614 KAPLAN AND RITTENBERG [CHAP, 23 



of calcium carbonate through the substitution of the sulfate by a sulfide and 

 then a carbonate. Lalou (1057, 1957a) found that the initial stage of decomposi- 

 tion in the reducing sediments was the formation of organic acids together with 

 the liberation of C()2. which caused a marked decrease in ])H. This solubilized 

 calcium and caused a twofold increase in its concentration in the sea-water. 

 The next step was the initiation of sulfate reduction with an increase in H2S, 

 until the sulfate disa])])eared. The pH at this stage rose to that at the start. Up 

 to this point little or no carbonate precipitated; only when the containers were 

 placed in the light was there an observed deposition. In the presence of light 

 there was growth of the photosynthetic purple bacteria and a rise in pH. The 

 conclusion that must be reached from this is that CO2 was removed by photo- 

 synthetic fixation displacing the equilibrium to the side of carbonate. There is 

 no indication from this work whether carbonate is actually removed from sea- 

 water or whether it is just a recycling of the carbonate in the mud. 



The necessity for photosynthesis suggests that those processes would occur 

 only at the relatively shallow depths of the euphotic zone. Rapid escape of 

 CO2, an increase in temperature and a high pH of the water would all help this 

 process. Shallow lagoons associated with atolls or fringing reefs would suggest 

 themselves as such environments; most of these, however, are highly oxj^genated. 

 Stagnant basins, particularly in temperate zones, could produce this pheno- 

 menon. In such a case autotrophic bacteria living at the oxidizing-reducing 

 interface could remove the CO2. Such a process may be responsible for the re- 

 working of carbonates in the Black Sea and other deep anaerobic basins. 



The problem outlined above deserves careful attention. There is need for 

 studies in situ rather than artificial culturing as has been done in the past. 

 Since bacterial metabolism liberates the light isotope of carbon as CO2, any 

 carbonate precipitated by this method should be enriched in ^-C. On the other 

 hand precipitation of carbonate by the biological adsorption of CO 2 will leave 

 the calcite enriched in ^^C. This may be one method of obtaining more in- 

 formation from the sediments, as well as from controlled laboratory experiments. 



Neritic marine basins, including fjords, lagoons and inland semi-isolated 

 seas cover only a small fraction of the total oceanic area. Their significance, 

 however, appears to be out of proportion to their extent. The epicontinental 

 seas that were so abundant during the Paleozoic have left ample evidence of 

 their importance in the deposition of neritic sediments. In particular, basins 

 located within continental shelves have played a special role in the formation, 

 preservation and accumulation of mineral resources. The two basic reasons for 

 this role are (a) the slow rate of deposition preventing a dilution of authigenic 

 minerals by terrigenous detritus, and (b) the isolation of the basin water from 

 the overlying water because of restrictions imjoosed by sill depth. 



The consequence of the al)ove two factors is to i)ermit relatively high contents 

 of organic matter to accumulate in the sediment. Because of a restricted ex- 

 change between basin water and surrounding water, dissolved oxygen is re- 

 moved by biological activity, mainly at the mud- water interface. In exchange, 



