SECT. 3] BASIN SEDIMENTATION AND DIAGENESIS 613 



ment, as now exists in marine basin sediments and occasionally in waters, as 

 described in this chapter. Under these conditions phosphate is released by dia- 

 genetic processes and may reach concentrations one hundred times as great 

 as in the overlying surface waters. At the same time the pH of the basin waters 

 is decreased (Rittenberg, Emery and Orr, 1955; Baas Becking and MacKay, 

 1956). The mixing of such water with oxygenated water at the sill or near the 

 rim of reducing basins with a resulting increase in pH should cause phosphate 

 to precipitate. As a consequence, areas in the open ocean surrounding such basins 

 would be enriched in insoluble phosphate. The shelves surrounding the Cariaco 

 Trench or the coastal regions of Norway where fjords are abundant could be 

 present-day environments where phosphate deposition is occurring. Another 

 possible mechanism could involve large-scale turbidity currents which might 

 introduce sufficient oxygenated water and divalent ions into a stagnant basin 

 to cause a precipitation of phosphate. In such an event phosphate would be 

 enriched in the surface layers of the coarser sediments. This may account for 

 the presence of phosphate in ancient basin sediments, 



E. Carbonate Deposition 



The deposition of carbonates on the ocean bottom has been a problem of 

 primary interest to geologists for many years. The source and mechanism of 

 precipitation of amorphous or microcrystalline carbonates is still not entirely 

 solved to the satisfaction of all workers. One of the earliest explanations was 

 given by Murray and Irvine (1889) who suggested an interaction between 

 ammonium carbonate liberated by microbial decomposition of proteinaceous 

 matter and the calcium sulfate in sea-water, producing calcium carbonate and 

 ammonium sulfate. Since then there have been numerous advocates of the 

 hypothesis of deposition of calcite by bacterial activity (Lalou, 1957). Much 

 evidence has been based on the experimental work of Drew (1913) who sug- 

 gested that denitrification was the primary cause of calcium carbonate de- 

 position. Lipman (1924) critically repeated the work of Drew and concluded 

 that the artificial conditions used in culturing the bacteria were so different from 

 those actually existing in the ocean as to make a direct comparison untenable. 

 The most recent support for bacterial precipitation comes from the work of 

 Lalou (1957, 1957a) who set up mixed cultures in aquarium tanks using re- 

 ducing mud covered with sea-water enriched with 0.5 to 1.0% glucose. 



The studies with bacterial cultures have many drawbacks, the major one 

 being the high content of nutrients and calcium added to the culture medium. 

 The processes of denitrification to which has been attributed the liberation of 

 ammonia are probably incorrect. Denitrification which results in the reduction 

 of nitrate to ammonia or nitrogen can occur only in the anaerobic environment. 

 In a highly anaerobic environment the nitrate content is low, and it is doubtful 

 how much ammonia actually forms compared to the molecular nitrogen liberated. 

 Deamination of the proteins in the mud is probably more important. 



Bacterial sulfate reduction has been proposed as a direct method of formation 



