604 KAPLAN AND RITTENBERG [CHAP. 23 



because of their quantitatively greater importance, ultimately dictate the condi- 

 tions. The larger organisms, particularly the worms, as well as some mollusks, 

 sea-urchins and other animals which burrow or disturb the surface of the sedi- 

 ment by their movements (Hartman and Barnard, 1958), may have a marked 

 influence in keeping the first few centimeters oxygenated. 



The role of the microbes is to utilize the oxygen for the metabolism of the 

 organic matter settling on the bottom and to release reducing substances. In a 

 basin in which water is not constantly renewed, the extraction of oxygen plus 

 the release of reduced substances lowers the Eh. From the work of Baas Becking, 

 Kaplan and ]\Ioore (1960) it appears that the removal of oxygen alone does not 

 lower the Eh. The most effective poising agent is hydrogen sulfide, or more 

 correctly the sulfhydril ion (HS~), Highly oxidized waters have an Eh of +500 

 mV, while stagnant hydrogen-sulfide-containing waters appear to have a mini- 

 mum of about -200 mV (Strakhov, 1959). This corresponds to the S04-~-HS~ 

 couple. Basin sediments on the other hand generally show lower Eh values 

 (Emery and Rittenberg, 1952; Rittenberg, Emery and Orr, 1955; Baas Becking, 

 Ka])lan and Moore, 1960); it is seldom that a potential exceeds + 300 mV under 

 maximal oxidizing conditions, and reducing values as low as — 400 niV have 

 been reached. This approaches the lower limit for the stability of water. 



In sea-water the pH generally remains within the range 7,5 to 8,5; the water 

 is buffered by the carbonate-bicarbonate system. Under conditions of extreme 

 evaporation, addition of fresh water or high biological activity, a greater 

 spread may result. The sediments of true marine basins also have a narrow pH 

 range. There is usually no marked trend, although, in general, a slight increase of 

 pH occurs with depth. There is often a change in pH between the sediment and 

 the overlying water. The direction of this change is unpredictable and usually 

 slight. In some basins there is a definite decrease in the pH of the sediment 

 relative to the water, e.g. Milford Sound, New Zealand. This may be due to 

 the release of CO 2 by biological activity and to a cation exchange process in 

 which the alkali metals are preferentially adsorbed. 



C. Subsurface Waters of Ancient Basins 



Subsurface waters of ancient marine basins are thought to be remnants of 

 sea- water trapped within the sediments at the time of deposition. It is, therefore, 

 of some interest to examine the ionic composition of connate waters in order to 

 gain a greater understanding of the processes causing diagenetic changes in the 

 interstitial waters of freshly deposited marine sediments. The recent review by 

 Chave ( 1 960) draws adequate attention to the pitfalls involved in accepting all 

 published data without critical evaluation. It may be added that because of the 

 heterogeneous character of sediments, even analyses on freshly deposited 

 material are often subject to insurmountable error. 



From the many thousand published results, Chave (1960) draws some con- 

 clusions on the chemistry of the ancient waters. The predominant ion is chloride. 

 There is always an increase of Cl~ with respect to sea- water and the chlorinity 



