THE CRITICAL LEADING EDGE OF GULF OF MAINE SALT MARSHES – INTERFACE WITH FRESHWATER WETLANDS

Down East coastal Maine possesses extensive bogs, some of which are intersected by the leading edge of salt marshes. Modern sea-level rise of ~2 mm/yr in eastern coastal Maine is 2-3 times the late Holocene average. While numerous studies have focused on the effects of sea-level rise on salt marshes, fewer studies have addressed their intersection with freshwater wetlands. We investigated rates of migration of the salt marsh–freshwater transition zone and the stratigraphic record left by this migration. Multiple techniques were employed to quantify succession rates of change of freshwater wetlands to salt marsh, to establish the stratigraphy, and begin to characterize the catalyst for transgression at the transition zone at four sites along the Down East coast. At Jones Marsh, Grand Marsh and Hay Creek, sequential aerial photography over the time span of ~50 years, where the freshwater wetland is buffered by a salt marsh, was examined for changes. Difficulties with co-registration and boundary uncertainties did not allow for detection of measurable change. The transgression rate of the salt marsh is not occurring on the order of decimeters of change per year, and thus the stratigraphy in that transition zone is >50 years old. At Carrying Place Cove, the only site that directly abuts the open marine environment, photographic comparison was successful, demonstrating erosion rates of up to a meter per year. Ground-penetrating radar (GPR) revealed general substrate stratigraphy and guided coring and sampling transects. The interface of freshwater wetlands and salt marsh at depth corresponds with a blanking of the GPR signal by saltwater. Dutch cores generally penetrate salt marsh over freshwater peat, and refuse in glaciomarine mud or sand. The results from GPR and coring suggest that the ongoing transgression results in a clear erosional unconformity. The base of the section shows a break in radiocarbon-dated samples from a few hundred to thousands of years between the salt marsh and the underlying bog. The freshwater bog is accumulating at much slower rates (~0.05 mm/yr) than the salt marsh (0.5 mm/yr). These rates of accumulation lag current and likely future rates of sea-level rise. The lateral rate of salt marsh-freshwater bog interface migration is not happening on the time scale of management interests.