Early Diagenesis of Carbon and Nutrients in Sediments of the Gulf of Manfredonia (Southern Adriatic Sea).

The aims of the research presented here were to investigate the carbon and nutrient cycles in sediments of the Gulf of Manfredonia and to understand their role in the water column chemistry. Four cores were collected in two sites of the gulf in early fall 2002 and late winter 2003. The cores were extruded for pore water (TCO2, DOC, alkalinity, nutrients, Si(OH)4, sulphate, Fe, Mn, Ca and Mg) and solid (grain size, organic and total C, total N, 210Pb excess, 137Cs, 234Th) analyses. Furthermore, fluxes at sediment-water interface have been measured by benthic chambers and calculated from pore water concentration profiles for O2, TCO2, DOC, alkalinity, nutrients, Si(OH)4, sulphate, Fe and Mn. Pore water data evidenced high inputs of reactive organic matter in the two stations, diagenesis of organic matter progresses through oxygen consumption, denitrification, Mn-Fe-oxy-hydroxide reduction and weak sulphate reduction. Degradation processes are more intense during the warm season. Bio-irrigation seems to be a consistent transport mechanism in both stations, with more evident effects in early fall in the outer station. Measured benthic fluxes showed no clear difference between sites with slight higher values in the offshore site in the warm season. Benthic flux comparison of the Gulf of Manfredonia with the northern Adriatic allowed evaluating the role of the gulf sediments in the chemistry of the south-western Adriatic Sea waters.