STORM DEPOSITS IN BACK-BARRIER MARSHES FROM ONSLOW BAY, NORTH CAROLINA

Reconnaissance gouge auger coring and foraminiferal analysis of eighteen cores taken from seven localities along Onslow Bay, North Carolina, revealed a total of seventeen storm deposits (five total storms) from the upper 3.0 m of back-barrier marsh strata. Coring localities included the back-barrier marshes of Caswell Beach, Kure Beach, Alligator Bay, Bear Island, Hope Pole Creek, Tar Landing Bay, and the southern Core Banks Islands. Only two localities, Alligator Bay and Tar Landing Bay, contained sediments that were enriched with offshore foraminifers that indicate storm deposition. Radiocarbon analysis of in situ marsh halophytes dates this deposition to the last 1,500 years.

Storm layers within the marsh strata were detected based on an increase in the number of offshore calcareous foraminifers. Common taxa in the offshore assemblages include Quinqueloculina seminulum, Nonionella atlantica, and Cibicides lobatulus. Elphidium spp. and Ammonia spp., while present in the offshore assemblages, were not considered indicative of an offshore source as they were also present in modern low marsh and estuarine assemblages.

Radiocarbon analysis of buried halophytes indicates sedimentation rates for these marshes of 2.03 to 2.27 mm/year (Tar Landing Bay), 1.31 to 1.91 mm/year (Alligator Bay), and 0.97 to 1.05 mm/year (Caswell Beach). Because the Caswell Beach marsh did not received (preserved) sediment via hurricane deposition during the last 2,000 years, the overall sedimentation rate is approximately half that of the other marshes.

Most storm deposits could not be correlated between sites. Nevertheless, a 10-cm thick storm layer, or series of bioturbated storm layers, was found between 85 and 105 cm below the surface in several cores from different localities, suggesting deposition from major hurricane strike(s) approximately 450 to 550 years ago. This deposition may represent a series of major hurricane strikes in a relatively short period of time or may be the result of better storm layer preservation. Interestingly, the cores contained little stratigraphic evidence of the period of increased hurricane landfalls to North Carolina during the late 1950's (Hurricanes Hazel, Connie, Ione, Helene, and Donna).