Inorganic Sediment Budget for the North Inlet Salt Marsh, South Carolina, U.S.A.

Abstract

Most salt marshes along the east coast of North America appear to accumulate sediment at a rate sufficient to keep pace with the rise in eustatic sea level and local subsidence. Thus, these marshes must be importing sediment from the coastal ocean and/or the adjacent estuaries. The sediment accumulating in the North Inlet salt marsh, South Carolina, is 80% inorganic, and, based on ²¹⁰Pb dating, is accreting at a rate of 0.098 g cm⁻² yr⁻¹, equal to a 2.7 mm yr⁻¹ vertical sedimentation rate. Tide gauge records show a relative sea level rise of 2.2 to 3.4 mm yr⁻¹, indicating this marsh is maintaining its elevation relative to mean sea level rise. The North Inlet salt marsh has two avenues of sediment exchange: (1) through the tidal inlet to the ocean and (2) through Winyah Bay, the adjoining estuary. Long-term inorganic suspended sediment flux through the inlet is calculated to be a net import of 1.35 kg s⁻¹, based on application of a tidal hypsometric flow model to seven years of daily suspended sediment concentrations. However, the import required to balance relative sea level rise is only 0.80 kg s⁻¹, implying an excess net import of 0.55 kg s⁻¹. The difference between import and accumulation is explained by the progradation of the marsh toward Winyah Bay, a conclusion which is supported by the geomorphology and stratigraphy of the marsh-estuary border. Thus, the North Inlet marsh imports sediment on the average through the tidal inlet, at a rate which allows for both vertical accumulation at a rate approximately equal to the relative sea level rise and also lateral expansion of the marsh.