Biogenic accumulations of uranium in recent seas

The objective of this study is to determine the geochemical role of molluscs in the distribution of uranium in the marine ecosystem. Biogeochemical studies are carried out on recent mollusc shells from the Caspian Sea, Sea of Japan,,Sea of Marmara, Aegean Sea, Black Sea, Mediterranean Sea, Baltic Sea, and Indian Ocean, which differ from each other in terms of physical, chemical, geographic, and geochemical characteristics. In this study, nine Gastropoda and fifty-four Pelecypoda shells of different species are analyzed to document variations of uranium in seasonal layers, which were formed by the seasonal carbonate-organic phase of molluscs during their entire lives. Shell used in this study principally comprises three layers: upper (outer) prismatic, middle prismatic, and inner, (mother-of-pearl) layers. In addition, when possible, the head, the middle, and the lower parts of the shells are used for analyses separately. Also, the biological accumulation rate values for each different mollusc species relative to the average uranium value in seawater are calculated. The BAR values of U in mollusc shells range between 11 and 216. Uranium concentrations in recent mollusc shells vary between 0.034 ppm and 0.65 ppm. Organic carbon values vary between 1100 and 9700 ppm in various mollusc species. The uranium and C-org concentrations in living mollusc shells are positively correlated. Both the uranium and C-org contents in mollusc shells are preferably dependent on the taxonomic characteristics of organisms. It has been observed that the uranium concentrations in the mineral-organic phase of molluscs are in good agreement with those of surrounding sediments. Therefore, the changes in the uranium concentrations in shell and sediment types are derived from the uranium concentrations in water.