Enhanced, Climate‐Driven Sedimentation on Salt Marshes

Abstract

With Arctic amplification (enhanced polar warming) possibly increasing periods of intense winter freezing and global warming producing more powerful extratropical storms, winter sedimentation on northern salt marshes may likely increase in the future. Here, we show that a large ice-rafting event in northern Massachusetts delivered the equivalent of 15 years of mineral deposition to the marsh surface in a single storm. During an intense extratropical cyclone in January 2018, sediment-laden ice was rafted onto the Great Marsh, Massachusetts, by an~1-m storm surge coinciding with high astronomical tides. The muddy sand content combined with abundant shells indicates that the sediment originated from proximal bays and tidal flats. Sediment layers delivered by individual ice rafts averaged 3.19 cm in thickness, 12 times the combined organic and inorganic yearly vertical accretion rate on high marshes. This previously underappreciated vector for sediment deposition is likely to help marsh resiliency to sea-level rise.Plain Language Summary We found that a single storm in 2018 moved ice onto a New England salt marsh from nearby bays and creeks, carrying the equivalent of 15 years of average mineral sediment deposition across the entire marsh. Salt marshes maintain elevation with rising sea level, in part, by trapping mud and sand. The storm deposit that we observed was a consequence of prolonged ice formation followed by a strong nor'easter coinciding with a very high tide. During the period of extended low temperatures, ice formed in shallow water, and mud and sand froze to its base at low tide. The cold spell combined with very low tides meant that the ice was especially thick and laden with sediment when the storm moved it on to the marsh. Recent studies suggest that climate change may increase the likelihood of extreme cold spells in winter in the northern hemisphere and nor'easters may strengthen. If these conditions persist, the amount of sediment added during winter to salt marshes in northern high latitudes (which is almost half of global marshes) may increase, helping salt marshes to be more resilient to sea-level rise.

Sediment Delivery to a Tidal Marsh Platform Is Minimized by Source Decoupling and Flux Convergence

“…The slumping of the bank creates accommodation space, which would allow for increased sediment deposition without the formation of a levee (Mariotti et al, 2016(Mariotti et al, , 2019. Other possible explanations for the lack of a levee could be historical changes in sediment supply associated with land clearance and dam construction (Kirwan et al, 2011) or delivery of sediment by ice rafting (Argow et al, 2011;FitzGerald et al, 2020) or major storm events that were not captured during our measurement period.…”

Grain-Size Analysis of Hurricane-Induced Event Beds in a New England Salt Marsh, Massachusetts, USA

“…Argow, Hughes, and FitzGerald (2011) determined that ice rafts have the potential to transport sediment loads more than 100 m from the source and that 97% of ice rafts carry sizeable sediment loads. Whereas ice rafts presently tend to form further north in New England (FitzGerald et al, 2020;Hardwick-Witman, 1986;Wood, Kelley, and Belknap, 1989), it is possible that cooler climates may have extended their range in the past. It is important to consider, however, that ice rafting is often patchy and is not wholly likely to contribute to traceable, widespread layers of sediment across .100 m of marsh.…”

“…It is important to consider, however, that ice rafting is often patchy and is not wholly likely to contribute to traceable, widespread layers of sediment across .100 m of marsh. For example, while an intense extratropical cyclone in 2018 led to the ice-rafting of 18,000 m 3 of sediment across the Great Marsh in northern Massachusetts, deposits-while spread throughout the marsh-were relatively patchy, particularly deposits of substantial thickness (FitzGerald et al, 2020). Complete inundation of the barrier during intense, surgedominated storms also influences sediment transport dynamics.…”

Biotic Recovery Following Ice-Rafting in a Salt Marsh

“…Ice-rafting (Fig. 1A, B), the physical movement of frozen sediments from intertidal mudflats to adjacent salt marshes during storm events, is a well-documented disturbance in the salt marshes of New England, USA (composed of the northeast Atlantic states: Connecticut, Massachusetts, Rhode Island, New Hampshire, Vermont, Maine) (Hardwick-Witman 1985;Wood et al 1989;Ewanchuk and Bertness 2003;Argow et al 2011;FitzGerald et al 2020;Moore et al 2021). Ice-rafting from winter storms occurs annually in this region, and it can have both positive and negative impacts on the salt marsh.…”

“…Ice-rafting Communicated by R. Scott Warren has been cited as one mechanism potentially driving pond or panne formation (Redfield 1972;Burns 2021), leading to decreased plant cover and marsh stability. Ice-rafting, however, also serves as an important secondary source of sediment to New England salt marshes (Wood et al 1989;Argow et al 2011;FitzGerald et al 2020), which are surrounded by estuaries with historically low sediment supply (Hopkinson et al 2018;Coleman et al 2020;Langston et al 2020). For instance, in the winter of 2018, New England experienced a historic extratropical cyclone, winter storm Grayson, which brought storm surges of ~ 1 m (FitzGerald et al 2020;Moore et al 2021).…”