ChemicalGeology, 85 (1990) 171-181 171 Elsevier Science Publishers B.V., Amsterdam [3] The oxidation state of manganese in ferromanganese nodules and deep-sea sediments from the Central Indian Ocean J.N. Pattan and A.V. Mudholkar National Institute of Oceanography, Dona Paula, Goa 403 004 (India) (Received June 1, 1989; revised and accepted December 28, 1989) ABSTRACT Pattan, J.N. and Mudholkar, A.V., 1990. The oxidation state of manganese in ferromanganese nodules and deep-sea sedi- ments from the Central Indian Ocean. Chem. Geol., 85:17 l- 181. 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 compo- sition, 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 is 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 compara- tively 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. 1. Introduction The ferromanganese nodules from the Cen- tral Indian Ocean were studied for chemical composition, mineralogy, morphology, depo- sition of ferromanganese oxide on biological hard parts, internal microfeatures and relation between sea-bed topography and nodule distri- bution (Banakar, 1987; Jauhari, 1987; Mu- khopadhyay, 1987; Rao, 1987; Pattan, 1988; Pattan and Kodagali, 1988 ). The valency of Fe in manganese nodules of the Central Indian Ocean has been examined previously by M/Sss- bauer spectroscopy by Pattan and Mudholkar (1990) who found that Fe is solely present as Fe(III). However, the oxidation state of Mn in manganese nodules, as well as the associ- ated sediments of the Central Indian Ocean, have not been studied. The oxidation state of manganese nodules is important for their min- eralogy and incorporation of trace elements (Dymond et al., 1984). Buchanan ( 1891 ) first measured oxidised manganese (O/Mn) or MnO, for Pacific nodules. Manheim (1965) concluded that the average oxidation state of Mn in manganese nodules decreased with de- creasing water depth. The bulk oxidation state of Mn in Pacific Ocean nodules, in nodules as- sociated with sediments and in surface sedi- ments was examined by Piper et al. (1984), Murray et al. (1984) and Kalhorn and Emer- son (1984), respectively. These studies re- vealed that almost 100% of Mn is Mn(IV). On the other hand, Wakeham and Carpenter (1974) used electron spin resonance (ESR) 0009-2541/90/$03.50 © 1990 Elsevier Science Publishers B.V.