Extension of the Late Holocene Sea-Level Record In North Carolina, USA

Abstract

Future sea-level rise will dramatically affect coastal landscapes and populations. The coast of North Carolina (USA) is particularly vulnerable to sea-level rise because its low-lying coastal plain is expansive, has a low gradient, provides significant ecosystem services and is economically important. In order to understand how future sea-level rise may affect the coast, it is necessary to study past sea-level rise. Widespread salt-marshes compose much of North Carolina's coastal system, providing an excellent environment from which to produce relative sea-level reconstructions using salt-marsh foraminifera, whose distribution is controlled by tidal elevation. Distinctive assemblage zones related to different tidal ranges can be recognized in salt-marsh foraminiferal assemblages, allowing them to be used as a proxy for reconstructing sea level as sea-level indicators. Foraminiferal assemblages from surface samples along two transects at Sand Hill Point on Cedar Island, North Carolina added to an existing modern training set of paired observations of foraminiferal assemblages and tidal elevation; these data provide local analogues for interpreting fossil assemblages using a locally weighted-weighted average (LWWA) regression model. Foraminiferal assemblages preserved in a radiocarbon-dated core of salt-marsh peat from Sand Hill Point were used to produce a continuous, high-resolution late Holocene relative sea-level reconstruction. The existing late Holocene RSL reconstruction from North Carolina is based on two sites: Sand Point on Roanoke Island and Tump Point on Cedar Island. The Sand Point record spans the last ~2200 years, but the Tump Point record spans only the last ~1000 years. Therefore, the sea-level history described from 200 BC to 1000 AD is based on only one site. The new sea-level reconstruction from Sand Hill Point extends the existing record from nearby Tump Point, NC by 1400 years, producing a high resolution, continuous record of sea-level change spanning 1500 BC – 1915 AD. This new record tests whether patterns and rates of late Holocene sea-level changes reconstructed elsewhere in North Carolina are consistent throughout the region. The calculated average rate of relative sea-level rise for Sand Hill Point of 0.7 mm/year is consistent with patterns of regional rates along the US Atlantic coast, which may be partly attributed to isostatic response to deglaciation of the Laurentide Ice Sheet.