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

THE CRITICAL LEADING EDGE OF GULF OF MAINE SALT MARSHES –

INTERFACE WITH FRESHWATER WETLANDS

By Margot Elena Mansfield

Thesis Advisors: Dr. Daniel F. Belknap and Dr. Joseph T. Kelley

An Abstract of the Thesis Presented
in Partial Fulfillment of the Requirements for the
Degree of Master of Science
(in Earth Sciences)
August, 2012

Courtesy of Daniel F. Belknap, 2011 

Sea level has risen at a rate of ~2 mm/yr in eastern coastal Maine. While numerous studies have focused on the effects of sea-level rise on the seaward edge of salt marshes, fewer studies have addressed the movement of the landward boundary of these marshes with adjacent freshwater wetlands. By shifting the landward boundary of salt marshes, continued sea-level rise will impose local geographic changes on the Maine coast. Given that sea level exerts such a strong influence on salt marshes, the chrono-stratigraphic record of marshes allows predictions of future changes at the edge of salt marsh and freshwater wetlands. Down East coastal Maine is unusual among the lower 48 states because it possesses extensive bogs, some of which are intersected by the leading edge of salt marshes. This study documents past behavior of the salt marsh–freshwater transition zone along the Down East coast in terms of rates of migration of the marine edge into terrestrial environments and the stratigraphic record left by this migration. Multiple techniques were employed to quantify succession rates of freshwater wetlands to salt marsh, establish the general stratigraphic signal, and begin to characterize the catalyst for transgression at the transition zone at several sites along the Down East coast (Jones Marsh, Mount Desert Island; Grand Marsh, Gouldsboro; Hay Creek, Jonesport; and Carrying Place Cove, Lubec). The results from the field suggest that the ongoing transgression results in a clear erosional unconformity. Dating the base of the section shows a break from a few hundred to thousands of years between the salt marsh and the underlying bog. Carrying Place Cove, the only site that directly abuts the open marine environment, experiences erosion rates of up to a meter per year. An age model from Hay Creek illustrates the distinct, order-of-magnitude difference in accumulation rates for the freshwater and salt marsh environments. The freshwater bog (~0.05 mm/yr) is accumulating at much slower rates than the salt marsh (0.5 mm/yr). These rates of accumulation lag current and likely future rates of sea-level rise. The rate of migration is not happening on the time scale of management interests.