sequent lower stands the brown layers formed 

 during high stands are buried under iron-and 

 manganese-deficient sediments and presumably 

 preserved as brown interlayers. Furthermore, 

 because iron is chemically more stable than 

 manganese in near-shore areas, or accumulates 

 more rapidly than manganese in these areas, it 

 is possible that sediments deposited at low 

 stands of sea level are more iron-rich than 

 those deposited in the same area at higher 

 stands of sea level. 



Eustatic control of the deposition of nonde- 

 trital elements in the Kara Sea is difficult to 

 evaluate at present because of the conflicting 

 evidence of sea level fluctuations in the Kara 

 Sea region during the Quaternary (Lazukov, 

 1964) and because of the few reliable estimates 

 of rates of sedimentation in the Kara Sea which 

 have been published. That sea level is a signifi- 

 cant factor is doubtful because the variability 

 in the Mn/Fe ratio in surface sediments (table 

 14) is relatively low compared with the varia- 

 bility of this ratio with depth in typical cores 

 (tables 12 and 13). If it has been the history of 

 regional variability of this ratio which pro- 

 duced its stratigraphic variability, then the or- 

 ders of magnitude of both variabilities (re- 

 gional and stratigraphic) should be similar. 

 Furthermore, development of brown surface 

 layers and interlayers does not seem to have 

 been limited to certain water depth ranges and 

 in fact extends to the greatest depths of the 

 Arctic Ocean (Strakhov, 1966; Belov and Lap- 

 ina, 1959). 



Considerable evidence has been gathered 

 (Murray and Irvine, 1895; Brujevicz, 1938a, b; 

 Mortimer, 1942; Bezrukov, 1960; Manheim, 

 1965; Lynn and Bonatti, 1965; Strakhov, 1966; 

 Anikouchine, 1967; Price, 1967; Presley, 

 Brooks, and Kaplan, 1967 ; Li, Bischoff, and 

 Mathieu, 1969) which suggests, and frequently 

 supports, postdepositional redistribution of 

 manganese where oxidizing upper layers are 

 underlain by reducing layers such as in the 

 Kara Sea (fig. 2). This model proposes that 

 manganese, incorporated into accumulating 

 sediments as the oxide, is remobilized after 

 burial by local reducing conditions and diff"uses 

 upward in response to the concentration gra- 

 dient produced by reprecipitation of manga- 

 nese in upper oxidized strata. The factors 

 which appear most important for the produc- 



tion of reducing environments in sediments are 

 the amount and type of decomposable organic 

 matter deposited contemporaneously with the 

 sediment and the rate of accumulation of the 

 sediment (Lynn and Bonatti, 1965). Dissolved 

 oxygen in bottom water, and that which dif- 

 fuses into the sediment, is probably responsible 

 for the reprecipitation of manganese in upper 

 strata although there is some evidence (Gabe, 

 Troshanov, and Sherman, 1965; Ehrlich, 1966) 

 that bacteria precipitate reduced manganese 

 ions from interstitial water. 



The upward enrichment of manganese in 

 typical Kara Sea cores is consistent with the 

 migration model outlined above. The compara- 

 tively fast rates of sedimentation (4 to 6 cm. 

 per 1,000 years, Kulikov, 1961) apparently 

 favor burial of enough organic matter to prod- 

 uce reducing conditions below the upper oxidiz- 

 ing layers. Reducing conditions in buried lay- 

 ers of Kara Sea sediments are supported by 

 (1) negative redox potentials (Eh) below the 

 upper few centimeters (Trofimov, 1939), (2) 

 Fe'*/Fe^* ratios ranging from 0.1 to 2 in 

 gray-green layers as compared with 2 to 20 

 (and as much as 56) in brown layers (Yermo- 

 layev, 1948a,6), and (3) the presence of hydro- 

 troilite in gray-green layers (Klenova, 1948, 

 cited in Strakhov, 1966). 



The upward enrichment of iron and its sepa- 

 ration from manganese (fig. 8) has apparently 

 not been observed in marine sediments before 

 but is also consistent with the migration model 

 when consideration is given to the greater sta- 

 bility of iron oxides under redox conditions fa- 

 voring solubilization of manganese oxide and 

 the greater ease of oxidation of reduced iron 

 compared with reduced manganese (Kraus- 

 kopf, 1957; Hem, 1963). Assuming that man- 

 ganese and iron have been released into the in- 

 terstitial water of gray-green layers and that a 

 gradient in oxidation potential exists between 

 the interface, iron should precipitate first from 

 an upward diffusing solution containing both 

 iron and manganese ions even if only inorganic 

 processes are operating. 



The models which require reduced rates of 

 sedimentation for formation of brown layers 

 (Arrhenius, 1963; Goodell, personal communi- 

 cation) are not inconsistent with postdeposi- 

 tional i-edistribution of iron and manganese. In 

 fact, such a rate reduction in the accumulation 



11 



