183-5 Novel Washover Fan Growth and Back-Barrier Ecosystem Response: A Multi-Century Record from Cape Cod, Massachusetts

Session: Recent Advances and New Voices in Marine and Coastal Geoscience (Posters)

Presenting Author:

Authors:

Abstract:

Since early 2021, the Duck Harbor beach and dune system in Wellfleet, MA, USA, has experienced persistent, near-monthly tidal overwash events during perigean spring tides, creating a unique washover fan that differs from typical storm-driven deposits. Chronic tidal overwash has caused extensive saltwater intrusion and vegetation mortality across approximately 48.5 ha, transforming the back-barrier environment from forest and freshwater wetland to salt marsh. Regular elevation transects taken since 2021 reveal a gradual widening and deepening of the dune breach. The salt-killed trees and shrubs were mechanically removed in 2023, and the low-lying basin was quickly inhabited by marsh vegetation, including Sueda linearis, Spartina alterniflora, Spartina patens, Distichlis spicata, Salicornia europaea, and Juncus gerardii. To understand how the Duck Harbor system evolved over longer timescales, five sediment vibracores were collected in summer 2024 to reconstruct the environmental history of this barrier-wetland complex over the past ~1000 years. Stratigraphic analysis reveals alternating periods of channel development and salt marsh evolution preserved as distinct sediment layers. Loss-on-ignition measurements show variations in organic content, ranging from <5% in sandy marine deposits to >40% in peat layers. Grain-size analysis indicates episodes of high-energy deposition interspersed with lower-energy marsh environments. Radiocarbon dating of organic material provides age estimates for environmental reconstructions. Preliminary results suggest that the current sequence of persistent tidal overwash shows an unprecedented departure from historical patterns. The sustained nature of recent overwash – potentially driven by a lack of sediment to enable beach and dune recovery as well as several other factors, including sea-level rise and eelgrass (Zostera marina) loss – appears to be driving permanent ecosystem change rather than temporary disturbance followed by recovery. This study provides insights into the conditions that trigger persistent tidal overwash in coastal systems and documents how chronic marine influence differs from episodic storm events as preserved in the sedimentary record. The findings also have important implications for understanding how similar barrier-wetland systems may respond to accelerating sea-level rise and increasing coastal flooding.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9370

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