246 



Sediments 



and bank tops. Sediments of the basins 

 (and also marshes) accumulate in quiet 

 water, and accordingly they mostly are fine- 

 grained. A decrease of grain size occurs 

 from the now-filled Los Angeles Basin to 

 existing basins halfway across the continen- 

 tal borderland in response to the size control 

 exerted by diffusion and to the restriction of 

 important turbidity currents to nearshore 

 basins. The far-offshore basins and the con- 

 tinental slope have somewhat coarser sedi- 

 ments owing to a relatively high percentage 

 of coarse foraminiferal tests, which nearer 

 shore were masked or diluted by the more 

 rapidly deposited detrital components. On 

 the deep-sea floor the grain size falls to its 

 lowest values because of great distance from 

 shore and nearly complete dissolution of the 

 calcareous component. 



Reference to the curves for calcium car- 

 bonate (Fig. 203) shows the relationship 

 required by the variation of grain size: A 

 steep and continuous increase of percentage 

 of calcium carbonate is presented by the 

 shallow areas because access to them by 

 coarse-grained detrital sediments is less than 

 that by shell debris. Probably the low cal- 

 cium carbonate content of beaches is due 

 also to the fact that the resistance of calcite 

 to the intense abrasion there is less than that 

 of quartz and feldspar. The content of cal- 

 cium carbonate in fine-grained basin sedi- 

 ments also increases seaward until a peak of 

 abundance is reached in offshore basins and 

 on the continental slope. Inshore of these 

 areas the rate of deposition of detrital sedi- 

 ments is so great as to dilute the calcium 

 carbonate, and offshore of them the rate is 

 so slow as to permit the calcareous material 

 to be exposed long enough for it to be dis- 

 solved in the bottom water, which is more 

 corrosive at these depths than at shallower 

 depths anyway. 



Organic matter is of low percentage in 

 coarse-grained sediments for several reasons, 

 one of which is that it is easily winnowed 

 away. Moreover, the low rate of deposition 

 on shelves and bank tops permits nearly 

 complete oxidation of organic matter. Only 

 in the basin sediments do physical processes 

 permit it to remain, but in nearshore basins 



the rapidly deposited detrital sediments 

 greatly dilute organic matter, and far from 

 shore the slowly deposited detrital sediments 

 fail to bury it before it becomes oxidized. 

 Comparison of the curves for calcium car- 

 bonate and organic matter in the basin sedi- 

 ments suggests immediately that organic 

 matter is more susceptible to solution or 

 chemical regeneration than is calcium carbo- 

 nate. Even more susceptible are pheophytin 

 and hydrocarbons, minor components of the 

 organic matter, as revealed by the relative 

 positions of the points of highest concentra- 

 tion of these materials (Fig. 203). Obviously, 

 the optimum concentrations of all four or- 

 ganic substances are also dependent on the 

 positions of areas of greatest productivity for 

 all overlying water, yet although the area of 

 greatest productivity for all should be iden- 

 tical or nearly so, their curves of abundance 

 in bottom sediments differ markedly. De- 

 tails regarding pheophytin and hydrocarbons 

 are reserved to a later section. 



If may be profitable to think of the sedi- 

 ments in terms made familiar by economists, 

 supply and demand. The greatest supply of 

 detrital sediments is the shore. The supply 

 of organic sediments is more involved. For 

 pelagic organisms the supply is related to 

 productivity of overlying waters, which is 

 greatest in areas of upwelling, for example 

 (Fig. 86). Benthic production of organic 

 matter occurs only where the bottom is shal- 

 low enough to be in the euphotic zone, but 

 benthic production of calcium carbonate 

 may also occur much deeper, except in ba- 

 sins that because of low oxygen content are 

 nearly barren of life. Demand for sediments . 

 is also complex. Mechanical demand is of 

 course greatest where there is the most tur- 

 bulence, in shallow water, and it may be 

 nearly nonexistent at the bottoms of basins. 

 In all areas chemical demand causes some 

 regeneration, but this demand is minor rela- 

 tive to supply where the supply is very great 

 or where toxic conditions markedly reduce 

 both biological and abiological chemical de- 

 mands. The varying roles of supply and 

 demand factors are responsible for the curi- 

 ously contorted curves of grain size plotted 

 against calcium carbonate and organic mat- 



