How Old Are Marshes on the East Coast, USA? Complex Patterns in Wetland Age Within and Among Regions

Sea-level dynamics, sediment availability, and marine energy are critical drivers of coastal wetland formation and persistence, but their roles as continental-scale drivers remain unknown. We evaluated the timing and spatial variability of wetland formation from new and existing cores collected along the Atlantic and Gulf coasts of the United States. Most basal peat ages occurred after sea-level rise slowed (after similar to 4,000 years before present), but predominance of sea-level rise studies may skew age estimates toward older sites. Near-coastal sites tended to be younger, indicating creation of wetlands through basin infilling and overwash events. Age distributions differed among regions, with younger wetlands in the northeast and southeast corresponding to European colonization and deforestation. Across all cores, wetland age correlated strongly with basal peat depth. Marsh age elucidates the complex interactions between sea-level rise, sediment supply, and geomorphic setting in determining timing and location of marsh formation and future wetland persistence. Plain Language Summary Marshes, a type of coastal wetland, form under stable, slow rates of sea-level rise (SLR). Although many marshes along the East Coast of the United States formed after the most recent slowing of SLR (approximately 4,000-6,000 years ago), there is evidence that sediment erosion from colonial deforestation expanded marshes. This study uses sediment cores to determine when and where marshes formed to understand what drives modern marsh formation. We found that most marshes did form following SLR slowing after around 4,000 years ago. We also found evidence of marshes forming during European colonization in the northeastern and southeastern United States. Finally, we found recent formation of marshes closest to the coast, possibly driven by storm events and associated overwash. Through this study, we outline a novel way to easily determine marsh age and to better understand why marshes form through future research.