ANOMALIES OF RARE ELEMENTS IN MANGANESE MICRONODULES FROM ETHMODISCUS OOZES IN THE BRAZIL BASIN OF THE ATLANTIC OCEAN
The composition of manganese micronodules from miopelagic clays and Ethmodiscus oozes of the central part of the Brazil Basin (station 1537, R/V Akademik Sergei Vavilov) is considered. Micronodules were recovered from >50 μm fraction of sediments from the depth intervals of 300 to 305, 405 to 410 and 442 to 452 cm below seafloor. The composition of micronodules was determined in separate size fractions of 50-100, 100-250 and 250-500 μm after dissolution in 0.5N NH2OH × HCl + 25% CH3COOH. The contents of Co, Ni, Cu, Ce, Pb, W, Th, and Bi in micronodules of miopеlagic clays were found to be higher than in micronodules from Ethmodiscus oozes. In the latter, the positive anomalies of Li, As, Mo, Cd, Tl, and U were revealed. The REE composition of micronodules in miopelagic clays is similar to the composition of hydrogenous crusts with a maximum in middle REE and a positive cerium anomaly. Micronodules of Ethmodiscus oozes have a positive Ce anomaly 2.8-3.8 and a deficiency of light lanthanides, similar to the composition of dissolved REEs in pore and bottom ocean waters. High accumulation of redox-sensitive elements in micronodules (As, Mo, V, and Cd) indicates an anaerobic stage in the post-sedimentary period in Ethmodiscus ooze caused by high biological productivity of waters. The formation of micronodules began during the period when the reducing conditions changed to oxidizing ones. Elements with low mobility (As, Mo, V, and Cd) accumulated in sediments and pore water during the anaerobic stage were then sorbed on Mn oxyhydroxides during the oxidation stage.