promise to help solve this problem in the future. Sampling 

 and extensive photography have disclosed the presence of 

 different grades of manganese nodules over great areas, 

 especially in the Pacific Ocean. 1 The distribution of the 

 nodules is somewhat patchy which is to be expected in view 

 of depth variations and the obstructing effect of major and 

 intermediate underwater topographic features. However, 

 regional patterns of surface nodules and encrustations do 



1.31 



occur. ' 



Menard has concluded from photographs and dredge 

 hauls that almost all rock outcrops in the open ocean are 

 covered with oxides. 1 Slabs of volcanic ash or consolidated 

 sediment encrusted with ferromanganese oxides (fig. 19) 

 are very common in the eastern Pacific. 2 



Manganese nodules occur in a wide variety of under- 

 water environments (table 1). In protected basins where 

 water circulation is poor, nodules do not usually form. In 

 fact it is not uncommon to find oxygen-starved sediments 

 which form a reducing environment. Evidence of chemical 

 reduction in sediments has been found in San Pedro Basin 

 on the southern California Continental Terrace. The 

 surface was mottled and smooth, and animal activity was 

 lacking. Microrelief is very low in such areas because of 

 the lack of organisms and of sediment-churning. Nodules 

 are normally found on ridges, highs, hills, large mounds, 

 in open deep-sea basins, and on abyssal plains (fig. 20) 

 where oxygen is in plentiful supply, unless a very high sill 

 cuts off water circulation. In some abyssal areas, nodules 

 occur on highs and in lows between hills as a relatively 

 uniform cover, but it is likely that most nodules are on 

 hills. 1 Sediment accumulation would normally be greater 

 in the lows than on the highs. 



Cores from Pacific areas indicate that fewer nodules 

 exist below the interface than at the interface. 1 To explain 

 the presence of nodules at the surface, the sediment must 

 accumulate at a slower rate than manganese oxide, although 

 some workers now feel that organisms can keep the nodules 

 at the surface in their search for food. The author doubts 

 the effectiveness of this mechanism, except in areas where 

 nodules are small and closely spaced (fig. 21), since many 

 nodules would be too large for any benthonic organisms to 

 move except downward by undercutting. 



41 



