Andrea D’Alpaos

Vegetation engineers marsh morphology through multiple competing stable states

Marshes display impressive biogeomorphic features, such as zonation, a mosaic of extensive vegetation patches of rather uniform composition, exhibiting sharp transitions in the presence of extremely small topographic gradients. Although generally associated with the accretion processes necessary for marshes to keep up with relative sea level rise, competing environmental constraints, and ecologic controls, zonation is still poorly understood in terms of the underlying biogeomorphic mechanisms. Here we find, through observations and modeling interpretation, that zonation is the result of coupled geomorphological–biological dynamics and that it stems from the ability of vegetation to actively engineer the landscape by tuning soil elevation within preferential ranges of optimal adaptation. We find multiple peaks in the frequency distribution of observed topographic elevation and identify them as the signature of biologic controls on geomorphodynamics through competing stable states modulated by the interplay of inorganic and organic deposition. Interestingly, the stable biogeomorphic equilibria correspond to suboptimal rates of biomass production, a result coherent with recent observations. The emerging biogeomorphic structures may display varying degrees of robustness to changes in the rate of sea level rise and sediment availability, with implications for the overall resilience of marsh ecosystems to climatic changes.

Variation in the Occurrence of Rainfall Events Triggering Landslides

We analyze the climatic features of the Vicenza Province (NE Italy) and the characteristics of the exceptional rainfall event that hit the area in November 2010, triggering a huge number of landslides. Our analysis aims at identifying the hydrological variable related to the triggering of the recorded instabilities and the recent variation in the occurrence of rainfall events inducing landslides.

Field migration rates of tidal meanders recapitulate fluvial morphodynamics

Significance Meandering tidal channel networks play a central role in the ecomorphodynamic evolution of the landscapes they cut through. Despite their ubiquitous presence and relevance to sedimentary and landscape features, few observations of tidal-meander evolution exist, and we lack a full understanding of the governing processes. Field analyses show that tidal meanders, traditionally viewed as stable landscape features, display modes of migration and migration rates per unit width quite similar to those characterizing their fluvial counterparts, with important implications for the characterization of the related sedimentary products. The results presented here contribute to our understanding of the morphological evolution of tidal landscapes. The majority of tidal channels display marked meandering features. Despite their importance in oil-reservoir formation and tidal landscape morphology, questions remain on whether tidal-meander dynamics could be understood in terms of fluvial processes and theory. Key differences suggest otherwise, like the periodic reversal of landscape-forming tidal flows and the widely accepted empirical notion that tidal meanders are stable landscape features, in stark contrast with their migrating fluvial counterparts. On the contrary, here we show that, once properly normalized, observed migration rates of tidal and fluvial meanders are remarkably similar. Key to normalization is the role of tidal channel width that responds to the strong spatial gradients of landscape-forming flow rates and tidal prisms. We find that migration dynamics of tidal meanders agree with nonlinear theories for river meander evolution. Our results challenge the conventional view of tidal channels as stable landscape features and suggest that meandering tidal channels recapitulate many fluvial counterparts owing to large gradients of tidal prisms across meander wavelengths.

Latest Holocene depositional history of the southern Venice Lagoon, Italy

Analyzing the signatures of landscape changes in the stratigraphic record is crucial to refine our knowledge of tidal landform dynamics and represents a first step toward the development of predictive morphodynamic models. The southern Venice Lagoon (Italy) is particularly suited to analyze modifications in the depositional environment and, consequently, in the sedimentary record because of the remarkable changes that occurred in the last centuries. We collected 25 cores along a NE–SW linear transect about 5 km long cutting through salt marshes, tidal flats, and subtidal platforms. High-resolution sedimentological analyses defined the spatial arrangement of four deposits along the transect (palustrine, salt marsh, lag, and tidal-flat/subtidal-platform deposits), whose cores were dated through radiocarbon, 210Pb, and 137Cs geochronological analyses. The study succession testifies an evolution from a palustrine freshwater environment to a lagoonal environment over the last 2000 years. The palustrine peat progressively evolved into salt marshes in the 14th century. Salt-marsh aggradation is characterized by different accretion rates over time and occurred in parallel with the decrease in the salt-marsh extent and tidal-flat expansion. Where salt-marsh deposits were locally flooded and impacted by wind waves, a lag deposit developed. As a consequence of the progressive water deepening, organic-rich mud accumulated above the lag. The results, as well as providing the first accretion model for the latest Holocene succession in the southern Venice Lagoon, highlight that the disappearance of salt marshes in this area has to be ascribed to the lateral erosion of their margins, rather than to their progressive drowning.