Distribution pattern and morphochemical relationships of manganese nodulesfrom the Central Indian Basin

In the Central Indian Basin manganese nodule abundance was variable in all sediment types, Mean abundance varied from 1.5 in calcareous ooze to 10.2 k g/m(2) in terrigenous-siliceous ooze sediments. Nodule grade and growth rat es are positively correlated only up to 10 mm/My (million years), and grade shows no distinct relationship with abundance. Relationships between the m orphochemical characteristics of the nodules and host sediment types are su btle. Both hydrogenetic and diagenetic nodules (with smooth and rough surfa ces respectively) occur on almost all sediments, but in variable proportion s. Thus, the overall distribution pattern shows that small nodules ( < 4-cm diameters) of lower grade (average value Ni + Cu + Co = 1.21 %) with smoot h surfaces are more common on red clay, terrigenous, and terrigenous-silice ous ooze transition-zone sediments. By contrast, large nodules (> 4-cm diam eters) of higher grade (average value Ni + Cu + Co = 1.80%) with rough surf aces are more prevalent on siliceous ooze, siliceous ooze-red clay, and cal careous ooze-red clay transition-zone sediments. This implies an enhanced s upply of trace metals from pore waters to rough-surface nodules during earl y diagenesis.