Interstitial Water 



265 



glass electrode are used, and for Eh a calo- 

 mel and a platinum electrode. Details of the 

 measurements are given by Emery and Rit- 

 tenberg (1952). 



Values of /?H in the water column gener- 

 ally decrease from about 8.3 at the surface 

 to 7.7 at some depth between 400 and 1000 

 meters, below which it again rises to about 

 7.8 (Rittenberg, Emery, and Orr, 1955). Bot- 

 tom water collected just above the cores 

 averages 7.5, indicative of a pW minimum 

 just above the sediment- water interface. 

 Usually the pW of the topmost layer of sedi- 

 ment is slightly greater than that of the im- 

 mediately overlying water, averaging 7.6. At 

 depth in the sediment the /jH typically rises, 

 in many cores to values of 8.5, although in 

 others it is lower or irregular with some 

 obvious dependence on grain size (Fig. 192). 



Values of Eh in the water column are 

 invariably positive with values mostly be- 

 tween -1-200 and +300 millivolts. The sur- 

 face of the sediments of all except parts of 

 two basins (Santa Barbara and Tanner) is 

 also characterized by positive values. At 

 depth the Eh decreases sharply or gradually 

 to zero values reached at depths averaging 

 about 2 meters except in San Pedro and 

 Santa Monica Basins and the deep-sea floor, 

 where, except for rare instances in the first 

 two areas mentioned, the Eh is never nega- 

 tive within the depths reached by coring. 

 Below the depth of zero Eh values become 

 as low as — 100 mv and even —300 mv. 



The variations of /'H and Eh in the sedi- 

 ments are attributed to diagenetic changes 

 produced by both biogenic and abiogenic 

 agents. For example, low/7H's, to 7.5, can 

 be produced by carbon dioxide liberated 

 during oxidation of organic matter, particu- 

 larly of carbohydrates and fats. Oxidation 

 of protenaceous and fatty organic matter 

 forms ammonia and hydrogen sulfide, both 

 of which tend to raise the pW, the latter be- 

 cause a strong acid (sulfate ion) is changed 

 into a weak one. If the ammonia and hy- 

 drogen sulfide work their ways to near the 

 sediment-water interface, they may be fur- 

 ther oxidized to nitrate and sulfate, thus 

 lowering the/7H in that area. Because of the 

 complexity of the question of which prod- 



ucts are formed first in the sediment and of 

 how they become altered, it is useless to 

 speculate on the cause of changes of pYi at 

 depth at the present low state of factual 

 knowledge. 



The decrease of Eh at depth in the sedi- 

 ments is also a very complicated problem. 

 Probably the chief cause of the decrease is 

 the oxidation of organic matter. The easiest 

 available oxidizing agent is the free oxygen 

 dissolved in the interstitial water, but as soon 

 as this has been used up the chief remaining 

 source is oxygen in sulfate ions. This is used 

 by sulfate-reducing bacteria whose activity 

 is revealed by a decrease in the ratio of sul- 

 fate ion to chloride ion (Fig. 192), beginning 

 at the same depth as the first appearance of 

 hydrogen sulfide and of negative values of 

 Eh. Because Eh depends on the ratios of 

 the concentrations of the oxidized and re- 

 duced forms of many chemical systems in 

 the sediment, the state of equilibrium, tem- 

 perature, /'H, and other factors (ZoBell, 

 1946a), Httle use can be made of absolute 

 measured values; however, the depth of zero 

 Eh is so closely identified with the first ap- 

 pearance of hydrogen sulfide that it can 

 safely be taken as an indicator of the replace- 

 ment by hydrogen sulfide of dissolved oxy- 

 gen in the interstitial water, and thus of the 

 change from aerobic to anaerobic conditions, 

 even though it is impractical to determine 

 oxygen on water squeezed from cores. 

 Measurements of Eh on nearly 400 samples 

 from 30 cores show that more than 90 per 

 cent of the samples have either a positive Eh 

 with no hydrogen sulfide or a negative Eh 

 with hydrogen sulfide (Fig. 214). Most of 



Figure 214. Frequency distribution of £/? measurements 

 in 30 cores. Note the grouping into two modal ranges, 

 to +300 and - 100 to -300 mv and the close depend- 

 ence of hydrogen sulfide on negative Eh. 



