In-situ loading experiments reveal how the subsurface affects coastal marsh survival

Over the past decades coastal marshes around the world have declined dramatically. Their deterioration is controlled by scarcity of sediments, erosion and accelerated rise of relative sea-level. The feedbacks between these processes control marsh evolution and determine their long-term survivability. Aggradation of a marsh to keep pace with relative sea-level rise mainly depends on the interplay between sedimentation and autocompaction, but their interactions are severely understudied. Here we present an in-situ loading experiment applied in the Venice Lagoon, Italy, to assess long-term autocompaction, with subsurface displacements and pressure monitored during loading cycles, up to ∼40 kN applied on a ∼4 m 2 surface. Two identical experiments carried out in inorganic and organic soil-dominated marshes provided unique insights on the spatio-temporal subsurface dynamics. The large differences in behavior and maximum compaction (6 vs 32 mm) underscore the crucial role of autocompaction and soil heterogeneity when predicting the fate of coastal marshes worldwide.