Salt Marsh Migration into Lawns Revealed by a Novel Sediment-Based Approach

Upslope migration will be critical to the future survival and stability of tidal salt marshes in an age of accelerated sea-level rise, yet only a small number of studies have measured historic migration rates. We developed a new approach for reconstructing salt marsh migration that can be used even in situations where that migration would be undetectable by aerial photograph analysis, such as the growth of marsh plants under trees or the movement of marsh ecosystems into mowed lawns. Our approach involves identifying a wedge of salt marsh peat (overlying the pre-existing upland soil) through the presence of foraminifera and then dating that wedge using Cs-137. We demonstrate our approach by calculating migration rates (1963-2016) for two lawn-adjacent salt marshes along the Connecticut coast of Long Island Sound. Both marshes showed substantial migration over this time period, although vertical migration rates differed dramatically between the two sites, perhaps because of the differential influence of large storms. Further, we demonstrate that the presence of foraminifera (an unequivocal signal of tidal inundation) constitutes a more accurate indicator of the migration wedge than high organic content. Our relatively inexpensive approach could be used to assess migration rates at multiple sites in order to better understand the factors controlling marsh migration.