QUANTIFYING ICE-RAFTING: THE MISSING SEDIMENTATION VECTOR ON NORTHERN SALT MARSHES AND ITS IMPACT ON BACK-BARRIER EVOLUTION

Coastal wetlands are threatened by projected sea-level rise (SLR). Nationally, most research has focused on southeastern Atlantic and Gulf coast marshes; however New England marshes may also respond dramatically to increased rates of SLR. Northern marshes are dominated by high marsh and may therefore flood catastrophically, rather than progressively, triggering unusually rapid coastal evolution.

Winter processes such as ice-rafting contribute up to 50% of the total volume of inorganic sediment delivered to New England high marshes. Ice-raft formation and dispersion redistributes sediment within the back-barrier system, enlarging creeks and pannes, and augmenting vertical accretion of the marsh surface. Ice rafts form from drift ice or in situ in tidal creeks and salt pannes. Adfreezing of sediment occurs during low tides. During spring or storm tides, the sediment-laden ice is rafted onto the marsh surface. In an enclosed backbarrier environment, rafts become stranded on the flood-dominated marsh surface, melt and deposit sediment.

Ice-rafted sediment is difficult to quantify precisely, but careful estimation yields useful approximations. Field survey data are analyzed relative to physical parameters including elevation and distance to tidal creek and upland, and compared to tidally deposited sediment measured seasonally using sediment pads deployed at each site. Measurements at four New England back-barrier salt marshes indicate that maximum ice-rafting occurs in the mid-back marsh and near tidal creeks, but annual variation in patterns of sediment deposition is significant. The net volume of ice-rafted sediment deposited on the high marsh surface varies with latitude, and decreases linearly to the south by a factor of 0.14 with decreasing degrees of latitude (r²=0.9).

Modal concentrations of sediment in surveyed ice rafts may be used to estimate regional ice-rafted sedimentation measured remotely, as M_sed = 0.01 V_ice – C; C = 2.36 for New England. In northern marshes exposed to winter processes, ice rafting is a significant vector in the backbarrier sediment budget, and may materially affect whether or not these supratidal marshes are able to accrete rapidly enough to keep pace with future acceleration in the rate of sea-level rise.