THE OXIDATION-STATE OF MANGANESE IN FERROMANGANESE NODULES AND DEEP-SEA SEDIMENTS FROM THE CENTRAL INDIAN-OCEAN

Ferromanganese nodules from different sediment facies including siliceous ooze, pelagic clay, sediments from transition zone of siliceous and pelagic clay and seamount crusts from the Central Indian Ocean were analysed for chemical composition, mineralogy and bulk oxidation state of Mn by the iodometric method. Despite considerable variation in chemical composition (Mn/Fe ratio times from 1.35 to 7.42) and mineralogy, the O/Mn ratio or MnOx (solid hydrous manganese oxide) does not vary much (from 1.73 to 1.81), suggesting most of the Mn in the manganese nodules in Mn (IV). The oxidised upper zone in the sediment cores decreases in thickness from 15 cm in the north (5° S) to 11 cm in the southern part (14° S) of the Central Indian Ocean. Iodometric titration of deep-sea sediments yields O/Mn ratios of 1.29-1.95. Sediments from the manganese nodule field have maximum solid Mn concentrations and lower O/Mn ratios in the core top (0-3 cm) with minimum Mn and O/Mn ratios at 11-cm depth and thus may reflect the mechanism controlling the supply of diagenetic Mn to the nodules. In contrast, sediments from the non-manganese nodule field have comparatively low solid-phase Mn concentrations in the core top (0-3 cm) and almost uniform O/Mn ratios with depth, indicating that Mn is not preserved in the sediments but may be remobilised and deposited somewhere else. © 1990.