Formation of ferromanganese microcrusts in relation to glacial/interglacial stages in Pleistocene sediments from Ampere Seamount (Subtropical NE Atlantic)

Abstract

Ferromanganese microcrusts were found in oxic sediments that are undisturbed between 60 and 480 cm bsf. Rhythmic alternations of muddy calcareous ooze and foraminiferal-nannofossil ooze make up the sediment sequence, which extends to 270 ka at 480 cm bsf where a hiatus of at least 1.3 Myr occurs. The bottom of the core is Pliocene. The occurrence of ferromanganese microcrusts mainly formed on foraminifera tests larger than 200 μm can be correlated with glacial stages 4, 6 and 8 and indicates enhanced metal fluxes. They do not form at the expense of carbonate material but derive their metals from the water column by hydrogenetic precipitation. Manganese was supplied as Mn 2+ mainly from the oxygen minimum zone (OMZ), where it is transported via detrital material and brought into solution by weakly reducing conditions promoted by the oxidation of organic matter. An enhanced detrital flux during glacials may also cause increased Mn availability in the OMZ during these periods. Mixing of oxygen-rich intermediate water with OMZ water during the glacial intervals caused Mn 2+ oxidation in the water column underneath the OMZ.