31-22 Shallow subsidence steals elevation from U.S. Atlantic Coast marshes

Session: Environmental Geochemistry and Health (Posters)

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Abstract:

Shallow subsidence (SS) can substantially reduce marsh elevation gain relative to sea level. While compaction and organic matter decomposition in marsh strata are well documented, direct measurements of net shallow subsidence remain sparse, and its spatial distribution across broad coastal gradients is poorly known. We analyzed SS rates measured from surface elevation table–marker horizon (SET-MH) records at 308 tidal marsh sites along the U.S. Atlantic Coast. SS rates are modest at most sites (25th–75th percentiles −2.4 to +2.6 mm/y), indicating that both subsidence and subsurface expansion occur within the coastal wetland landscape. Spatial patterns are pronounced. In the South Atlantic (North Carolina to Florida), 68% of sites are actively subsiding (25th–75th percentiles −0.1 to +2.6 mm/y). The North Atlantic (Massachusetts to Connecticut) is dominated by expansion, with only 34% of sites subsiding (−3.8 to +1.0 mm/y). The Mid-Atlantic (New York to Virginia) is intermediate, with 47% of sites subsiding (25th–75th percentiles −2.5 to +3.7 mm/y). This regional contrast likely reflects variation in soil composition and organic matter dynamics. One possibility is that southern marshes are more prone to decomposition and autocompaction under warmer temperatures and higher microbial activity, while cooler conditions, slower decomposition, and denser root structures sustain soil volume in northern marshes. An alternative, not mutually exclusive, explanation is that regional differences in mineral content govern the pattern, since southern marshes tending to be more minerogenic, with higher mineral fractions increase effective stress and compaction potential. Distinguishing between these mechanisms requires paired measurements of soil organic matter and bulk density across the latitudinal gradient. Regardless of cause, these patterns are consistent with conceptual frameworks in which shallow subsidence is driven by processes acting within the root zone and upper stratigraphic profile, meaning surface accretion alone does not reliably indicate vertical stability.

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