Comparative geochemistry of Mn nodules and micoodules from of the Gulf of Mexico, the Atlantic and the Pacific oceans

Ferromanganese microconcretions (FMC) were hand picked from four sieved size fractions (100, 200, 325 and 400 mesh) of sediment samples from four depths in a one meter gravity core taken in the Mid-Atlantic Ridge FAMOUS Area. Scanning electron microscopy and electron microprobe analysis were used to relate surface morphology and metal content to depth of burial, size fraction and apparent sediment redox conditions. Upper sediment (0-2 cm) FMC's were typified by rounded, botryoidal forms while FMC's taken from the redox boundary region (42-46 cm) showed signs of dissolution and had depressed Fe to Mn ratios (1.32 at the surface, 0.62 at 42-44 cm). Well crystallized pyritihedra were found in samples taken from the base of the core as well as in samples from the redox boundary.