Organic Carbon Geochemistry in the North-western Black Sea–Danube River System

The isotopic and chemical composition of organic matter from sediments collected on the north-western shelf of the Black Sea and the Danube River are discussed. The 13C distribution pattern in organic carbon from surface sediments (0–1 cm) of the western part of the Black Sea has been established. It reveals a rather complicated picture, reflecting the superposition of several factors: local marine primary productivity, terrestrial input to the Danube River discharge and possible contribution from anaerobic microbial activity. The analysis of organic carbon by a pyrolysis-chromatography technique showed that the H/O indices of organic matter from marine sediments are in correlation with 13C values. This is an indication of the mixed origin of the organic carbon in the littoral sediments. However, samples from the zone where H2S conditions prevail deviate from the correlation line of 13C vs H/O indices. We believe that this is due to the contribution of the biomass of chemosynthetic bacteria in the sediments. Thus, we argue that in the Danube–Black Sea system several consecutive zones are distinguished. River discharge delivers organic carbon with 13C values from 28 to 26 (PSU is used). Mixing of the land-derived material with autochtonous marine primary production gives 13C values of about 26 to 23 for the organic carbon in coastal sediments. On the shelf area, beyond significant influence of both terrestrial and sulphide regime factors, plankton material dominates as a source of organic carbon in sediments. In the hydrogen sulphide zone, chemosynthetic bacteria produce additional amounts of organic matter with hydrogen to oxygen indices similar to those of plankton, but with different isotopic composition, which results in the appearance of relatively isotopically light organic carbon in the deep-sea sediments.