SECT. 1] THE OCEANS AS A CHEMICAL SYSTEM 17 



reactions occurring under oxidizing conditions at the sea-water-sediment 

 interfaces in the pelagic and certain in-shore areas. Here, the ubiquitous ferro- 

 manganese minerals accumulate in sites of limited sedimentation. Menard and 

 Shipek (1958) estimate that between 20 and 50% of the deep-sea basement in 

 the southwestern Pacific is covered with these minerals, on the basis of bottom 

 photography and their occurrence in cores. They are found as nodular con- 

 cretions which exist in sizes from millimeters to meters, as coatings about 

 rocks and shells and as fine-grained dispersions in unconsolidated sediments. 

 The nodules often form about a nucleus of phillipsite, pumice, shell fragments, 

 lithified sediment or the refractory biogenous remains, ear-bones of whales or 

 sharks' teeth. Growth is usually radial about the nucleus with easily visible 

 concentric layers. 



The rates of build-up of the ferromanganese minerals are extremely low — of 

 the order of hundredths of millimeters to millimeters per thousand years. For 

 example ionium/thorium geochronological measurements on a nodule from 

 the Blake Plateau (29° 18'N, 52° 20'W; depth 5400 m, Lamont Theta Trawl 

 No. 4) indicated a rate of accumulation of 0.1 mm/10 3 years. Perhaps such 

 chemical growth, representing one of the lowest chemical reaction rates as- 

 certained in nature, can better be expressed in terms of atomic layers per day 

 in which the results range between 1 and 100. 



The two principal metals in the minerals are iron and manganese, occurring 

 as oxides or hydroxides, in similar amounts, although, less generally, either 

 may be dominant. These phases act as hosts for a suite of metals including 

 copper, nickel, cobalt, lead and the rare earths. These guest metals are mod- 

 erately to highly reactive in sea-water and their assimilation by these wide- 

 spread minerals may well account for their states of undersaturation in 

 sea- water and their rather modest residence times. 



Their rather unusual chemical composition (Table V) unlike that of any 



Table V 



Average Composition of Ferromanganese Minerals from the 

 Pacific Ocean 



