Winter processes on New England salt marshes

The back-barrier marshes of New England form a fragile ecosystem that is threatened by future sea-level rise (SLR). Salt marshes maintain their elevation relative to rising sea level by vertical accretion, a function of bioproductivity and inorganic sediment influx. New England marshes are particularly sensitive to SLR due to their supratidal platform-like hypsometry and limited sedimentation. If accretion rates are lower than projected rates of SLR, this morphology may cause the marsh to flood rapidly, triggering accelerated coastal evolution. A thorough understanding of the processes controlling the elevation of supratidal marshes on varied timescales is critical to accurate predictions of future geomorphic change in coastal New England.; Tidal inundation, especially during storms, has long been assumed to be the dominant source of inorganic sediment to salt marshes. However, studies conducted on three New England marshes indicate that winter processes contribute significantly to the total volume of inorganic sediment delivered to the marsh. Ice raft formation redistributes sediment from tidal flats, channel beds and salt pannes to the marsh surface, producing deposits diagnostic of their environment of origin. Ice rafting accounts for over 50% of annual inorganic sedimentation, based on in-situ measurements of ice-rafted sediment, seasonal tidal sedimentation patterns, and marsh accretion over a three-year period. Morphologic analyses reveal that the distribution of ice rafts is highly stochastic; however maximum deposition occurs in the mid-back marsh and near smaller tidal creeks, and is correlated with marsh surface elevation, hydromorphology, and vegetation type. The total volume of ice-rafted sediment varies with latitude, increasing to the north.; In contrast to sediment influx via ice-rafting, winter processes such as ice erosion, peat dehydration, and shallow compaction from ice build-up lead to a loss of marsh surface elevation. In-situ experimentation demonstrated that compaction from thick overlying ice (equivalent mass) significantly lowers surface elevation (e.g. 46 cm "ice" produced 6.1 +/-0.5 mm of compaction, equivalent to three years of average accretion). However, compaction is short-lived; nearcomplete rebound occurs with spring peat rehydration, implying that ice accumulation does not degrade the marsh surface in New England. This process may prove to be significant at more northerly latitudes.