264 



Sediments 



value is about five orders of magnitude 

 smaller than the direct measurements of per- 

 meability, it is obvious that permeability 

 does not Hmit the expulsion of water during 

 compaction and that compaction must in- 

 stead be limited by the characteristics of the 

 solid phase of the sediment. The most rea- 

 sonable of these characteristics is the resist- 

 ance of the grains toward repacking and 

 deformation, both of which are controlled 

 by pressure transmitted from grain to grain 

 and thus by weight of overburden or depth 

 of burial. 



Under steady-state conditions last year's 

 layer originally had the same water content 

 as this year's layer, and next year it will have 

 the same content as now held by the layer 

 deposited the year before last. Thus, every 

 year each layer discharges its water to the 

 overlying layer and receives the water that 

 was held by the layer under it. If the sedi- 

 ment at some depth were compacted to zero 

 water content, the same water would move 

 endlessly upward in the sediment column, 

 and the total amount of water which passed 

 through any given annual layer would equal 

 the total volume of water above that layer 

 and to the left of the porosity-depth curve 

 (Fig. 213C). This volume is shown for all 

 depths by Figure 213H, from which we can 

 see that during the time since the layer now 

 at 500-cm depth was deposited, about 3500 

 years ago, a total of 385 ml of water has 

 passed through every square centimeter of 

 its area. Using an extension of the porosity- 

 depth curve to a depth of 1500 meters, we 

 learn that about 32 liters of water would 

 have passed through each square centimeter 

 of a layer deposited about 3,800,000 years 

 ago. It is not unreasonable to suppose that 

 such a vast flushing of water through the 

 sediment may have caused some chemical 

 alteration of it. 



From this view of the process of compac- 

 tion we can recognize that if the sediments 

 at some great depth have a zero water con- 

 tent, there need be no escape of water from 

 the surface and no entrance of new water. 

 In reality, however, even at great depth the 

 sediment retains some water (in the Los 

 Angeles Basin, about 5 per cent by volume). 



As a result of this loss from the bottom of 

 the steady-state curve of water content ver- 

 sus depth there must be a continuous addi- 

 tion of some new water at the top of the 

 sediment column. The annual amount of 

 new water could be as small as the amount 

 of water at final compaction contained in an 

 annual layer, about 0.0017 ml. To trace a 

 given average molecule of interstitial water 

 through space, we can visualize it as rising 

 upward with respect to sediment layers, 

 owing to its expulsion from the layers by 

 compaction, but as slowly sinking with re- 

 spect to the rising sediment surface. In other 

 words, none of the interstitial water need 

 ever escape into the overlying water. In 

 reality, however, some of the water does 

 escape — through the stirring activities of 

 burrowing organisms, through channeling, 

 through following along sandy, more per- 

 meable, layers that crop out. In Santa Bar- 

 bara Basin neither burrowing organisms nor 

 sandy layers nor channels have been dis- 

 covered in cores. According to Kullenberg 

 (1952) the coefficient of diffusion of salts in 

 sediments of the Baltic Sea is about 20 per 

 cent of that in the overlying water, about 

 2 X 10~*^ sq cm/sec. Although the coeffi- 

 cient of diffusion for interstitial water is not 

 known for the basin sediments, the existence 

 of differences in concentration of some salts 

 between interstitial water and overlying wa- 

 ter must cause movement of the salts across 

 the sediment-water interface by diffusion, 

 even if there is no bodily movement of the 

 water itself. 



pH and Eh 



The symbol pH stands for the negative 

 logarithm to the base 10 of the hydrogen ion 

 concentration in moles per liter, being to 

 7 for acids and 7 to 14 for bases. The oxi- 

 dation-reduction potential Eh is a measure 

 of the tendency of the system to accept or 

 give up electrons relative to the standard 

 hydrogen electrode, being either positive or 

 negative. Both pH and Eh were measured 

 in sediments (usually slurried with freshly 

 boiled distilled water) by means of a Beck- 

 man pH meter. For pH a calomel and a 



