258 



percentage composition of all sediments de- 

 posited annually is as follows: 



Detrital 



Calcium carbonate 



Organic matter (at depth) 



Total 



84.2 per cent 

 13.2 

 2.6 



100.0 per cent 



Interstitial Water 



Percentage 



Usually water content is expressed as per- 

 centage dry weight of sediment, paralleling 

 the expression of concentration of other 

 chemical constituents. Values in excess of 

 100 per cent are common, so for some pur- 

 poses water content may be preferred in 

 terms of percentage of wet weight of sedi- 

 ment. Conversions are simple: W = 

 100Z)/(100 + D), otD = 100Pr/(100 - W), 

 where W is per cent water by wet weight 

 and D is percent water by dry weight. 



The simplest and most accurate method 

 of measuring the water content of fresh sed- 

 iment samples is to find the weight loss on 

 drying to 110°C. An alternate method that 

 can be used for samples which have become 

 partially dried before analysis is that of ex- 

 tracting the chloride ion with hot distilled 

 water and titrating it with silver nitrate. 

 Tests of this method made on five fresh 

 cores showed that the chlorinity of the in- 

 terstitial water averaged 19.3%o (Emery and 

 Rittenberg, 1952), nearly the same as the 

 chlorinity of the overlying basin water, 

 19.1%o. Evidently, changes in the chloride 

 content of interstitial water are absent or 

 minor during diagenesis. 



The top 5 to 20 mm of most cores of fine- 

 grained sediments exhibits a thin soupy 

 character. This material flows so readily 

 that it sometimes is referred to as the mobile 

 layer. Its mobihty makes proper sampling 

 difficult, and frequently it is entirely lost 

 from core samples. The water content in 

 samples of the top 5 cm from twelve typical 

 cores ranged between 180 and 670, with a 

 mean of 230 per cent by dry weight. Thus 

 water at the mean value has a weight more 

 than twice that of the sediment grains and a 

 volume about six times as great. 



Sediments 



At depth the water content decreases 

 markedly just below the surface and more 

 gradually farther down. Little similarity, 

 however, is exhibited by water-depth curves 

 for sediments of diff^erent areas (Fig. 209). 

 For simplicity the curves of this figure omit 

 the irregularities produced by sand layers, 

 which consistently have less water than over- 

 lying and underlying clays and silts, as il- 

 lustrated by Figures 192 and 224, as well as 

 by many others presented by Emery and 

 Rittenberg (1952) and Orr, Emery, and 

 Grady (1958). Quite clearly, water con- 

 tent is inversely related to grain size. How- 

 ever, comparison of the curves given in Fig- 

 ure 209 shows that at a depth of 3 meters, 

 well below the surface region of rapid 

 change, the water content must also be con- 

 trolled by the rate of deposition, varying di- 

 rectly with it. For example, even though 



Figure 209. Typical curves of water content versus depth 

 in basin sediments. For comparison a curve from the 

 very rapidly deposited sediment of Lake Mead near 

 Hoover Dam is included (Gould, 1954). Note the sharp 

 decrease of water content near the sediment surface and 

 the fact that the water content at depth is a function 

 of grain size, rate of deposition, and probably of other 

 factors. 



