SALT-MARSH POOLS, MARSH-SEDIMENT BUDGETS, AND ACCELERATED SEA-LEVEL RISE: A LOOSING SCENARIO FOR NORTH-TEMPERATE, MESO-MACRO-TIDAL MARSHES?

Salt marshes, including those in backbarrier settings, are vulnerable to rising sea level in response to climate change. Studies in the Gulf of Mexico and Mid-Atlantic regions suggest that one response of salt marshes to sea-level rise is an irreversible state change through the expansion of pools in interior marsh sections, leading to the rapid conversion of once-vegetated surfaces to open water over short time periods. In Maine’s meso-macro-tidal marshes (tidal range 2-6 m), previous work shows that a dynamic exchange between pools and tidal creeks is one mechanism that reverses this process at the local scale through pool drainage and subsequent revegetation. Lead-210 and ¹³⁷Cs dating of sediments in two paired cores collected through revegetated pools and adjacent high-marsh surfaces from Wells and Brunswick, ME, indicate that revegetated pools rapidly accumulate sediment at 2-4.5 times the rate of the neighboring marsh platform. This result is consistent with other modeling results that predict that disturbed, recently revegetated salt-marsh patches accrete faster than undisturbed vegetated patches. Our results have important implications for the overall sediment budget of salt-marsh systems, especially in the northeast where pools are prominent features, and for the capacity of these systems to sustain themselves in response to rising sea level. With accelerated sea-level rise, two predicted system-level responses are an increase in the tidal prism and channel deepening, both of which will increase the hydraulic potential between pools perched on the salt-marsh platform and deepened tidal creeks, in places like Maine. In a system with a limited or finite sediment supply, pools may then capture a disproportionate amount of the sediment load available to the marsh surface. This, in turn, may starve other portions of the marsh platform from receiving this load, creating a positive feedback system where pools drive surficial marsh dynamics and the marsh platform is unable to sustain itself over the long-term.