EVENT SEDIMENTATION AND LONG-TERM ACCUMULATION IN A SALT MARSH: ST. LOUIS BAY, MISSISSIPPI, NORTHERN GULF OF MEXICO

Marshes are important to coastal settings because they buffer inland regions from the impacts of storm events and sea level rise, and they serve as nurseries for many species of marine organisms. However, studies from recent decades show that coastal marshes are suffering from losses due to natural subsidence, accelerated sea-level rise, and a host of human activities. Therefore, it is important to understand the natural processes that influence vertical accretion and lateral development of the marsh surface. A series of push cores (1 m x 10-cm i.d.) were taken from channel and high marsh settings in a brackish marsh environment in St. Louis Bay, MS, to investigate the relationship between long-term marsh accretion rates, and sediment transport associated with tropical storms and hurricanes. Cores were analyzed for gamma density, and magnetic susceptibility (using a multi-sensor core logger), as well as grain size, organic carbon, and ⁷Be-²¹⁰Pb-¹³⁷Cs geochronology.

Two upward-fining sandy layers with relatively sharp basal contacts and low organics were found interbedded in a matrix of organic-rich marsh muds, and are inferred to be event layers associated with major storm events. Accumulation rates estimated from ²¹⁰Pb and ¹³⁷Cs distributions are ~2.4 mm/y, and indicate that the two event layers were produced by major hurricanes that made landfall within 10 km of the study area in 1947 and 1969. These long-term accumulation rates are roughly comparable to rates of sea-level rise in the northern Gulf of Mexico. Because the event layers are each 1-4 cm thick, and compose at least ~25% of total accretion during the past fifty years, sediments deposited by storms in this region appear to be an important component of the long-term sediment budget.