X-Ray absorption spectroscopy of uranium at low ppm levels in coral skeletal aragonite

Abstract

We performed X-ray absorption spectroscopy of uranium in three coral specimens and present results of analysis of U L-II edge XANES (X-ray absorption near edge structure) spectra. The XANES of two specimens, Pavona gigantea (collected live at Champion Island, Galapagos) and Pavona clavus (collected live from Bartolme, Galapagos) were most similar to XANES of model compounds of U 6+. In contrast, the XANES of a specimen of Montastrea annularis (Pleistocene age from an exposed reef terrace on Marie Galante, Guadeloupe) showed some features typical of spectra of model compounds of U 4+. These data and interpretations suggest that uranium is incorporated into coral skeletons chiefly in its oxidized U 6+ form. The uranium may be subject to postmortem subaerial reduction to U 4+, or may be replaced or enhanced by addition of U 4+ compounds.