Seasonal Influence of the Front Area on Sediment Deposition in the Northwestern Adriatic Sea

Abstract

In order to improve our knowledge of short time-scale variations of seasonal particle fluxes into sediments, 28 short sediment cores were collected along six transects in two areas influenced by the frontal system formed by the Po River waters moving into the Adriatic Sea. Radioisotopes ²³⁴Th and ¹³⁷Cs were measured in surficial sediment sections and used as tools to evaluate the rates of sediment deposition, accumulation and mixing. Sedimentological and mineralogical parameters were also analysed.

The ¹³⁷Cs distribution points out that the area south of the Po Delta, characterised by muddy sediment scarcely colonised by macrofauna, is directly influenced by the input of fine material delivered by the Po River. Here the surficial ²³⁴Th activities are lower in winter than in summer. This suggests a front influence that in the warm season couples a higher concentration of suspended matter with an enhanced capacity of interaction with particle-reactive species. The lithology of the area in front of the Marche Region is otherwise characterised by an alternance of silt and clay. Together with the presence of thanatocoenoses, this pattern reflects pulsed sedimentation with colonisation stages and sudden inputs of sediment that bury the benthic fauna. High values of ³³⁴Th activities were found at the stations close to the front, where the amount of particulate material is high. Behind the front line, thorium activities are lower, due to both the shallow environment and the kind of particles. The maximum values of ²³⁴Th activities, characteristic of the two offshore stations, reflect the high depth of the water column, the enhanced residence time of particles and the very fine grain size of the sedimentary material.