PELAGIC SEDIMENTS 365 



amounts are so great that, including the considerable percentage 

 of cobalt and nickel, eventual recovery and utilization seem 

 possible. 



Coring generally indicates that finely disseminated manganese 

 is in greater amounts in underlying sediments (of pre-Quaternary 

 age in part, at least) than near the surface. The coring also indi- 

 cates, however, that the nodules are more concentrated at the 

 present sediment surface. A suggestion is offered that may account, 

 in part at least, for the exceptional amounts occurring as nodules 

 on the present ocean floor. 



The large accretions as nodules from the low percentage of 

 manganese in solution in ocean water are obviously in regions of 

 slow sediment accumulation. Although the average of sediment 

 accumulation has been indicated as now more rapid than in the 

 pre-Quaternary, there is considerable evidence that the character 

 and rates are now less uniform over the ocean floor than in the 

 pre-Quaternary. 



Water temperatures were higher and more uniform in the 

 Tertiary, from data of Emiliani (1954) and others. This should 

 result in a more uniform areal distribution of the supply of calcium 

 carbonate from surface waters. The greater compensation depth 

 with warmer water would greatly reduce the areas of solution of 

 carbonate below this depth. Less cold polar bottom water with its 

 higher carbon dioxide content, would also have less influence on 

 the differences in solution related to topography and depth than 

 followed the advent of glacial times. Sorting and grain size differ- 

 ences related to topography also appear greater in the recent 

 sediments, and suggest stronger bottom water motion than that 

 usual in Tertiary time. This also results in differences of accumu- 

 lation rate between areas. 



Summarizing this too brief consideration of evidence, it suggests 

 that the high proportion of manganese accumulation as large 

 nodules is now abnormal because rates of sediment accumulation, 

 though now greater than average, also show much greater differ- 

 ences between areas. Those present day areas of relatively slow 

 accumulation may thus permit deposition of more of the manganese 

 as large nodules, rather than as the disseminated micronodules 

 that are in larger proportion in the Tertiary. 



