DECAPITATED PALEO-CHANNELS AND GROUNDWATER DISCHARGE: IS THERE A CONNECTION? ONSLOW BAY, NORTH CAROLINA

Many rivers drain the East Coast of the United States today, and these same rivers would have extended across the continental shelf during the last major low stand in sea level (i.e., the Last Glacial Maximum (LGM)). The LGM pathways of most paleo-rivers across the shelf remain unknown. During Late Pleistocene and Holocene sea-level rise, these systems likely were filled, decapitated and otherwise modified by coastal processes. The heterolithic fill of these features should provide a detailed record of sea-level rise, and may cause them to function as modern groundwater conduits, with significant impact on continental shelf ecosystems. In this study decapitated channels on the North Carolina shelf were identified and examined with geophysical and radiochemical methods to determine their morphology and evaluate their potential role as groundwater conduits. More specifically, a Reson 8101, an Edgetech 512i and a Rad-7 flow-through radon system were employed to collect bathymetry/backscatter, sub-bottom profiles, and water-column Rn-222 activity. An abundant distribution of shallow and deep channels is evident on the portion of the shelf surveyed. Most of the channels imaged were covered by shoreface sands, but some, those most seaward, have surface expression and appear to impact shelf sediment distribution. Collectively, the data provide convincing evidence for the association of elevated Rn-222 activities with channel locations, suggesting their role as conduits of groundwater. However, a similar relationship could result from channel fill of U-rich sediments. More research is needed to evaluate the magnitude and the nature of groundwater exchange to the shelf.