RETREAT OF MID-ATLANTIC (US) BARRIER SHORELINE LINKED TO CHANGES IN OCEAN CIRCULATION 2,900 YEARS AGO

We analyzed and dated backbarrier deposits recovered in vibracores from an infilled paleochannel offshore Shackleford Banks, North Carolina. Based on onshore GPR and offshore seismic surveys, the paleochannel is inferred to be a relict Holocene tidal inlet. The three studied vibracores (from ~9.5 m water depth) retrieved up to 5 m of sand, shelly gravel and silty mud; the mud units were up to 2 m thick. Four samples of Elphidium excavatum (estuarine benthic foraminifera) from the mud intervals were radiocarbon dated and yielded midpoint calibrated ages of 3030, 3150, 3240 and 3290 yrs BP. The observed lithostratigraphy and C-14 dates require the Shackleford barrier island shoreline to have moved approximately 2 km landward since around 3,000 yrs BP.

Our data do not place a younger boundary on the suggested shoreline retreat, but Mallinson et al. (2008; Quat. Res., v. 69) suggest the beach ridges in Kitty Hawk, NC formed 2,900-2,500 yrs BP, immediately after a rapid shoreline retreat of >2 km. If the Shackleford and Kitty Hawk shorelines responded to the same forcing, then the retreat event is constrained between 3 and 2.5 ka, an interval during which numerous paleoclimate studies report abrupt northern hemisphere cooling. Although its cause is uncertain, the climate change pattern is consistent with a reduction in the strength of the Atlantic meridional overturning circulation, and paleoceanographic studies report changes in deepwater composition around this time. We hypothesize that the observed shoreline retreat was a response to persistently elevated water levels along the east coast of North America associated with a dynamic, local sea-level response to the slowing of the Gulf Stream that in turn resulted from reduced deepwater formation in the North Atlantic.

Recent observations and modeling studies have shown that the dynamic sea-level rise component associated with reduced Gulf Stream velocity may be on the order of 20 cm or more along the East Coast of the US by 2100 AD due to changing ocean circulation; this rise is in addition to global steric effects, global ice volume and regional gravitational effects on sea level. We suggest that the ancient shoreline retreat event in North Carolina may provide an analog for future impacts of sea-level change on US East Coast barriers.