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Saltmarsh evolution is closely linked to sea-level rise (SLR) and sediment supply. In a regime of accelerating SLR, saltmarsh survival depends on the ability of the marsh platform to grow vertically through organic and inorganic accumulation, and laterally via transgression over adjacent uplands. In formerly glaciated settings, the formation and maintenance of a saltmarsh are complicated by steep uplands, low sediment availability, isostatic rebound, changes to tidal amplitudes, and anthropogenic alteration. While much work has examined the development of large saltmarshes in New England since the mid-Holocene and Anthropocene, little is known about the smaller isolated saltmarshes that form in pockets along the glaciated coastline. We use radiocarbon dates from newly uncovered index points (basal peat) within Boston Harbor to date one of the oldest saltmarshes in the region (∼4.2 ka), and calculate the polynomial relative sea-level curve (RSLR) for the lower Gulf of Maine taking into account tidal amplification throughout the Holocene. This study informs the historical persistence of isolated saltmarshes under varying rates of sea-level rise (magnitude as much as 2 mm yr−1). Additionally, we refine the timing and rates of the mid-Holocene sea-level deceleration in this region: between 2.8 and 3.3 ka, slowing from 2.1 mm yr−1 to 0.5 mm yr−1. RSLR determined from the past century of tide gauge data is 2.86 mm yr−1, five times greater than historic rates over the past 2000 years. Recent vertical accretion rates (determined from marker horizons and 210Pb data) indicate that portions of Boston Harbor's saltmarshes are able to keep pace with current RSLR. However, historical diking and ditching practices appear to have resulted in elevation loss and conversion from high saltmarsh platforms to low saltmarsh platforms, largely influencing the amount of inorganic sediment needed to help maintain marsh surfaces. These results highlight that knowledge of historical anthropogenic modifications and changes in hydroperiod are critical when interpreting vertical accretion results. More importantly, anthropogenic modification within sediment starved saltmarshes may be the cause for cannibalization of marsh shorelines, whereby inorganic sediment eroded from marsh edges is recycled and delivered to the surface, facilitating vertical accretion that offsets RSLR. Under modern accelerated sea-level rise and decreased sediment supply, this cannibalization process may now be the only pathway for saltmarsh survival. • New index points from basal peat refine the sea-level curve for the lower Gulf of Maine. • Marsh cannibalization is an important process for saltmarshes facing SLR. • Anthropogenic modification impacts elevation dynamics in saltmarshes. [ABSTRACT FROM AUTHOR]

Accelerating sea-level rise (SLR) threatens salt marshes globally, and thus understanding how these ecosystems respond to this stress is vital to increasing their resiliency. In order to maintain their surface elevation as sea level rises, salt marshes must add below ground biomass and accrete sediment. In many cases, biota can significantly affect marsh accretionary and erosional processes by stabilizing or helping to mobilize sediment, which in turn can profoundly affect the morphological evolution of the marsh. However, these effects are poorly understood. Bioturbation by dense populations of the marsh crab Sesarma reticulatum has been found to facilitate expansion of tidal creeks and creek bank erosion in multiple areas. The influence of S. reticulatum bioturbation on sediment erodibility has not yet been quantified, due to difficulties involved in measuring the processes in a field setting. In this study, we used a laboratory flume to examine the effects of burrowing by common marsh crab species S. reticulatum and Uca pugnax on sediment surface roughness and erodibility. Measurements and observations of surface elevation, flow velocities, and sediment movement indicate that burrowing and feeding by S. reticulatum , and to a lesser extent, Uca pugnax, increase surface roughness and decrease the threshold velocities and shear stresses required for sediment erosion. Erosion associated with observed burrowing is of a similar magnitude to mean annual marsh surface accumulation, and, consequently, this mechanism for sediment loss in heavily burrowed areas has the potential to produce large-scale morphological changes. Ecological engineering, Coastal setting, Salt marsh, Geomorphological aspects, Soil erosion. USA, Texas, Galveston. • We determine the impact of Sesarma reticulatum and Uca pugnax on erosion thresholds. • S. reticulatum increase surface roughness, more so than other crab species. • Crab burrowing decreases velocity and shear stress thresholds needed for erosion. • Extensive burrowing by S. reticulatum facilitates headward creek erosion and marsh landscape evolution. [ABSTRACT FROM AUTHOR]