Spatial variations of scavenging rates within the central and northern Adriatic Sea: Use of U-Th disequilibria

Dissolved and particulate concentrations of 234Th, 210Pb and 210Po were measured in surface waters of the central and northern Adriatic Sea. The half-lives and biogeochemistry of 234Th (t1/2 = 24.1 days), 210Po (t1/2 = 138.4 days) and 210Pb (t1/2 = 22.3 years) are ideal for studying particle dynamics in the upper water column. It has been observed that the residence time of dissolved 234Th with respect to removal on biogenic particles is inversely proportional to primary productivity and may be used to predict export from or new production in the euphotic zone. Model-derived 234Th fluxes can also be used to help validate sediment trap collections in the upper water column. The aim of the present study was to gain an insight into the intensity and variability of active scavenging and particle removal processes in the central and northern Adriatic Sea. This semi-enclosed sea is strongly influenced by fresh water runoff from rivers carrying large quantities of dissolved nutrients and suspended particles. These inputs of nutrients and suspended particles are expected to give rise to large biological and biogeochemical gradients across the Continental Shelf and offer a range of environments for studying particle scavenging and removal processes. The authors have examined the relationships between 234Th scavenging rates and residence times of dissolved and particulate phases as a function of other physical and biological parameters (primary productivity, POC, chlorophyll, C/N and total suspended matter). Particular reference is made to the two contrasting environments of the Adriatic--a eutrophic zone near the Po River outflow and in the largely oligotrophic waters of the Jabuka Pit