Summary
Shallow marsh stratigraphy along Assateague Island highlights variations in marsh emergence along a barrier island system. In some cases marshes become protected by foredunes, and the marsh accretes with limited sediment input from the marine environment. In other examples, the marsh emerges from relict inlet features that remain low and easily overwashed. Overall, these data highlight the importance of storms as a vector to exchange sediment between adjacent environments (ocean and estuary). Pre-existing geomorphic and geologic conditions as well as the storm characteristics largely drive the contrast in emergence patterns observed at these sites.
1. Geomorphic control: The pre-existing geomorphology of the barrier island greatly determines the role that overwash has on back-barrier marsh emergence. In many cases the back-barrier environment remains too low and too frequently overwashed for the marsh to become established (for example, site 11). In the event that the marsh does emerge and become established, then the development of foredunes and geomorphic highs seaward of the marsh are required to maintain an active marsh area (Smith and others, 2008).
2. Thickness of washover deposit: One physical component of the overwash-marsh interaction that is evident in the marsh cores from Assateague Island but requires further work is the importance of washover fan thickness on marsh recovery. The cores show multiple overwash events, some potentially impeding ecosystem function (evident by low to no organic matter) while others appear to have temporally little impact on vertical accumulation and organic matter burial. When event deposition is large (that is, magnitude greater than the annual vertical accretion), mass loading onto the marsh substrate from washover sediment can cause subsidence and lowering of the marsh surface. Similarly when deposition is large, the marsh system could require a long period of time to recover and never re-establish vegetative growth. This is apparent in cores 04M and 14M as well as in the aerial imagery comparison for 1966 and 2013 (fig. 2B), where some areas remained low and overwashed while others showed signs of apparent recovery.
3. Provenance: The impact of Hurricane Sandy and potentially other recent storms on 7 marsh sites in Assateague Island and the adjacent vicinity indicate that multiple exchange pathways exist between the ocean, barrier island, and estuary. Sediment texture in a subset of the cores suggests that marine overwash from Hurricane Sandy or other recent storms is less influential to the marsh evolution. The texture is fairly constant through time at a number of the sites, which favor a non-marine source. The next most probable source of sediment is the adjacent estuary.
Further studies could help differentiate these sediment sources, attempt to quantify the amount of deposition, and evaluate the pre-existing geomorphology to better define how marine and estuarine overwash processes may influence the sustainability and vulnerability of coastal marshes.
