IMPACT OF SEDIMENTARY AND GEOLOGIC PROCESSES ON SEDIMENT DYNAMICS AND SALT MARSH ACCRETION IN THE SOUTHEASTERN U.S.

Expansive salt marshes are a key feature of the Georgia coast, providing a variety of ecosystem services such as storm protection, carbon sequestration, and vital nursery habitat. However, the longevity of these marshes is threatened by accelerated sea level rise (SLR) and decreased sediment inputs. Decreasing sediment supply hinders the ability of the salt marshes to naturally accrete and maintain the ideal position within the tidal frame. The study area is Camden County, GA, which has a smaller tidal range (1.83 m at Fernandina Beach vs. 2.11 m at the Savannah River) and a lower rate of SLR (0.23 ± 0.02 cm/y at Fernandina Beach vs. 0.36 ± 0.03 cm/y at the Savannah River). This lower rate of SLR in Camden County is due to the lower rate of land subsidence (-0.03 cm/y in Camden County vs. -0.54 cm/y on average for the rest of the Georgia coast). With a majority of marsh accretion data for Georgia coming out of areas with high subsidence rates, the goal of this study is to determine the impact of variation in sea level rise and subsidence on salt marsh resilience and future persistence in Camden County, GA.

Fifteen push cores were collected throughout the county. Sediment composition is dominated by clay (50-80%), with less silt (10-40%) and sand (0-20%), reflecting the dominantly muddy nature of sediment supplied to and transported within the back-barrier system. Lead-210, a naturally occurring radionuclide, was measured using gamma spectroscopy and documents marsh accretion rates of ~0.2 cm/y (0.06 g/cm²*y). This accretion rate is very similar to the rate of sea level rise measured at Fernandina Beach, FL, suggesting that marshes in Camden County are keeping up with sea level rise. This is not the case along the majority of the Georgia coast, where the rate of SLR is outpacing marsh accretion. Short-term mass accumulation, measured by filters deployed over 2 high tides, exhibit rates ranging from ~1-3.6 g/cm²*y, depending on location. Monthly vertical accretion rates, measured from fixed hard artificial substrates in the marsh, range from ~0.6-11.6 cm/y. These short-term rates are orders of magnitude greater than what we see on 100-year timescales, indicating the ephemeral nature of much of the material delivered to the marsh on daily and monthly timescales.