of detrital material may favor development of 

 the elemental distributions such as observed in 

 the present study. Under the influence of a con- 

 stant, comparatively rapid (4 to 6 cm./lO^ 

 years), rate of sedimentation, the interface be- 

 tween oxidizing and reducing conditions vv^ould 

 be continuously destroyed and new interfaces 

 constructed at progressively higher strati- 

 graphic levels. Presuming that dissolution and 

 diffusion processes in clayey marine sediments 

 are extremely slow, no significant enrichment 

 of iron or manganese in the upper oxidizing 

 layers could take place under conditions of con- 

 stant rapid sedimentation. On the other hand a 

 period of reduced sedimentation would permit 

 diffusion processes to concentrate the more mo- 

 bile elements in the upper layers before burial 

 and reduction can remobilize them. In addition, 

 the reduced deposition of detrital matter would 

 permit a relative increase in the primary accu- 

 mulation in surface layers of nondetrital sub- 

 stances such as ferromanganese colloids and 

 their associated "scavenged" (adsorbed) trace 

 metals. Thus the models proposing primary 

 processes for the development of brown layers 

 and those proposing postdepositional processes 

 need not be mutually exclusive. 



It is doubtful that only inorganic processes 

 are important in the postdepositional redistri- 

 bution of iron and manganese. The activities of 

 bacteria and other microorganisms are thought 

 to play a main role in the decomposition of or- 

 ganic matter and thus production of reducing 

 conditions in sediments (ZoBell, 1946). The ex- 

 periments of Yermolayev (1948a,6) involving 

 the inversion of a typical Kara Sea core clearly 

 demonstrate the role of bacteria in the mobili- 

 zation of iron and manganese in sediments iso- 

 lated from gaseous exchange with overlying 

 water. The work of Perfilev et al. (1965) and 

 Ehrlich (1966) also supports the importance 

 of bacteria in the production of reducing condi- 



tions in sediments, thus solubilizing the iron 

 and manganese, and provides some evidence as 

 to the role of bacteria in the oxidation or pre- 

 cipitation of iron and manganese when the sed- 

 iments are oxidized. The work of Gabe, Tro- 

 shanov, and Sherman (1965) is particularly 

 pertinent to the present study because they ob- 

 served color sequences and iron-manganese dis- 

 tributions in a core from Lake Valk-Yarvi 

 (table 4) identical to those observed in the 

 upper layers of cores E-26 and N-148 from 

 the Kara Sea. In addition, they found abundant 

 development of microbes, Metallogeniw7t and 

 Siderococcus, known to derive energy from the 

 oxidation of reduced iron and manganese, in 

 the respective sediment layers enriched in 

 these elements. Whether similar microbes are 

 responsible for the oxidation of iron and man- 

 ganese in Kara Sea sediments remains to be es- 

 tabli.?hed. 



The existence of secondary (buried) layers, 

 weakly enriched in iron and manganese, im- 

 plies that not all the nondetrital iron and man- 

 ganese have been released into interstitial 

 water below the oxidizing zone and migrated 

 upward. This may be due to a nonuniform dis- 

 tribution of organic matter as has been sug- 

 gested by Oppenheimer (1960) to account for 

 color banding in Texas marine bay sediments. 

 Another possibility is that the iron and manga- 

 nese in the secondary brown layers have been 

 irreversibly oxidized, as Yermolayev (1948a,&) 

 has proposed, but are still readily reduced by 

 the hydroxylamine hydrochloride-acetic ac'd 

 treatment. The occurrence of the iron phos- 

 phate, vivianite, as coprolitic concretions in the 

 secondary brown layers of cores E-31 and 

 N-77 is almost certainly authigenic and may 

 imply higher biological productivity, possibly 

 in response to an influx of warmer waters as 

 proposed by Yermolayev (1948a,6), at the time 

 these layers were deposited. 



Conclusions 



This textural, mineralogical and geochemi- 

 cal study of Kara Sea sediments allows the fol- 

 lowing conclusions and generalities to be 

 drawn regarding the significance of color vari- 

 ations in the upper layers : 



1. The color sequences observed in typical 



Kara Sea cores are not related to stratigraphic 

 variability in texture or detrital mineralogy. 



2. Nondetrital iron and manganese com- 

 pounds are the constituents of Kara Sea sedi- 

 ments which impart the brown color to the 

 upper layers. 



12 



