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
|
