Abandoned tidal channels as hotspots of Blue Carbon

Tidal marshes, recognized as “Blue Carbon ecosystems” for their high carbon sequestration rates, owe their carbon storage potential to primary production and rapid surface accretion driven by complex feedbacks among hydrodynamic, morphological, and biological processes. Tidal channel networks cutting through tidal wetlands exert a first-order control on ecogeomorphological dynamics by critically controlling fluxes of nutrients, sediments, and particulate matter. Although these networks have been traditionally seen as stable features, recent studies have shown that they are in fact highly dynamic systems. In particular, lateral channel migration, coupled with high drainage density, leads to frequent channel abandonment through meander cutoffs and channel piracies (i.e., stream captures). These processes significantly impact sediment dynamics, since reduced flow velocities within abandoned channels promote particle settling and channel infill, thereby providing ideal conditions for rapid organic matter deposition and trapping. To characterize the depositional processes occurring in abandoned tidal channels and investigate their role in blue carbon sequestration and storage, we analysed several sediment cores retrieved from abandoned tidal channels in the microtidal Venice Lagoon, Italy. Cores were sampled every 5 cm for soil dry bulk density, organic matter, and organic carbon content. Organic matter content was estimated as the difference in weight before and after the Loss-On-Ignition (LOI), while organic carbon was directly measured using an elemental analyser. Sedimentary facies analyses allowed for identifying the deposits accumulated during the abandonment phase, while aerial and satellite image analyses facilitated the evaluation of the temporal evolution of the channel infill process, enabling the estimation of the related infill rate. Combining infill rate and organic carbon density, we estimated the carbon accumulation potential of abandoned tidal channels, as well as its variability, comparing it to surrounding marshes. Preliminary results show that even if channel fill deposits are characterized by slightly lower organic matter content relative to marsh deposits, they feature significantly higher carbon accumulation rates owing to higher sediment deposition rates. These findings suggest that abandoned tidal channels could represent key hotspots for blue carbon accumulation. Consequently, a better understanding of depositional processes and carbon accumulation in abandoned tidal channels, as well as their characteristic spatiotemporal dynamics, can critically enhance the assessment of blue carbon sequestration and stock in coastal wetlands, providing crucial insights for effective conservation and restoration strategies.