Saltmarsh sustainability throughout the Holocene in Boston Harbor: A new sea-level curve for the lower Gulf of Maine and implications of recent anthropogenic alteration

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.