<p>Inorganic sediment supply is a critical component of a salt marsh&#8217;s ability to vertically aggrade in response to relative sea level rise, yet there remains significant uncertainty on the primary sources, timing, and rates of sediment delivery to marshes. This is particularly true for the Northeastern, U.S. Atlantic coastline where the magnitude and sourcing of sediment varies widely due to post-glaciated landscapes. Here we present results from a 3-year study between 2020 and 2023 designed to inform management and restoration decisions related to northeast marshes through the development of a scalable method for assessing the availability and distribution of inorganic sediment to and within marshes, including the identification of thresholds of inorganic sediment delivery required to maintain a stable marsh platform under various rates of sea level rise for the region. Field investigations involved instrumental observations, deployment and recovery of seasonal sediment traps, and the collection and analysis of marsh core samples. The study targets 12 marsh systems spanning environmental gradients for the region that allowed us to examine different sources and delivery mechanisms of sediment. Our compilation of existing data reveals spatial variability in marsh accretion rates, but also highlights regional trends and the general agreement among rates determined through a variety of different methodologies and time spans. Our instrumental observations and sediment trap deployments confirm differences in sediment delivery among marshes. Back barrier marshes with relatively small watersheds predominantly accumulated inorganic sediment during the fall in response to large storms and wave activity suspending coastal and offshore sediment deposits (marine sources) that are carried into marshes through tidal advection. In contrast, marshes proximal to large rivers (>10,000 km<sup>2 </sup>watersheds) have higher accumulation rates and receive the bulk of their inorganic sediment in response to fluvial delivery of terrestrial sediment during spring freshet events. Among our 12 study marshes, only one experienced its highest rate of sediment accumulation during summer months, which we attribute to substantially greater crab herbivory promoting internal recycling of sediment. Overall, we have measured sediment accumulation in over 450 individual traps across spring, summer and fall seasons in twelve marshes. The results from the analysis of these samples represents the largest dataset of its kind for the region and enable defining regionally appropriate input variables for modeling the spatial variations of sedimentation across marsh surfaces as a function of tidal inundation and distance to the nearest channel, as well as providing defined sedimentation limits needed to sustain healthy marsh growth under future sea level rise and various potential restoration pathways.</p>
<p>Elevation is a key control on the frequency and duration of flooding experienced by a salt marsh over the course of the tidal cycle, which in turn modulates the deposition of sediment onto the marsh surface. The amount of sediment deposited onto the marsh surface is an important factor in the development of the salt marsh and its ability to withstand sea level rise. Human interference in the form of agricultural practices (e.g., ditching and embayments) and mosquito control significantly altered the structure and function of salt marshes throughout New England with lasting impacts on marsh platform elevation and, consequently, the persistence of salt marshes in the face of sea level rise. This study establishes the present-day distribution of elevation and vegetation zones for a salt marsh in Maine, United States, and compares these baseline measurements to past estimates of elevation made using carbon stable isotopes (&#948;<sup>13</sup>C) measured in sediment cores. A LiDAR scan and a series of multispectral air photos were collected from a representative salt marsh in Maine (Cousins River Marsh, Yarmouth, ME). The LiDAR scan is processed to create a digital elevation model (DEM) of the marsh and the air photos are converted into a 2D digital model of the marsh platform. In New England salt marshes, an elevation-mediated gradient in vegetation exists across the marsh surface, with the most salt-tolerant species residing in lower-elevation areas. Different species of marsh grasses produce varying &#948;<sup>13</sup>C values, and once incorporated into the marsh peat, can potentially be used to identify changes in vegetation cover through time. Sediment cores collected from Cousins River are sub-sampled and analyzed for down-core variations in &#948;<sup>13</sup>C to assess salt marsh paleovegetation. Short-term radioisotopes <sup>210</sup>Pb and<sup> 137</sup>Cs are used to produce age-depth models by integrating sedimentation over ~100 and ~70 years, respectively, and are correlated to stable carbon isotope results for an approximation of salt marsh elevation change. Results will inform our understanding of the relative influences of sea level rise and human-driven landscape alteration on salt marsh morphodynamics along the coast of Maine, with implications for salt marshes throughout New England.</p>
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