FORMATION OF FERROMANGANESE MICROCRUSTS IN RELATION TO GLACIAL INTERGLACIAL STAGES IN PLEISTOCENE SEDIMENTS FROM AMPERE-SEAMOUNT (SUBTROPICAL NE ATLANTIC)/

Ferromanganese microcrusts were found in oxic sediments that are undis turbed between 60 and 480 cm bsf. Rhythmic alternations of muddy calca reous ooze and foraminiferal-nannofossil ooze make up the sediment seq uence, which extends to 270 ka at 480 cm bsf where a hiatus of at leas t 1.3 Myr occurs. The bottom of the core is Pliocene. The occurrence o f ferromanganese microcrusts mainly formed on foraminifera tests large r than 200 mu m can be correlated with glacial stages 4, 6 and 8 and i ndicates enhanced metal fluxes. They do not form at the expense of car bonate material but derive their metals from the water column by hydro genetic precipitation. Manganese was supplied as Mn2+ mainly from the oxygen minimum zone (OMZ), where it is transported via detrital materi al and brought into solution by weakly reducing conditions promoted by the oxidation of organic matter. An enhanced detrital flux during gla cials may also cause increased Mn availability in the OMZ during these periods. Mixing of oxygen-rich intermediate water with OMZ water duri ng the glacial intervals caused Mn2+ oxidation in the water column und erneath the OMZ.