Department or Program



Salt marshes and tidally controlled wetlands are known to have a carbon (C) sequestration potential up to 40 times greater per unit area than many terrestrial ecosystem types (Chmura et al., 2003; McLeod et al., 2011; Schlesinger and Bernhardt, 2013). Despite their immense potential as a C-sink, fresh and brackish water margins of salt marshes produce and emit methane (CH4), a powerful greenhouse gas (GHG) with a 100-year radiative forcing nearly 40 times that of carbon-dioxide (CO2) (Balcombe et al., 2018; Bridgham et al., 2013). The magnitude of CH4 emissions from salt marshes is highly variable, but decreased salinity from tidal restriction and variable hydrology is known to increase CH4 production and export. (Kroeger et al., 2017; Poffenbarger et al., 2011). In this study we use gas chromatography to quantify monthly CH4 emissions from the freshwater margin of the Sprague Marsh, a tidally controlled salt marsh in Phippsburg, Maine. Per-cent lost on ignition (%LOI), dry bulk density (DBD), and a lead (Pb) horizon-based accretion rate are used to estimate overall C-sequestration, and hydrogeological monitoring is used to assess the effects of porewater and groundwater salinity regimes on the calculated C-sink capacity of the freshwater margins of glaciofluvial marshes.

Our findings indicate that although mechanisms of CH4 production in salt marshes are complicated, there is a significant linear inverse relationship (y = -2.916x + 49.217, R2 = 0.5247, P < 0.05) between porewater salinity and CH4 efflux. Total CH4 emissions estimated at 1.16 mols hr -1 , negating roughly 14.7% of the calculated C-sequestration capacity of the area of study. Furthermore, groundwater salinity regimes at the tidal margins of salt marshes are dominated by freshwater runoff from the surrounding upland, with limited upgradient migration of saline tidewater even during astronomical and daily high tides. Porewater salinity responds more readily to surface processes than to groundwater dynamics, and thus appears decoupled from groundwater salinity, but further study of this relationship is necessary.

There is growing recognition for the importance of restoring tidally controlled wetlands to reduce atmospheric CO2 concentrations, yet the global extent of salt marshes has shrunk 25% in the past two centuries (Duarte, 2002), and New England salt marshes face existential threats in the face of global sea level rise (SLR) (Gedan et al., 2011). As the effects of global climate degrade and destroy valuable ecosystems across the globe (IPCC, 2019), and atmospheric greenhouse gas concentrations reach the highest levels in nearly a million years (WMO, 2020), increasing our understanding of GHG dynamics in salt marsh systems is hugely important.

Level of Access

Restricted: Embargoed [Open Access After Expiration]

First Advisor

Beverly Johnson

Date of Graduation


Degree Name

Bachelor of Science

Number of Pages



Available to all on Friday, August 16, 2024