SEDIMENTARY ROCKS/Oceanic Manganese Deposits 119 Indian Ocean In the Indian Ocean, Mn-, Ni-, and Cu-rich nodules are present in the Central Indian Ocean Basin between about 5 and 15 S They are largely diagenetic in origin and rest on siliceous sediments below the CCD under high productivity waters The deposits show north–south compositional variability with the highest grades occurring in the north Atlantic Ocean In the Atlantic Ocean, diagenetic Mn-, Ni-, and Cu-rich nodules occur most notably in the Angola Basin and to a lesser extent in the Cape/ Agulhas Basin and the East Georgia Basin These three areas have in common elevated biological productivity and elevated organic carbon contents in their sediments which, coupled with their depth near or below the CCD, would help to explain the composition of their nodules However, Ni and Cu contents are lower in them than in areas of diagenetic nodules in the Pacific and Indian Oceans Distribution and Compositional Variability of Ferromanganese Oxide Crusts Crusts generally accumulate on sediment free hard rock substrates, and thus their regional distribution is related to that of seamounts, plateaux, and other sediment free areas In a major study on crusts by Hein et al (2000) it is pointed out that the main substrata on which crusts form include basalt, phosphorite, and limestone However, other than serving as nucleating surfaces for precipitation to occur, the substrata not contribute to the formation or the composition of the crusts to any significant degree Ferromanganese oxide crusts (excluding hydrothermal ones) are generally less variable in composition than manganese nodules In a large-scale study on crusts in the South Pacific, Verlaan et al (in press) have shown that over the depth range from which the analysed crusts were sampled (650–5853 m), Co, Mn, and Ni increase as depth decreases, while Fe and Cu increase as depth increases However, the relationship between crust composition and depth may be more complex than this, as analysis of crust composition versus depth in 500 m depth intervals, shows that in certain intervals the correlations between individual elements and depth differ from their overall correlations with depth These differences are mainly found between three depth segments, above 1500 m (shallow), 1500–3000 m (middle), and below 3000 m (deep) Particularly noteable are the relationships (or lack of them) between elements in crusts and depth in the shallow segment in comparison with those in the deeper segments There is an absence of any correlation with depth in the shallow segment for Co and Cu, and there is an opposite correlation with depth in the shallow segment for Mn compared with that in the middle and deep segments Also noteable is the disappearance in the deep segment of any depth correlation for Ni and Fe, and the weakening of the correlation between Cu and depth Investigations on the regional variability in crust composition in the South Pacific by Verlaan et al (in press), show that Co increases overall towards the equator Manganese also increases from south to north and is generally low south of the 12th parallel Nickel likewise increases northwards towards the equator, while Fe increases to the south-west, away from the equator Copper shows little regional variation in crusts in the South Pacific Regionally, Co, Mn, and Ni maintain an opposite behaviour to that of Fe throughout the South Pacific, over the full depth range of the samples collected Furthermore, the overall equator-ward increase in Co, Mn, and Ni remains evident in each depth segment The opposite trends in Co, Mn, and Ni enrichment, on the one hand, and Fe enrichment on the other, start from about the 10th parallel, which is the approximate latitude dividing the the high from the low biological productivity regions in the area studied, suggesting that the latitudinal compositional variations in crusts are at least partly productivity influenced Longitudinally, Co, Mn, and Ni show a tendency to increase to the north-west and Fe towards the south-west, but these variations are much less pronounced than the latitudinal variations Economic Potential Interest in manganese nodules commenced around the mid-1960s and developed during the 1970s, at the same time as the Third United Nations Law of the Sea Conference However, the outcome of that Conference, in 1982, was widely regarded as unfavourable for the mining industry This, coupled with a general downturn in metal prices, resulted in a lessening of mining company interest in nodules About this time, however, several government-backed consortia became interested in them and this work expanded as evaluation of the deposits by mining companies declined Part 11 of the 1982 Law of the Sea Convention, that part dealing with deep-sea mining, was substantially amended in an agreement on 28 July 1994, which ameliorated some of the provisions relating to deep-sea mining The Convention entered into force in November 1994 During the 1980s, interest in manganese nodules and crusts in exclusive economic zones (EEZs) started to increase An important result of the Third Law of the Sea Conference, was the acceptance of a