Sedimentary characteristics of abandoned-channel deposits in shallow tidal environments

Tidal channel networks control tide propagation and, therefore, fluxes of water, sediment, nutrients, and particulate matter in wetlands and low-lying coastal areas. Furthermore, tightly linked wetland-channel systems deliver multiple ecosystem services, among which blue carbon sequestration is critically important. While carbon fluxes associated with both vertical and lateral dynamics of salt marshes have been extensively studied, the role of tidal channel abandonment still needs to be further investigated. Reduced flow velocities promote rapid particle settling within abandoned channels thus rapidly storing large volumes of inorganic and organic sediment, both from autochthonous and allochthonous sources. Hence, a better understanding of the processes that lead to the abandonment of active tidal channels and the characterization of the related sedimentary deposits are critical steps to assess their potential sequestration capacity and storage of blue carbon.

Towards this aim, here we investigate the sedimentary features and the related depositional processes in abandoned tidal channels by analyzing several undisturbed sediment cores retrieved in the microtidal Venice Lagoon, Italy. Cores were cut longitudinally and photographed for classical sedimentary facies analysis and description of the main sedimentary units. In each core, soil subsamples were taken every 5 cm and were prepared for different laboratory analyses. 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 analyzer.

The deposits accumulated during the abandonment phase and the related infill volumes were identified thanks to sedimentary facies analyses. The combination of the infill volume and organic carbon content allowed us to estimate the carbon stock potential of abandoned channels. Preliminary results show that, although the organic matter content in abandoned channel deposits is lower than that of the surrounding salt marshes, high infill rates make the carbon accumulation rate comparable between these different depositional systems. Moreover, the analysis shows that there is a very high spatial variability in sedimentary features of abandoned channel deposits, which needs to be taken into account to assess the potential of abandoned tidal channels as carbon sinks.