Date of Award

2008

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Daniel F. Belknap

Second Committee Member

Joseph T. Kelley

Third Committee Member

Andrew S. Reeve

Abstract

Sea-level rise, presently about 2 mm/yr in Maine, affects the entire coast and is expected to rise over the next century. Even slight sea-level rise may have dramatic impacts on low-lying areas by changing soil salinity, eroding the shoreline, or causing insitu drowning. Salt marshes and adjacent wetlands often form in these low, susceptible areas. Although many studies have investigated what may happen at the ocean to saltmarsh boundary, few seem to have focused on the transition between salt marshes and adjoining freshwater environments. One UK study suggests that bogs collapse when they encounter the sea. This study examines this question, the possibility of preservation, and assesses the transgression rates occurring at the salt-marsh to freshwater transition in four sites within Downeast Maine. Carrying Place Cove in Lubec exposes both a transgressing beach on its seaward side and a rapidly eroding peat bluff on its northern tidal-flat margin. Hay Creek in Jonesport shows an unusually abrupt transition of a tidal creek and salt marsh intersecting a raised bog, representing an ecological succession driven by salinity and frequency of flooding. Grand Marsh in Gouldsboro consists of a broad, high salt marsh gradually transitioning into a brackish-to-fresh marsh at the inflow of a minor stream. Two hypotheses are tested to determine whether an interfingering onlap or erosional unconformity happens between the two habitats. Testing includes ground-penetrating radar (GPR) to examine salinity changes in the groundwater and the stratigraphic contact layers down to the Presumpscot Formation, a glaciomarine clay deposited during the last sea-level highstand. Hand-auger (Dutch) cores groundtruth the GPR profiles and provide further analysis of the contact between salt and freshwater peat deposits. Saltwater intrusion appears to be leading the surficial vegetation changes. Cores show the transgression currently occurs so rapidly that no significant amount of salt marsh peat has accumulated below the surface near the transition zone. Examination of the contact supports the erosional transgression hypothesis with no interfingering between environments. The dominant factor initiating the erosion, whether physical or ecological, can not be resolved from the data. Carbon-14 dating of salt-marsh peat overlying freshwater peat suggests marsh accumulation rates are increasing with increasing sea-level rise. Time-series aerial photographs analyzed with a Geographic Information System (GIS) program measure transgression rates and show land losses. Adjacent freshwater wetlands do not appear capable of equivalent accretion and subsequently are being transgressed.

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