to the back-barrier Pamlico Sound. The first significant environmental change occurred between 2004-2005 when 35% of the unvegetated overwash sand was removed, presumably by deflation, mostly from the ocean side of the transect. From 2005-2008 the overwash deposit remained virtually unchanged and the surface continued to be barren of marsh plants and foraminifera. In 2008-2009, the marsh grass Spartina patens began to populate the surface in small, isolated patches. Typical agglutinated salt marsh foraminifera were still absent. However, samples at the highest elevations of the transect contained abundant live specimens of Trichohyalus aguayoi, a calcareous taxon not previously recorded north of the Caribbean. In 2009-2010, verified tide data reveal 30 instances when wind-generated tides sustained elevations 0.3 m above mean higher high water (four times more frequently than any of the previous years). Much of the remaining overwash sand was removed, predominantly from the sound side of the transect. Only 20% of the original overwash deposit now remains, mostly on the oceanside of the transect. The sound side is now below the elevation of mean higher high water and the surface is covered by the marsh grass Spartina alterniflora. Live foraminifera are abundant, the assemblage is completely agglutinated, and dominated by the salt marsh taxa Arenoparrella mexicana and Trochammina inflata. The average elevation of the undeflated portion of the overwash deposit is still ~0.15 m above the tidal frame. Although now entirely covered by Spartina patens, marsh foraminifera have not yet repopulated this part of the fan. The calcareous species Trichohyalus aguayoi is still abundant. We do not expect salt marsh foraminifera to recolonize this part of the overwash fan until its elevation is reduced to ca. mean higher high water – unlikely in the near term due to the dense vegetation.