Short-term dynamics of sedimentary organic matter in a littoral area of the North Adriatic Sea (Italy): Evidence from ²³⁴Th and ⁷Be

¹ Interdepartmental Centre for Environmental Science Research, Bologna University, via S. Alberto 163, 48100 Ravenna, Italy
² Department of Physics, Bologna University, Viale Pichat 6/2, 40127 Bologna, Italy

Abstract

In summer of 2007, short sediment cores (<20 cm) were collected in a littoral area of the North Adriatic Sea, Italy. Downcore 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 D_b 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.