EVENT-DRIVEN SEDIMENTATION IN MASSACHUSETTS AND VIRGINIA SALT MARSHES

Coastal salt marshes help to maintain their elevation relative to sea-level rise through positive eco-geomorphic feedbacks: plants trap suspended sediment delivered by tidal waters contributing inorganic matter to the marsh soils thereby promoting vertical accretion and stability. Inorganic sediment is delivered to the marsh surface during regular tidal flooding; however, event-driven sedimentation has been shown to play a substantial, and perhaps outsized, role in vertical accretion. Using Pb²¹⁰ and Cs¹³⁷ isotopic dating of sediment cores, we explore two mechanisms of event-driven sedimentation—storm overwash and ice rafting—from salt marshes in Virginia (Cedar Island) and Massachusetts (Plum Island). Multi-decadal accretion rates for low marsh backing Cedar Island range from 1.8 to 4.7 mm yr^-1 with an average rate of 3.5 mm yr^-1. Similarly, rates of accretion in the wave-protected backbarrier high marsh in Plum Island range from 1.8 to 4.9 mm yr^-1 and average a rate of 2.9 mm yr^-1. Backbarriers behind both islands are characterized by relatively low suspended sediment concentrations due to their geographic setting distal from a major river sediment source (Cedar) or lack of fine sediment derived from a previously glaciated drainage basin (Plum). However, marshes in both systems benefit from event-driven sedimentation: cores at both sites show evidence of 4-cm thick radioisotopic inversions coincident with sediments characterized with high inorganic content. At Cedar Island, these deposits are found in select island-proximal cores and are interpreted as overwash deposits, formed through storm erosion of the beach and dunes and deposition along the landward margin of the island. At Plum Island, these deposits are found distal from the island and interpreted as ice-raft deposits sourced from nearby tidal flats, tidal creeks, and ponds. Our results demonstrate the utility of combined sedimentologic and radioisotopic analyses in identifying event-driven deposition in salt marshes. We confirm the role of both overwash (contributing tens of m³/m of sediment to barrier-connected marshes) and ice-rafting (contributing ~ 5% of vertical accretion annually) in the long-term resilience of salt marshes to rising sea level.