RARE-EARTH ELEMENTS IN MANGANESE NODULES FROM THE SOUTH ATLANTIC-OCEAN AS INDICATORS OF OCEANIC BOTTOM WATER-FLOW

The mineralogy and geochemistry of a suite of nine manganese nodules f rom the South Atlantic have been determined. The Ce/La ratios of the n odules were investigated to see if they could be used as redox indicat ors to trace the oxygen content of the ambient water mass and the flow path of the Antarctic Bottom Water as has previously been successfull y carried out in the Pacific Ocean. The Ce/La ratios of the nodules de crease in the sequence Lazarev Sea, Weddell Sea (10.4 and 9.7)>East Ge orgia Basin (6.5 and 7.1)>Argentine Basin (5.0), but then increase in the Brazil Basin (6.2) and Angola Basin (9.8 and 15.1). A further decr ease was observed in the Cape Basin (7.6). An extremely high Ce/La rat io of 24.4 had already been determined for nodules sampled north of th e Nares Abyssal Plain in the western North Atlantic. These data reflec t the more complicated pattern of bottom water flow in the South Atlan tic than in the South Pacific. The penetration of more oxygenated Nort h Atlantic Deep Water into the South Atlantic accounts for the higher Ce/La ratios in the nodules from the Angola and Brazil basins. Based o n this study, the flow path of the Antarctic Bottom Water could only b e traced as far north as the Argentine Basin. The unique geochemistry of nodules from the central Angola Basin thigh Mn/Fe and Ce/La ratios, high contents of Ni, Cu, Zn and Mo) appears to be a function of the n ature of the overlying water mass and of the multiple diagenetic sourc es of metals to the nodules. (C) 1998 Elsevier Science B.V. All rights reserved.