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Early diagenesis can be investigated by pore water analyses while benthic fluxes can be determined by direct measurements or pore water modelisation. From the integration of past researches different diagenetic environments and benthic fluxes have been individuated in Adriatic and Ionian seas; they resulted from different: particulate and dissolved continental inputs; distances from sediment sources (mainly the Po River); bottom sediment composition (carbonatic or silicoclastic); organic matter composition; depth; oxygenation of water column. In Northern and Central Adriatic Sea It is possible to distinguish: 1) the area on the North of the Po River, characterized by carbonate sediments, low reactive organic matter inputs and carbonate precipitation. In front of main rivers higher organic matter contents are present and create oxic and suboxic environments; 2) the proximal area in front of the Po River, characterized by high organic matter and silicate inputs producing strong early diagenesis processes and benthic fluxes. In this area frequent anoxic environments are reached; 3) the distal costal area South of the Po River, where lower fresh organic matter and silicate inputs originate weaker early diagenesis processes and nutrient benthic fluxes; 4) the Mid-Adriatic Depression area, characterized by oxic conditions in uppermost centimetres of the sediment consequent to very low inputs of fresh organic matter and low sediment accumulation rates. The Southern Adriatic basins are characterized by oxic and suboxic-non sulphidic organic matter degradation. The most intensive degradation processes occur in the Otranto Channel. In the Ionian basin the sediment remineralisation processes takes place mainly by means of oxic reactions. Calculated benthic fluxes reflect the early diagenesis processes: they show a northern-southern and shallow-deep trend characterized by decreasing remineralisation processes with progressive weaker oxygen fluxes into the sediments and DIC fluxes outside the sediments while ammonium and nitrate fluxes are complicated by the nitrification/denitrification processes.

In order to evaluate the origin and the transformation of organic matter on the shallow shelf of the NW Adriatic Sea, organic carbon, total nitrogen and stable isotope ratios of organic carbon were analysed in riverine suspended matter and sediments as well as in marine suspended and sedimentary organic matter, in marine phytoplankton and zooplankton. The deposition of organic matter is influenced by fine sediment concentration. Surface sediments were characterised by highly variable biogeochemical conditions on the sea floor, whereas sub-surface sediments showed a more homogeneous hypoxic/anoxic environment. Low C org /N ratio and high organic carbon and nitrogen concentrations in riverine suspended organic matter indicate an important contribution of freshwater phytoplankton within rivers, particularly during low flow regimes, which adds to the marine phyto- and zooplankton at shelf locations. In order to evaluate the importance of terrestrial, riverine and marine sources of OM in shelf sediments, a three end-member mixing model was applied to shelf surface sediments using 13 C/ 12 C values for organic matter and N/C ratios. The model showed an elevated contribution of terrestrial organic substances at intermediate depths (10–15 m), mostly corresponding to an area of coarser grain-size, whereas the riverine and marine organic fractions were mainly accumulating near the coast and offshore, respectively.

In this paper we report some preliminary results from an experiment to evaluate oxygen and nutrient benthic fluxes performed in April 1999 in the Primo Seno of the Mar Piccolo of Taranto. The basin is liable to periodic anoxic crisis induced by high nutrient inputs. The sediment was characterised by grain-size, mineralogical and biogeochemical analyses (organic and inorganic C, total N, organic and inorganic P) carried out in a sediment core collected in the basin. In the same site dissolved benthic fluxes (PO4, NH4, NO2, NO3, Si(OH)4, TCO2, O2, Fe, Mn, Ca, Mg) were carried out by benthic chamber. The sediment is mainly silty-clay (73%) with organic carbonate fragments and high organic matter (organic C= 4%) still very reactive (C/N=9.9). Benthic fluxes were positive for NH4, NO2, TCO2, PO4, Si(OH)4 and Mn and negative for O2 and NO3. Furthermore benthic chamber data showed a difference between daytime and night fluxes for O2, NO3, NO2, e TCO2. Besides benthic fluxes indicated reactive organic matter degradation processes, due to dissolved oxygen and nitrate consumption, and dissolution of diatom skeletons, due to positive fluxes of Si(OH)4. The Mn positive flux indicates reducing environment little down to the sediment-water interface.

A sedimentological and geochemical study was carried out on a continental shelf affected by water-based drilling fluid and cutting discharges from an offshore platform. Investigation of the distribution of the cuttings and drilling mud over time and space in proximity to the dumping site provided information on mechanisms of sedimentation, including resuspension and redistribution of the natural and anthropogenic sediment in this marine area. The study area is at a depth of about 15 m in the Northern Adriatic Sea. Surface and subsurface sediment samples were collected during six surveys from 1985 to 1993 by box-corer, altogether 201 box-corer were collected. Grain size, total Ba and Cr content and chemical bonding between Cr and the solid matrix were determined from sediment samples. Coarser sediment fractions, Ba and Cr can be considered as tracers of the discharged drilling sludge. Furthermore they provide information on sedimentation and resuspension processes and are good detectors of the main seasonal, annual and multi-annual sea bottom currents in the investigated area and on continental shelves in general. They showed that local marine hydrodynamics were modified by the platform structure that induced higher turbulence in its vicinity. Out of the influence of the platform these tracers highlighted the presence of drift and tidal bottom currents, directed parallel to the coastline, and storm currents acting mainly perpendicularly to the coastline. Partitioning of Cr in the solid phase showed an ageing process of chemical bonds that confirm the repeated and continuous resuspension and resedimentation processes of the fine sediments.