Early diagenesis and benthic fluxes in the Adriatic and Ionian seas

Various research projects investigated early diagenesis and benthic fluxes in Adriatic and Ionian seas in the past (IGM-CNR Bologna projects, MAST, EUROMARGE, PRISMA1, PITAGEM and VECTOR). In these projects early diagenesis were investigated by pore water analyses, while benthic fluxes were determined by direct measurements (benthic chamber deployments and on deck incubations) or pore water modelization. From the integration of these previous data different diagenetic environments and benthic fluxes have been identified: they resulted from different particulate and dissolved continental inputs, different distances from sediment sources (mainly the Po River), different bottom sediment (carbonatic or silicoclastic) and organic matter (fresh marine or old continental organic matter) composition, different depths (increasing reworking and pressure with depth), different oxygenation of water column. In the Northern Adriatic it is possible to distinguish: 1) the central area on the North of the Po River, characterized by carbonate sediments, low upward phosphate fluxes, due to authigenic apatite precipitation, low ammonia fluxes and downward DIC fluxes due to low reactive organic matter inputs and carbonate precipitation; 2) areas in front of northern rivers, characterized by terrigenous and low reactive organic matter inputs that produce low phosphate, ammonia and DIC fluxes by oxic and suboxic or even anoxic (in lower oxygenated waters) organic matter mineralization; 3) areas in front of the Po River mouths, characterized by high ammonia, phosphate and DIC fluxes due to the high sedimentation rate and reactive organic matter inputs that degrade in anoxic and even sulphidic environments. In this area, when bottom waters reach anoxic conditions, also Fe, Mn and phosphate fluxes increase due to the dissolution of Fe and Mn oxi-hydroxide surface layer; 4) the Po River prodelta distal area, South of the Po River until Ancona offshore, where partially degraded organic matter and silicate inputs originate weak diagenesis processes and nutrient benthic fluxes. In the Central Adriatic it possible to discriminate: 1) the coastal Western muddy area, characterised by sediments and partially degraded organic matter coming from Po and Appennine rivers that generate higher DIC and ammonia fluxes, mainly by sub-oxic mineralization; 2) the Mid-Adriatic Depression area, characterized by oxic mineralization due to the low reactivity of already degraded organic matter and low accumulation rates; this produces low upward fluxes of all nutrients, with the exception of nitrate flowing into the sediment; 3) a central area, extending also in the northern Adriatic, characterized by low sedimentation rate, no sedimentation or even erosion, with low and irregular benthic fluxes of organic matter degradation products. As regard the Southern Adriatic and Ionian seas only some regional differences can be highlighted. The Gulf of Manfredonia is characterized by high benthic fluxes of nutrients indicating consistent inputs of reactive autochthonous organic matter attributable to local high primary productivity. The Southern Adriatic basin, characterized by oxic and suboxic-non sulphidic organic matter degradation with higher strength occuring in the Otranto Channel. Calculated benthic fluxes in these areas show weak oxygen fluxes into the sediments and weak DIC fluxes outside the sediments while ammonium and nitrate fluxes are complicated by nitrification/denitrification processes occurring in the oxic zone. In the Ionian Sea sediments remineralisation processes takes place mainly by oxic reactions consequent to refractory organic matter inputs, for the low productivity of the basin, the greater water column thickness and the higher distance from riverine inputs. In the Ionian Sea higher nutrient benthic fluxes are encountered in basin area with respect to slope ones due to higher accumulation rates of partially reactive organic matter.