Presence of Rare-Earth Elements in Black Ferromanganese Coatings from Vântului Cave (Romania)

This study examines the rare-earth elements (REEs) found in ferromanganese coatings covering both sandy alluvium and submerged boulders in an underground stream from Vântului Cave, Romania. The black ferromanganese sediments are mainly composed of birnessite and other poorly-crystallized man-ganese oxide and hydroxides (pyrolusite, romanechite, todorokite, rhodochrosite) as well as goethite and kaolinite. Scanning electron microscope and EDX analyses performed on the black ferromanganese sed-iments show the material to have concentrated considerable amounts of REEs (La, Ce, Sm, Nd) in iron-rich spheres that build up botryoidal-like aggregates. The correlation of 143Nd/144Nd ratio for 6 different samples indicates that the REEs were concentrated in the cave environment after being leached from bauxitic and red residual clays from above the cave. Based on our observations, we conclude that an increase in pH resulted in adsorption of REE onto the surface of ferromanganese minerals. This study demonstrates the potential of using Nd isotopes as a tool for paleochemistry studies of the cave environ-ment. The REEs have been used in several recent studies of oceanic manganiferous deposits in order to identify possible sources of the elements and, specifically, to assess seawate