Detecting the Delayed Signatures of Changing Sediment Supply in Salt-Marsh Landscapes: The Case of the Venice Lagoon (Italy)

Marcella Roner,Martina Vannacci,Massimiliano Ghinassi,Adrian Gilli,Andrea D’Alpaos,Nathalie Combourieu-Nebout,Adele Bertini,Lucia Liccioli,Mariaelena Fedi,Luigi Vigliotti,Alvise Finotello,Laura Tommasini,Luca G Bellucci,Sandra Donnici

doi:10.3389/fmars.2021.742603

Abstract

Many salt-marsh systems worldwide are currently threatened by drowning and lateral erosion that are not counteracted by sufficient sediment supply. Here we analyze the response of a salt-marsh system to changes in sediment availability and show that, contrary to what would have been expected, marsh dynamics in the vertical plane can be insensitive to large sediment supply. We integrate sedimentological, geochronological, paleoecological, geophysical, and chemical analyses of salt-marsh sediments accumulated over the past six centuries in the Southern Venice Lagoon (Italy), and suggest that a time lag exists between enhanced river-fed clastic sediment input and its signature in the salt-marsh succession. This time lag is likely caused by the stocking of the sediment along the margins of pre-existing marshes, which started to significantly expand horizontally – rather than accrete vertically – when sediment input increased. When sediment input drastically decreased, wind waves re-suspended the river-fed deposits and distributed them over the marsh platform, eventually allowing for vertical accretion. Understanding the response of salt-marsh systems to changes in sediment supply has important implications for the management of tidal landscapes and the prediction of their evolution under the effects of natural and anthropogenic forcings. Our results highlight that the study of ultra-recent sedimentary successions needs to be carried out on the basis of a deep understanding of specific depositional dynamics.

Highlights

Salt marshes are valuable ecosystems of great social, economical, ecological, and geomorphological importance (Barbier et al, 2011)
The effects of natural changes and human interference on the subtle equilibrium between vertical accretion and rates of relative sea-level rise (RSLR) have often resulted in irreversible transformations, leading to a significant decrease in salt-marsh extent worldwide during the last century (Castillo et al, 2000; Carniello et al, 2009; Gedan et al, 2009; FitzGerald and Hughes, 2019)
High rates of RSLR and the lack of clastic sediments are key factors driving salt-marsh drowning worldwide (Morris et al, 2002; Marani et al, 2007; Gedan et al, 2009; Valiela et al, 2009; Mudd, 2011; D’Alpaos and Marani, 2016), whereas the effect of wind-wave erosion on salt-marsh margins has been highlighted as the main process responsible for their lateral retreat (Mariotti and Fagherazzi, 2010, 2013; Marani et al, 2011; Leonardi and Fagherazzi, 2014; Leonardi et al, 2016; Finotello et al, 2020)

Summary

Introduction

Salt marshes are valuable ecosystems of great social, economical, ecological, and geomorphological importance (Barbier et al, 2011). The accumulation of inorganic and organic sediments (e.g., Morris et al, 2002; D’Alpaos et al, 2007; Mudd et al, 2009; Roner et al, 2016) allows salt marshes to face rates of relative sea-level rise (RSLR) up to a given threshold The effects of natural changes and human interference on the subtle equilibrium between vertical accretion and rates of RSLR have often resulted in irreversible transformations, leading to a significant decrease in salt-marsh extent worldwide during the last century (Castillo et al, 2000; Carniello et al, 2009; Gedan et al, 2009; FitzGerald and Hughes, 2019). Other second-order processes, such as for example the establishment and expansion of salt pans and ponds, can lead to significant loss of marsh surfaces (e.g., Mariotti, 2016; Ortiz et al, 2017; Schepers et al, 2020; Wang et al, 2021)

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