Woods Hole Oceanographic Institution

Dr Rob. L. Evans

»Electrical Structure of the Central Cascadia Subduction Zone: The EMSLAB Lincoln Line Revisited
»Electrical Lithosphere Beneath the Kaapvaal Craton
»Gulf Of Mexico Gas Seep
»MELT MT Results
»Wrightsville Beach Geophysics and Hydrology
»MELT Area Off-Axis Structure
»Karst Formation off North Carolina
»Review of Shallow Offshore EM Work
»Towed EM System
»EPR MMR Experiment
»Offshore MT and Subduction Systems
»Shallow Porosity Structure on the Continental Shelf
»Oceanic and Continental Mantle Resistivity
»New Jersey EM Survey
»Eel River EM Survey
»Impact of groundwater on EM data
»Electrical structure of Slave Craton
»Report of Shoreline Change Workshop

Rob. L. Evans and Dan Lizarralde, Geophysical evidence for karst formation associated with offshore groundwater transport: an example from North Carolina, G3, 2003

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 karstificaion. 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 U.S., where carbonate aquifers are prevalent.

FILE » 2003GC000510_7965.pdf

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