Characteristics of manganese nodules from the Central Indian Basin: Relationship with the sedimentary environment

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The hydrogenetic and diagenetic manganese nodules from the CIB show an unequivocal genetic relationship with Neogene sedimentological patterns. Hydrogenetic nodules were observed at the top of the sedimentary column and interstratified in thin sequences of biosiliceous sediments and underlying Miocene(?) red clays. In cores from topographic highs, Early to Late Pliocene radiolarian fauna were found at the base of the biosiliceous deposits. These condensed biosiliceous sediments are less than 4.0 m thick, and periods of non deposition and/or erosion may have occurred. In the central deep area (11–15° S), significant biogenic deposition began during the Early to Late Pliocene as a result of the development of a high biogenic productivity belt to the south, producing the favourable conditions for diagenetic accretionary processes for Mn nodules. This indicates growth of hydrogenetic nodules prior to the Late Pliocene and the subsequent inception of diagenetic accretion. Formation of the most recent diagenetic nodules is associated with higher rates of sedimentation in the deep-sea troughs, where the siliceous sediments are much thicker. This should promote the diagenetic mobilization of Mn and its precipitation under oxic and suboxic conditions. Nevertheless, diagenetic nodules are restricted to the upper 40 cm of sediments younger than 400,000 years, suggesting fast-growing diagenetic nodules and the dissolution of buried nodules below the first 50 cm of siliceous oozes. Hydrogenetic nodules are formed by dense layers of δMnO 2Fe(OH) 3 and amorphous silica, and they show smooth texture and botryoidal shapes. Analysis of bulk and selected portions of layered crust or rind shows very low Mn Fe ratios (< 2) and relatively high Co and Ti contents. In most cases, the cores of hydrogenetic nodules are fragments of altered ash layers from the Indonesian Volcanic Arc, which is located more than 2500 km to the east. The ash layers are also interstratified with Miocene(?) red clays. Diagenetic nodules show a granular texture and comprise concentric layers with dendritic or columnar structures, mainly of 10 Å manganate. Higher Mn, Ni and Cu contents are observed in this phase. Late diagenetic enrichment of these nodules is continuous within the upper 40 cm of Recent (< 0.4 m.y.) biosiliceous sediments, which favours (a) the replacement of the silicate phases (biogenic silica, clay minerals and volcanic glass) by Fe-Mn oxides, (b) a metallic uptake (Cu > Ni) that enhances the stability of early diagenetic 10 Å manganate (buserite), and (c) the formation of todorokite. Some nodules, referred to as mixed in the literature, show hydrogenetic internal layers which grade outward to a diagenetic crust. This transitional character suggests slow changes in the sedimentary environment as growth advances. The nodules of the CIB and the East Equatorial Pacific (EEP) are similar in terms of both metallic composition and accretionary patterns. However, the nodules of the CIB are considerably younger and present a continuous accretionary history. The history of the manganese nodules from the CIB is related to the post-Late Miocene sedimentary and geodynamic evolution of this basin.