Role of sedimentary environment in the development of hypoxia and anoxia in the NW Adriatic shelf (Italy)

Abstract

In order to understand to what extent inner shelf sediments can feed or even induce bottom hypoxia and anoxia events frequently recorded in the NW Adriatic (NWAd) shelf, we analyse sediment lithology, water content, oxidation and 7Be and 137Cs as well as differences (as Δx values) of grain size, Eh, pH, organic carbon (OC), total nitrogen (TN), C/N and δ13C between surface (SUP) and sub-surface (SUB) layers at 28 sampling sites south of the Po Delta. The SUP level represents the active sedimentation still “mobile” and 2) the SUB level represents the already consolidated deposition, essentially definitive. In order to eventually identify the shelf areas with a predisposition to hypoxia, environmental indicators are obtained by the differences between the two layers of grain size (as % of SUB), OC and TN (as % of SUP) concentrations as well as of Eh, pH, C/N and δ13C absolute values. The results show a NW zone characterised by coarse sediments with sand concentrations of 46–90% dry weight (d.w.), low OC, TN and fine sediment contents in presence of C/N < 10 and ΔEh > 200 mV. A central area, where we found a reactive bottom environment due to the accumulation and burial along the coast of fresh, reactive organic matter (OM) probably formed within the Reno River and in the Sacca di Goro. In the southern area, the coastal stations are characterised by clay sediment (50–67% d.w.) at shallow depths. The distribution patterns of the two radioisotopes indicate that riverine material deposited close to the Po Delta has a relatively “younger” component whereas river-derived sediments are relatively “older” on the SE shelf. The high activity of 7Be and 137Cs found along the ER coast appears also to confirm that river-derived organic-rich fine sediments remain inshore. We therefore argue that this material, whose accumulation and burial is probably favoured by the presence of mussel farms as well as by local morphology and hydrodynamic conditions, contribute to the lowering of redox potential of SUP as well as SUB sediments. We conclude that the persistence or even the increase of anoxia in the NW shelf area could be due to the role of hypoxic, reactive shelf sediments which favours the fast onset of anoxia even with limited inputs of new fresh OM and/or during short period of water stratification.