Short-term sediment dynamics in a littoral area of the North Adriatic Sea (Italy): evidence from 234Th and 7Be

In summer of 2007, short sediment cores (<20 cm) were collected in a littoral area of the North Adriatic Sea, Italy. Down-core measurements for radioactive tracers (234Th, 7Be) and organic matter signatures (OC, TN, C:N ratio) have been used to elucidate potential temporal changes in fluxes and cycles of organic matter in the area. Tracer distributions suggest that surface deposits throughout this area are rapidly reworked and this is supported by the findings of burrowing bivalves. Excess 234Th was present at all stations, with surface activities (0-1 cm) ranging from 12 to 20 Bq/kg and inventories of 46 and 166 mBq cm-2 with penetration depths of about 10 cm. Steady-state particle mixing coefficients Db estimated using excess 234Th ranged from 60 to 150 cm2 yr-1. The cores collected throughout the littoral system show a decrease in OC and TN down-core, and organic carbon signatures (OC and C:N) measured in sediment layers are indicative of multiple sources of sedimentary organic matter, where marine and estuarine phytoplankton is likely an additional important source of the direct input of organic matter.