INAA investigation of major and trace elements. Vertical distribution in lacustrine sediments from the Danube Delta

By using the Instrumental Neutron Activation Analysis (INAA), the vertical profile of 4 major (Na, K, Ca and Fe) and 21 trace (Rb, Cs, Ba, Sc, La, Ce, Sm, Eu, Tb, Yb, Lu, Hf, Th, U, Ta, Cr, Co, Zn, As, Sb and Br) elements have been determined in sedimentary cores collected from three lakes (Lung, Mesteru and Furtuna), all of them located in the fluvio-lacustrine part of the Danube Delta. All determinations have been performed in correlation with the radiometric measurement of the 137 Cs vertical distribution as well as sediment dry bulk density. The last category of data allowed us to determine the sedimentation rate, and thus to reconstruct the geochronology of four potential polluting elements, i.e. Zn, As, Sb and Br. A previous lithologic study has showed a great similarity between the Lung and Mesteru lakes sediments, both of them being classified as lacustrine in opposition with the Furtuna lake whose sediments presented a significant fluvial character. The major elements present a relatively uniform trend, irrespective to the sediment origin. In the case of trace elements, we noticed some peculiarities about the Rare Earth Elements (REE) vertical distribution. Therefore, the total REE versus Sc diagram as well as REE normalized to the Upper Continental Crust (UCC) concentrations, in spite of their relatively similitude, presented different pattern for Lung and Mesteru lake sediments on one hand and Furtuna lake on the other. The La to Th ratio presented, in the limit of experimental errors, close values to 3.25, characteristic for the UCC. All these data pledge for a continental origin of the investigated sediments. As, Sb and especially Zn presented maximum concentrations beneath the upper sediment surface. Based on these values as well as on the sedimentation rates determined from the 137 Cs vertical profiles, the time evolution of above elements concentrations have been reconstructed for the last seventy years. By analyzing these findings, it results that the pollution process became more significant at the and of years '50s and beginning of '60s. By starting with the first years of the last decade of this century, this process has marked some regression. All these modification could be connected with the industrial activity of the Central and Eastern European Countries which begun to increase by the end of the '50s till the end of '80s when slowly declined. This fact can also be correlated with the decrease of the industrial activity of the same countries, as a consequence of the recent political changes. (authors)