Early diagenesis and benthic fluxes in Adriatic and Ionian seas
Spagnoli F; G. Bartholini; P. Giordano;
Various research projects studied early diagenesis processes and benthic fluxes in Adriatic and Ionian seas from eighties: IGM-CNR Bologna projects, MAST, EUROMARGE, PRISMA1 and VECTOR. In these projects early diagenesis have been 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 researches different diagenetic environments have been distinguished; they resulted from different particulate and dissolved continental inputs, different distances from main sediment sources (mainly the Po River), different bottom sediment composition (carbonatic or silicoclastic), different organic matter (fresh marine and old continental organic matter), different depths (more or less resuspension events, increasing pressure with depth), different oxygenation of water column. As regard the Northern and Central Adriatic Sea it is possible to discriminate: 1) the area on the North of the Po River, characterized by carbonate sediments, low upward or downward phosphate fluxes, due to authigenic apatite precipitation, low ammonia fluxes and downward DIC fluxes for low reactive organic matter inputs and carbonate precipitation. In this area, sediments in front of main rivers are higher in organic matter content producing slowly higher fluxes of phosphate and DIC by oxic and suboxic organic matter degradation; 2) the proximal area in front of the Po River, characterized by high ammonia, phosphate and DIC fluxes for the high organic matter and silicate inputs. In this area, when bottom waters reach anoxic conditions, also Fe, Mn and phosphate fluxes increase for dissolution of Fe and Mn oxi-hydroxide surface layer; 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 consequent to very low inputs of fresh organic matter and low sediment accumulation rates; this produces low upward fluxes of all nutrients with the exception of nitrate flowing into the sediment. As regard the Southern Adriatic and the Ionian basins some regional differences can be highlighted: the Southern Adriatic basins, more distant from main sediment sources and at greater depths of the northern areas, are characterized by oxic and suboxic-non sulphidic organic matter degradation. The most intensive degradation processes occur in the Otranto Channel sediments. In the Ionian basin the sediment remineralisation processes takes place mainly by means of oxic reactions. This means that inputs of reactive organic matter are lower for the lower productivity of the basin, for the greater water column depth and for the higher distance from river inputs. Calculated benthic fluxes reflect the early diagenesis processes, they show a northern-southern and shallow-deep trend characterized by lowering of remineralisation processes, this generate weaker oxygen fluxes into the sediments and DIC fluxes outside the sediments. Furthermore, in very low accumulation rate or erosion areas DIC fluxes are towards the sediment. Ammonium and nitrate fluxes are complicated by the nitrification/denitrification processes occurring in the oxic zone. Trace element stable isotopes (?98/95Mo, ?74Ge, ?56Fe) and also other stable element isotopes (?13C, ?15N, ?34S) related to early diagenesis and associated fractionation studies are completely lacking in the Mediterranean so investigation in this field could be very interesting.
2010 - Abstract in atti di convegno
GEOTRACES Mediterranean Planning Workshop, pp. 42–42, 4-6/10/2010
Keywords: early diagenesi, benthic fluxes