Saltmarshes in a backbarrier lagoon: sediment source and variability

In backbarrier lagoons, saltmarshes represent a depositional sub-environment with peculiar characteristics and management demands. With saltmarshes covering approximately 760 ha, the Grado and Marano Lagoon is an important wetland in the Northern Adriatic (Italy). Here, the extent of saltmarshes has decreased over the last fifty years due to a combination of natural and anthropogenic factors. Two short cores of sediment were collected from each of the twelve sample sites, which were chosen to represent different saltmarsh environments. Their stratigraphic description was combined with the recent morphological evolution (1910–2010), as determined by cartography, aerial photos, and topographic surveys. The facies description has been improved by using grain size and mineralogical-compositional data determined by a laser diffraction particle size analyser and the powder diffraction method, respectively. In the inner part of the lagoon, silt content is dominant even in cores situated in different morphological contexts, such as isolated and channel-fringing saltmarshes or abandoned fish farms. The facies transition from the tidal flat sub-environment to the saltmarsh sub-environment is frequently discernible and is marked by small grain size differences, both fining and coarsening upward. These different trends attest to the role of local changes in environmental and hydrodynamic conditions, as well as human action. Sand is the main component in the backbarrier saltmarshes, where the grain size variability along the core is evident due to the mixing of tidal and coastal processes. The mineralogical composition is geographically distributed in accordance with the morphology and hydrodynamics of the lagoon. The highest carbonate concentrations were only found in saltmarshes near or directly connected to tidal inlets, thus emphasising the importance of the external sedimentary source (Isonzo and Tagliamento rivers), with calcite playing a discriminant role. Our findings and interpretations contribute to a better understanding of the depositional model of saltmarshes in a back-barrier lagoon, particularly in different erosive or depositional contexts and with significant human interferences.