THE ORIGIN OF SEDIMENTARY ORGANIC-MATTER IN THE ADRIATIC

Organic C, N and P contents, and C-13/C-12 and C/N ratios of organic matter in surficial sediments of the Adriatic Sea were used to study the distribution, origin and diagenetic transformations of sedimentary organic matter. Using the differences in deltaC-13 values between autochthonous marine (planktonic) and allochthonous (riverine) organic matter in the Adriatic was possible to determine the origin of recent surficial sedimentary organic matter and that from some cores in the northern and central Adriatic indicating the past environmental conditions in the sea. In constructing the mixing model for determination of marine and terrestrial contributions to sedimentary organic matter we used the deltaC-13 values of different end-members, i.e. phytoplanktonic (deltaC-13 = -21.0 parts per thousand) and riverine organic matter (deltaC-13 = -28.0 parts per thousand) in the Adriatic area. The possible shift of deltaC-13 values of about 1.5-2 parts per thousand due to transformations occurring in the water column and early diagenesis in surficial sediments was also considered. The geographical distribution of a higher terrestrial contribution to sedimentary organic matter, also reflected in the higher organic C, N and P contents, is restricted to the western part of the Adriatic strongly influenced by the river Po and other Italian river inflows, and to southeastern part of the Adriatic affected by local (Albanian) riverine inputs. This distribution is a consequence of the general water circulation system and sedimentological properties of the Adriatic Sea. The area of high organic C content in the Jabuka Pit in the central Adriatic is, on the other hand, also a consequence of higher biological production in this area. The organic C content and the deltaC-13 values within the cores collected in the northern and central Adriatic indicate that the deltaC-13 values matched the organic C variations and that the organic C content is the function of allochthonous organic C input. The higher C content and lower deltaC-13 value observed in the horizon from about 15,000 years B.P. during the last post-glacial period, was therefore a consequence of a larger terrestrial contribution by increased river flow into the central Adriatic.