High resolution studies of late Holocene relative sea -level change (North Carolina, USA)
Records of late Holocene relative sea level (RSL) change are critical for understanding the mechanisms that drive current changes in RSL and for placing future scenarios of sea-level rise in an appropriate context. This study provides a high resolution (decadal and decimeter scale precision) record of RSL change from North Carolina for the last 500 years using salt marsh foraminifera and composite chronologies to elucidate the timing and magnitude of a recent acceleration in the rate of sea-level rise.
A modern dataset of salt marsh foraminifera from the Albemarle–Pamlico Estuarine System (APES) of North Carolina was used to investigate their utility as sea-level indicators in comparison to alternative lithological, biological (diatoms) and geochemical (δ13C) proxies. I defined ecological zones from ten salt marshes, which demonstrated that foraminifera have a strong relationship to elevation within the tidal frame. Salt marsh foraminifera are accurate and precise (± <0.1m) sea-level indicators and provide a reliable basis for reconstructing subtle changes in RSL. The principal biozones of foraminifera have distinctive spatial distributions, which may migrate over short intervals of geological time in response to the ephemeral nature of inlets. Regional-scale datasets that incorporate the dynamic nature of the APES are most suitable for RSL reconstruction. Diatoms are less reliable due to a lack of analogy between high diversity modern and fossil samples. δ 13C values differentiate between sediments of C3 and C 4 floral origin, but currently have limited precision as a sea-level indicator.
Cores of salt marsh peat were recovered from two sites in the APES. Age-depth models were produced using composite chronologies developed from AMS, high precision and bomb-spike radiocarbon coupled with 210Pb and 137Cs dating and a pollen chrono-horizon. I developed a transfer function to estimate former RSL from foraminifera preserved in each core. Reconciliation with tide gauge records demonstrated that this approach can bridge the gap between instrumental and geological records of RSL change. I identified a 2.2mm/yr increase in the rate of rise that began in the late 19th century. This acceleration is broadly synchronous with other studies from the Atlantic coast, although the magnitude is larger and indicative of a latitudinal trend along the US Atlantic coast.