ESTABLISHMENT OF PROXY INDICATORS OF MANGROVE AND SALT MARSH DEPOSITION AND APPLICATION TO SOIL CORES

Florida’s coastal saline wetlands contain a diverse mosaic of woody mangroves and herbaceous salt marsh vegetative communities, with mangrove communities abundant in the lower latitudes, generally below ~29°, and salt marsh communities abundant at higher latitudes. Physiological tolerance to cold is a primary control on the position of the ecotone between mangroves and salt marshes and logic suggests that decreases in the occurrence in killing frosts, as associated with anthropogenic climate warming, will favor mangrove expansion in Florida and, more generally, across the subtropical world. Sediment cores at the current mangrove/salt marsh ecotone provide an opportunity to test a hypothesis of modern mangrove expansion by determining the past abundance of mangrove versus salt marsh deposition. Our research establishes sediment proxy indicators of salt marsh and mangrove environments through analysis of surficial deposits of each ecosystem type. Mangrove deposits contain woody fragments and have generally lower δ¹³C values whereas Spartina alterniflora - dominated salt marsh deposits contain grass fragments with generally higher δ¹³C values. These proxies are applied to radiocarbon-dated soil cores from intertidal wetlands in St. Augustine, FL, which lies at the approximate current northern limit of significant mangrove expansion on the east coast of Florida. Initial data suggest that the mangrove stands that we cored established only recently, with one of the cores showing recent mangrove presence after ~2000 years of salt marsh sedimentation.