Humic acids contribution to sedimentary organic matter on a shallow continental shelf (northern Adriatic Sea)

The shallow northern Adriatic Sea receives large river runoff, predominantly from the Po River, which is the main allochthonous source of nutrients and organic matter. The origin and quality of organic matter deposited in the sediments can influence the degradation processes and oxygen consumption in the bottom waters as well as the fate of many pollutants. Therefore the humic acids (HA) were quantified in surface and sub-surface sediments collected in an area of the north-western Adriatic platform south of Po River. HA showed to have a relevant contribution to sedimentary organic matter. HA content in sediments were positively correlated with the organic carbon concentration and negatively with redox potential and pH, particularly in sub-surface reduced sediments, suggesting their important role in the diagenetic processes taking place in anoxic conditions. Elemental composition of HA extracted from surface and sub-surface sediments showed a wide range of variation of the Corg/N ratios which could be due to a mixed (terrestrial and marine) origin and/or an elevated bacteria degradation of nitrogen during diagenesis processes in sediments. The spectroscopic ratios A2/A4 and A4/A6 of HA confirmed a mixed origin with a high degree of condensation of the HA extracted from sediments.