Organic Carbon and its Isotopic Composition in Surficial Sediments From the Western Adriatic Continental Shelf

Continental shelves play an important role in the global carbon cycle because they represent the largest modern repository of particulate organic carbon. However, recent studies indicate that the role of shelves as sinks for C_org is still poorly known in details, suggesting that more understanding of sedimentary processes affecting the distribution and preservation of organic matter on the continental shelves is required. The western Adriatic continental shelf receives inputs of allochthonous, terrestrial organic carbon via rivers and autochthonous, marine organic carbon from coastal biological productivity. In the northern region, the Po river dominates the supply of organic carbon. The central coast is characterized by a series of small, distributed fluvial sources draining the Apennine mountains. In this region, modern sedimentation occurs on a clayey-silt shelf clinoform oriented parallel to the coastline. The organic carbon content of surface sediments collected during 2 cruises (April 2002 and October 2002) follows the same pattern of fine sediment dispersal. The highest values (from 0.60 to 1.71 wt%) were recorded close to the Po river delta, while in the central Adriatic the contents range from 0.11 to 0.91 wt%, with slightly higher values toward the bottomset region and negligible along-shelf variability. Riverine contributions measured on sediment collected along the axis of some rivers (Po, Uso, Marecchia and Aso), are characterized by intermediate C_org contents (from 0.56 to 0.82 wt%), and very negative δ ¹³C_org values ranging from -25.39 to -26.47‰ . The δ ¹³C_org distribution of surface sediments near the Po delta shows a clear increasing trend with distance from the river mouth, as a consequence of the decrease in the contribution of terrestrial organic carbon and the increase of marine carbon. The signature of terrestrial C_org is almost lost at about 9 km offshore the Po river's mouths. The sedimentary δ ¹³C_org values in the central Adriatic Sea suggest an overall dominance of the organic matter of terrestrial origin and a stronger influence of terrigenous organics close to the mouths of the Chienti, Pescara and Sangro rivers. Small seasonal variations of the carbon stable isotopes were observed and tentatively related to hydrodynamical conditions. Across-shelf variability of organic carbon accumulation rates, based on ²¹⁰Pb chronologies, is fairly small in front of the Chienti River, and well apparent offshore the Tronto River, with maximum values of 45 and 84 g C m^-2y^-1, respectively, occurring in the foreset of the modern clinoform. The isotopic composition of organic matter in marine sediments is often used to identify its source and transport pathways. Many workers have determined the relative proportions of marine and terrestrial organic carbon in sediments from the study of its isotopic composition. On the basis of suspended matter and sediment collected both in fluvial and marine environments, we have determined δ ¹³C_org values of -18.5‰ as end member for marine organic matter and -26.5‰ for terrigenous.