SECT. 3] 



PELAGIC SEDIMENTS 



685 



Table V — continued 



Sample no. 



Mineral type Environment 



U, 



ppm 



Description 

 of sample 



16 195-8.2 Fish-bone apatite Oxidizing 



16 193-10 + 

 + 16 194-6 



Oxidizing 

 but less than 

 in 16 195-8.2 



16 181-1 



Weakly reduc- 

 ing 



1.5 From low organic produc- 

 tivity area, 17-21°N in 

 East Pacific; brown clay 

 sediment with numerous 

 manganese nodules 

 8.5 From siliceous - calcareous 

 sediments below Equa- 

 torial Current system. 

 Ferrous iron preserved 

 in zeolites, 1-3 mmole 

 NH4+/I. of interstitial 

 solution 

 16.3 Modelo Formation, Cajif. 

 (Tertiary). Compara- 



tively rapidly accumulat- 

 ing epicontinental sedi- 

 ment. Organic matter 

 present at the level of 

 2-4% carbon in the sedi- 

 ment; ferrous iron pre- 

 served or generated, no 

 sulfides 



Goldberg and Parker (1960) have proposed that the low oxygen content of the 

 bottom water in areas with a high rate of deposition of organic matter is a 

 causal factor in the precijDitation of francolite. The widespread occurrence 

 of this mineral, deposited under oxidizing conditions as well as under reducing 

 ones, would seem to point instead to a reaction which is insensitive to the 

 redox state of the system. 



The high concentrations of phosphorus, often attaining values of 1-2% 

 P2O5 in slowly accumulating clay sediments and zeolitites on the deep-ocean 

 floor, were previously attributed to precipitation of iron phosphate (Gold- 

 schmidt, 1954); it now seems quite evident that the phosphate prevails in 

 the form of skeletal debris of fish (Arrhenius, Bramlette and Picciotto, 1957). 

 Apatite is soluble at the deep-ocean floor, so that delicate structures such as 

 scales and minute bone fragments are preserved only in the most recent parts 

 of rapidly accumulating calcareous deep-sea sediments. 



In deep-sea sediments, including manganese nodules, which accumulate 

 slowly, only the most resistant structures with a large crystallite size, such as 

 teeth of fish and earbones of whales, remain undissolved. The dissolution of 

 skeletal apatite is counteracted by the incorporation into the crystal structure 

 of large quantities of the rare earth elements thorium and zirconium, adsorbed 

 from solution in sea-water. The ensuing highly insoluble phosphates eventually 

 remain as microscopic crystals after the disappearance of the original skeletal 

 debris in the most slowly accumulating t3rpes of deep-sea sediments, such as 

 manganese nodules. 



