SALINITY EXTREMES: THE DYNAMIC HISTORY OF AN OUTER BANKS ESTUARY

The salinities of estuaries along the Outer Banks barrier island system are highly dependent upon distance to the nearest active tidal inlet. Back Bay, Knotts Island Bay and Currituck Sound are the northern-most estuaries along the Outer Banks, located at the Virginia/North Carolina border at the former site of Old Currituck Inlet.

We conducted a field study of surface salinity measurements at 44 locations throughout Back Bay, Knotts Island Bay and Currituck Sound during the summers of 2002 and 2003 to determine modern conditions. A study of microfossil assemblages contained within 6 vibracores from an abandoned flood tidal delta in Knotts Island Bay produced a historical record of salinity. Through direct measurements, microfossil interpretations and a compilation of historical salinity records, we reconstructed a >500 year salinity history of this estuarine system. We find a 3-stage history of salinity extremes that mirror the behavior of Old Currituck Inlet. Working in reverse chronological order, we assign approximate salinities to post-inlet (since 1731), open-inlet (~1585 to 1731) and pre-inlet (prior to ~1585) stages.

Post-inlet, or modern, annual salinities hover near 2ppt but are heavily influenced by rainfall, storm activity and human intervention, and range from ~0.5ppt during years of above-average precipitation to ~10ppt during years of unusually high storm activity. Modern foraminiferal assemblages record salinities of 1-7ppt. Open-inlet salinities were likely comparable to modern estuarine salinities near Oregon Inlet, NC. Our foraminiferal assemblages suggest salinities at the inlet throat of ~34ppt. Pre-inlet salinities were much fresher and comparable to modern conditions. Our foraminiferal assemblages indicate a pre-inlet salinity of <6ppt.

The natural variation in salinity of Back Bay, Knotts Island Bay and Currituck Sound is extreme, both between stages, where the salinity ranges from near fresh to normal ocean salinity, and within stages where salinity is affected by rainfall, storm activity and human intervention.