Abstract:
Pathways of exchange between the shelf and slope in the Mid-Atlantic Bight were investigated
using a combination of radiochemical tracer and hydrographic measurements.
The motivation was to provide evidence of transport routes for shelfwater that could be
important to the balance of shelf-slope exchange, as well as to the biogeochemical fluxes
across this crucial ocean boundary. The four radium isotopes, with half-lives of 4 days to
1600 years, a coastal source, and conservative properties in seawater, were used as coastal
water mass tracers. The final study was comprised of data from 5 cruises, with a total of 8
cross-shelfbreak transects. Two areas were studied, a northern Mid-Atlantic Bight transect
south of Nantucket Shoals, and a southern Mid-Atlantic Bight series of transects off the
coast of Delaware. In addition, data were collected from the shelfbreak at Cape Hatteras
crossing the western wall of the Gulf Stream to help determine sources of anomalous 224Ra
enrichment which was observed on several of the shelfbreak transects. Combined with the
hydrographic data, radium measurements suggested a pathway for exchange in the Mid-
Atlantic Bight that was not a direct advection of shelf water toward the slope. Rather, the
evidence suggested limited direct exchange of surface shelf water across the shelfbreak front.
This provides observational evidence that is consistent with models (e.g., Gawarkiewicz and
Chapman, 1991) which predict the shelfbreak front will impede exchange. Furthermore,
224Ra activity on the upper slope points to a rapid transport pathway for bottom water
from the Cape Hatteras shelf via the Gulf Stream onto the Mid-Atlantic Bight slope. The
radiochemical and hydrographic evidence suggests that recirculation around the slope sea
gyre may be a more important pathway than direct cross-shelf transport.
