Geophysical evidence for karst formation associated with offshore groundwater transport: An example from North Carolina

Marine geophysical data from Long Bay, North Carolina, involving a novel combination of electromagnetic and high‐resolution Chirp seismics, show evidence of submarine karst formation associated with what has been inferred to be a site of high‐flux submarine groundwater discharge (SGD) a substantial distance offshore. Recently observed temperature and chemical signals from wells in this area provide the basis for the interpretation of the high‐flux SGD here, and they also suggest a terrestrial source for the groundwater and thus a potentially important route for nutrient transport to the oceans. Our data indicate that karstification is localized to the high‐flux zone, and we suggest that mixing of the chemically distinct (but saline) groundwater with seawater has resulted in the karstification. As karstification increases permeability and flux, a positive feedback would tend to progressively enhance submarine groundwater discharge. Our data reveal a significant local anomaly in apparent porosity: a dense block that may have initiated the local focusing of groundwater flow. Conditions favorable to the formation of similar locally punctuated sites of high‐flux SGD are likely to exist along the mid to inner shelf of the southeastern United States, where carbonate aquifers are prevalent.