Uranium and Thorium in Phosphatic Bone Debris from the Ocean Bottom

Uranium and thorium content, as well as their distribution patterns are studied in biogenic phosphates from the Atlantic and Indian oceans. The material studied is represented by differently lithified fish remains (bones, scales, teeth) and marine mammal bones (ribs, vertebras, earbones) collected from both reduced shelf sediments and oxidized pelagic ones. The U content in the material varies from 0.7 to 700 ppm, and the Th content ranges from less than 0.5 to 14 ppm. The U/Th ratio varies from 0.16 to 400. Contents of both elements increase with the lithification of biogenic phosphates. The U concentration is more intense on shelves, whereas the thorium concentration increases in pelagic areas. A partial positive correlation of U and Th with Fe but a negative correlation of U with organic carbon are noted. The latter corresponds to increasing lithification of biogenic phosphates. Calcium phosphate, which is transformed from hydroxyapatite to fluorcarbonate-apatite serves as the main carrier of U, while transformed organic matter is a minor agent. Thorium is mainly bound with Fe.