SECT. 3] BASIN SEDIMENTATION AND DIAGENESIS 605 



may range from 19 to 200 %o. A decrease in chlorinity is interpreted by Chave as 

 being due to contamination by meteoric water. The other principal anions, 

 bromide and iodide, are also enriched. Sulfate is generally depleted (presumably 

 by bacterial reduction), often to vanishingly small quantities, and sulfide may 

 substitute for it. Enrichment in sulfate is thought to be due to the addition of 

 liyjjersaline evaporite water. The cations also show a variation. Calcium is 

 enriched, probably as a result of the solution of limestone and replacement by 

 magnesium in the formation of dolomite. This j^rocess, and the adsorption of 

 magnesium by clays during montmorillonite and illite formation, results in the 

 depletion of magnesium. Potassium is also depleted, presumably by the ]3ro- 

 cesses of exchange in clay-mineral formation. Strontium and barium are 

 enriched, probably through the release by fine sediments into sulfate-poor 

 water. 



Data on these ions in recent sediments in present-day basins are not as well 

 established. This is partly due to lack of effort and partly to the inability of 

 getting sufficiently long cores until relatively recently. The decrease of sulfate 

 in reducing environments has been confirmed by many workers. The change in 

 chloride is not so straightforward. Emery and Rittenberg (1952) and Ritten- 

 berg, Emery and Orr (1955) found no change with depth down to 200 cm; 

 Bien (Chave, 1960) also apparently could not detect any significant changes. 

 Shishkina (1959), however, studying the Black Sea sediments, found that a 

 considerable decrease occurred with depth, amounting to 43% at about 900 cm 

 in one core. Bromide was also found to decrease, but not to the extent of chloride. 

 Potassium, sodium and magnesium were all found to diminish by Shishkina. 

 In some cores, potassium almost disappeared at depths of 800-900 cm, although 

 in others the changes to depths of 300-400 cm were not ]3ronounced. Calcium 

 generally showed an increase with depth. 



These results, especially the analyses of anions in the interstitial waters of 

 recent sediments, can be seen to differ from the measurements on connate 

 waters. The results of the Black Sea measurements may not be truly repre- 

 sentative of present-day conditions, having been influenced by post-glacial 

 deposition in an environment of lower salinity. There is, however, a serious 

 lack of data from jjresent-day neritic basins. 



4. Isotopic Studies Related to Biological Processes in Sediments 



The recent interest in stable isotopes (Rankama, 1954) has proved of great 

 value in elucidating many problems in nature, including the ocean. Surprisingly 

 little work, however, apart from the oxygen isotope work, has so far appeared 

 in the literature attempting to interpret mechanisms in oceanic sediments 

 generally and basin sediments in particular. 



The studies of Richards and Benson (1959) on denitrification were briefly 

 mentioned earlier. By measuring the 1^^/14;^ ratio in the dissolved molecular 

 nitrogen of the Cariaco Trench, they were able to show that approximately 3% 

 of the nitrogen was derived from denitrification processes occurring in the 



