CONCOMITANT ENRICHMENTS OF URANIUM, MOLYBDENUM AND ARSENIC IN SUBOXIC CONTINENTAL-MARGIN SEDIMENTS

Elemental analyses were carried out by Neutron Activation Analysis on eleven sediment cores from the North-East Tropical Atlantic, collected at two sites of the French EUMELI programme. Total uranium, molybdenum and arsenic concentrations increase from the surface to about 20 cm depth in cores from the eutrophic site, but remain constant in cores from the mesotrophic site. These profiles clearly highlight an enrichment of the three elements in the sediments from the eutrophic site, linked to the suboxic conditions encountered in these sediments. This entrapping of uranium in suboxic sediments, observed in previous studies (Barnes and Cochran, 1990; Klinkhammer and Palmer, 1991), is thus confirmed at the EUMELI eutrophic site. Furthermore, other elements, such as molybdenum and arsenic, known to be trapped in anoxic sediments, are found to exhibit a similar behaviour in the suboxic sediments. Simple calculations allow the evaluation of the flux of these three elements into suboxic continental margin sediments and the assessment of the importance of this removal in their oceanic budgets. The results confirm the important removal of uranium into suboxic sediments, and suggest, on the other hand, moderate and low removals for molybdenum and arsenic, respectively. Furthermore, the concomitant increase of uranium, molybdenum and arsenic concentrations may be used as a tool to highlight changes in the paleoproductivity of ocean margins.