Coastal salt marsh survival detected by novel in-situ experiments: the subsurface perspective

Our recent experiments on salt marshes in the Lagoon of Venice demonstrate the importance of the subsurface in controlling the delicate balance that determines coastal marsh survival under sea-level rise.
Published in Earth & Environment
Coastal salt marsh survival detected by novel in-situ experiments: the subsurface perspective
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Over the past decades, coastal marshes around the world have declined dramatically, contributing to a substantial loss of important ecosystem services. Scarcity of sediments, erosion, and accelerated rates of relative sea-level rise cause the deterioration of coastal marshes. These processes and their feedbacks control the long-term evolution of the marsh elevation.

There is wide consensus that vertical accretion of coastal marshes is controlled by physical, biological and chemical processes such as sediment deposition during high tides, plant productivity, and decomposition. All these processes depend on the relative elevation between the salt marsh platform and mean sea level and, therefore, influenced by the rate of sea-level rise and natural autocompaction, the process of compression of older underlying sediments under their own weight. Although autocompaction is being studied in many coastal marshes worldwide with dedicated monitoring instruments, its prediction under future changes is still largely unknown. 

The experimental set-up at the Lazzaretto Nuovo site in the Venice Lagoon (Italy), July 2019. 

In our paper we address this problem by designing and implementing a new in-situ loading experiment aimed to characterize shallow compaction of salt marshes in their natural setting. The loading experiment was applied on two salt marshes of the Venice Lagoon, in Italy. To create the load, we used eight 500-liter tanks, occupying ~ 4 m2, which were cyclically filled and emptied out with an increasing percentage of tank filling, using the available lagoon water. The maximum weight applied, with all the tanks completely filled, was ~ 4000 kg. An appropriate monitoring system was set in place to monitor the vertical displacement of the marsh at different depths and locations below the loaded area. To understand the different behaviors and correctly interpret the loading experiment outcomes, it was also essential to characterize the subsurface composition of the two studied salt marshes and reconstruct their evolution through time. To do so, we collected several sedimentary cores performing sedimentological and geotechnical investigation, with grain-size and organic-content analysis.

The experimental set-up at the La Grisa site in the Venice Lagoon (Italy), October 2020. 

There is a huge variability for the two salt marshes, both in terms of the loading experiment results and their deposits. One salt marsh mainly consists of silty deposits, with relatively low organic content (i.e., roots and plant debris). It is located at the margin of a tidal channel close to an inlet and formed on top of a bare tidal flat. This marsh shows a maximum vertical displacement of ~ 6-7 mm at the ground level and of ~ 3 mm, 50 cm below the surface. The second marsh is characterized by a higher organic content. It is located along the inner margin of the lagoon toward the mainland, and formed on top of a deltaic sand body deposited by a paleoriver channel. Vertical displacements are higher and reach a maximum value of ~ 32 mm at the ground level and of ~ 1 mm, 40 cm below the surface.  

Salt marsh coring at the La Grisa site, September 2020.

Our results show that autocompaction can spatially be highly variable and largely depends on the deposits that compose the subsurface of the marsh and the type of depositional environment the marsh is located.

Our work allowed us to make a direct connection between subsurface characteristics and autocompaction, a process that occurs below the surface, thus hidden from our eyes, but detectable by monitoring the change in marsh elevation. We tested the tendency of the marsh to compact in the field, without sampling any core for laboratory compression tests. With our experiment we successfully brought the lab into the field, enabling us to measure compaction in the natural setting. Hereby we could avoid the drawbacks otherwise occuring during in lab tests on these highly compressible and easily disturbed sediments. 

The portable experiment during a recent initial field test at Le Saline site in the Venice Lagoon (Italy), September 2022.

The in-situ loading experiment is a promising cost-effective methodology to characterize the hydro-geomechanical properties of marsh soils. It is quite demanding from a technical standpoint because of its installation in soft soil and the handling of encumbering material in marshy areas. For this reason, we are designing and testing a smaller version of the experimental setup, which is easily transportable and can be replicated in many sites to better capture the heterogenity of autocompaction behavior in marshes. This furthermore enables to carry out this kind of experiment in a standardized way in marshes in different locations in the world. This will increase our understanding of subsurface behavior and its implication for future sustainability of marshes and other depositional environments around the world, often under pressure of relative sea-level rise.

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