LATE HOLOCENE SALTMARSH PEAT ACCUMULATION AND SEA LEVEL RISE IN CORE SOUND AND SOUTHERN PAMLICO SOUND, NORTH CAROLINA, USA

Peat accumulation allows coastal saltmarshes to persist in place as sea level rises, yet continuous peat build-up requires a dynamic equilibrium among rates of sea level rise, organic matter production, and mineral sediment supply. These ecogeomorphic processes underwent large variations over the last few centuries along the East Coast of North America. In an effort to isolate some of the factors controlling marsh persistence, we analyzed saltmarsh peat sequences deposited prior to significant human alteration of the adjacent coastal landscape. We obtained multiple cores from sites in eastern Carteret County, North Carolina: near Sand Hill Point on West Bay in southern Pamlico Sound, and Smyrna Creek (at head of Jarrett Bay) and Oyster Creek in Core Sound. At all three sites Juncus roemarianus dominates both the surface vegetation and the underlying peat material, the tidal range is less than 0.5 m, and salinities range from 10 to 28 ppt. Any differences in suspended sediment among the sites likely result from their proximity to the open waters of Core and Pamlico Sounds; while the marshes at Sand Hill Point and Smyrna Creek/Jarrett Bay are more sheltered, the Oyster Creek marshes are more exposed. Dry bulk density (BD) and organic matter content (OM) were analyzed at 2cm intervals in cores obtained using a Russian peat sampler. Analyses were made by weighing oven-dried, volume-specific samples, and by measuring mass loss-on-ignition after 4 hours at 550°C.

Preliminary findings show transitions from basal mineral-rich sediment (BD ≥ 0.8 ± 0.1 g/cm³; OM ≤ 10 ± 2%) upwards into saltmarsh peat (BD = 0.2 ± 0.11 g/cm³ and OM = 56 ± 16%) as deep as 3m below present MSL in isolated spots near Sand Hill Point and Smyrna Creek/Jarrett Bay. Across broad areas of the present marshes, the basal sediment-to-peat transition is found at a depth of 1.6m and peak OM content values (> 55%) occur just above this depth. At around 140cm and higher, however, a consistent decrease in OM and increase in BD is observed. Though more radiocarbon dates are pending, we interpret this as a response to accelerated local relative sea level rise, from 0.7mm/year up to 1.3mm/year, associated with onset of the Medieval Climate Anomaly. These findings imply that under certain conditions saltmarsh peat lithology is sensitive to subtle changes in sea level rise.