Changes in the composition of organic matter from prodeltaic sediments after a large flood event (Po River, Italy)

The Po River (Italy) experienced a 100-year flood in October 2000. Surface sediments (0-1 cm) from cross-shelf transects were collected in the Po prodelta area (Adriatic Sea) in December 2000, in order to describe the distribution of organic matter (OM) along the main sediment dispersal system immediately after the flood event. Stations were subsequently reoccupied in October 2001 and April 2002. This sampling program provided a special opportunity to characterize the initial surficial flood deposit and the evolution of its associated OM over the course of 2 years. CuO oxidation, elemental, delta C-13, Delta C-14, and grain-size analyses were carried out to characterize the source, age, and spatial variability of sedimentary OM. Statistical analysis (PERMANOVA) was then applied to investigate temporal changes in different portions of the Po prodelta area. Isotopic and biomarker data suggest that the sedimentary OM in the flood deposit was initially dominated by aged (Delta C-14(Dec-00) = -298.7 +/- 56.3%,), lignin-poor OM (Lambda(Dec-00) = 1.96 +/- 0.33 mg/100 mg OC), adsorbed on the fine material (clay(Dec-00) = 72.1 +/- 4.8%) delivered by the flood. In the 2 years following the flood, post-depositional processes significantly increased the content of lignin (Lambda(Oct-01) = 2.19 +/- 0.51 mg/100 mg OC; Lambda(Apr-02) = 2.61 +/- 0.63 mg/100 mg OC); and coarse material (silt and sand), while decreasing the contributions from aged OC (Delta C-14(Oct-01) = -255.7 +/- 32.8%.; Delta C-14(Apr-02) = -213.2 +/- 30.4%.) and fine fraction (clay(Oct-01) = 54.8 +/- 9.5%; clay(Apr-02) = 44.6 +/- 13.3%). The major changes were observed in the northern and central portions of the prodelta.