Influence of distributary channels on sediment and organic carbon supply in event-dominated coastal margins: the Po prodelta as a study case
Authors: Tesi T. , Miserocchi S. , Goñi M. A. , Turchetto M. , Langone L. , De Lazzari A. , Albertazzi S. , Correggiari A. 
Published: October 2010

Abstract. From November 2008 through May 2009, the North Italy experienced the highest precipitation period recorded over the last century. As a result, a long series of flood events occurred in the Po river (North Italy). This series of events ended with a large flood in early May 2009. An event-response sampling was carried out in the Po prodelta in April–May 2009 to characterize this latter event and to investigate the strata preservation in the stratigraphy record as a result of this series of floods. The water sampling was carried out during two periods of the flood, including early in the event under conditions of moderate river flow (~5000 m3 s−1) and 24 h later during the peak discharge (~8000 m3 s−1). At each station, profiles of conductivity, transmittance, and fluorescence were acquired whereas surface and bottom waters were sampled to collect sediments in suspension. In addition, sediment cores were collected in the Po prodelta before and after the peak flood. Biogeochemical compositions and sedimentological characteristics of suspended and sediment samples were investigated using a multi-proxy approach that included bulk and biomarkers analyses. Furthermore, 7Be down-core profiles and radiographs were used to analyze the internal stratigraphy of sediment cores. During moderate discharge, the water column did not show evidence of plume penetration. In surface waters, suspended sediment concentrations were found to be similar to low river discharge periods whereas the main OC was autochthonous. After 24 h, during the peak flood, water column properties and biogeochemical parameters exhibited marked changes indicating significant penetration of the turbid plume. However, suspended sediment concentrations and terrigenous OC content in surface waters were still less then expected based on the discharge. These results suggested that, since material enters the Adriatic as buoyancy-driven flow with a reduced transport capacity, settling and flocculation processes result in trapping a significant fraction of land-derived material prior to reaching the subaqueous prodelta. In spite numerous floods occurred from November 2008 through April 2009, sediment cores collected in late April 2009 did not exhibited significant evidence of event-strata preservation. Since these floods were ordinary (2–3 y return period), the lack of preservation indicates that most of the sediment supply during these oridinary events does not reach the subaqueous prodelta. However, it is likely that modest sediment deposition occurs during these oridinary floods but thicknesses of these event-strata are not sufficient to compete with post-depositional processes. Stations in the north and central prodelta were re-occupied after the peak of the May 2009 flood. Based on 7Be and radiographs, we estimated that 17 and 6 cm event-layers, respectively. Selective trapping of coarse material occurred in the central prodelta likely because of the geomorphologic setting of the central outlet characterized by an estuary-like mouth. Despite these settling processes, lignin-based parameters indicated that sources of the terrigenous OC were fairly homogenous throughout the channel network and between size-fractions.