224 



Sediments 



core depths ranging throughout most of the 

 Pleistocene; he attributed this relationship to 

 a high rate of production during times of 

 temperature minima. Foraminiferal studies 

 by Ericson and Wolhn (\956b), on the other 

 hand, showed that in the Atlantic a change 

 from low to high percentages of calcium car- 

 bonate corresponds to a change from cold- 

 to warm-water species. Verification was 

 given by oxygen isotopes as reported by 

 Emiliani (1955). Radiocarbon dates (Eric- 

 son, Broecker, Kulp, and WoUin, 1956) 

 showed that the change from low to high 

 carbonate content occurred about 11,000 

 years ago. Using these and additional radio- 

 carbon dates, Broecker, Turekian, and Hee- 

 zen (1958) proved that, although the per- 

 centage of calcium carbonate in an Atlantic 

 deep-sea core was less prior to 1 1,000 years 

 ago than afterward, the absolute rate of de- 

 position was more than twice as great prior 

 to 1 1,000 years ago than later. Clearly, the 

 problem is a complex one. 



Changes in sediments taking place more 

 than about 15,000 years ago cannot be in- 

 ferred from many of the cores off southern 

 Cahfornia because few of them are long 

 enough to reach so old a date. Only three 

 (West Cortes, No Name, and the continental 

 slope) are much older than that, but all these 

 cores show an increase of calcium carbonate 

 at about 10,000 to 15,000 years ago, perhaps 

 similar to that exhibited by some deep-sea 

 cores. On the other hand, sediments were 

 deposited in the basins off southern Califor- 

 nia much faster than were those of the deep- 

 sea floor, so trends exhibited by them yield 

 detailed data for the past few thousand years 

 that cannot be obtained from the very thin 

 layer of sediment deposited on the deep-sea 

 floor during the same period. The change at 

 about 3000 years, interpreted in the light of 

 studies of deep-sea sediments, may suggest 

 that the oceanic temperature increased 3000 

 years ago and has remained high since then. 

 However, other factors that may well have 

 influenced the rate of deposition of calcium 

 carbonate are (1) the blocking of the circu- 

 lation pattern of surface water by exposure 

 of the Santa Rosa-Cortes Ridge when sea 

 level was lowered during glacial stages (and 



reduction of upwelling and thus of produc- 

 tion of plankton), (2) the reworking of cal- 

 careous sediments on the then exposed 

 shelves and bank tops, and (3) the great 

 restriction in total area of shallow-water 

 sites for population by the highly calcareous 

 mollusks during glacial stages. Owing to the 

 complexity of the problem and the insuffi- 

 ciency of existing data, further speculation 

 on the cause of the decrease of calcium car- 

 bonate with depth in cores of basin sedi- 

 ments is considered too hazardous to be 

 justified. 



D. Depth distribution of grain size. As 

 soon as long cores began to be collected, 

 sand layers were noted interbedded between 

 greater or lesser thicknesses of the normal 

 green mud of the basins (Revelle and Shep- 

 ard, 1939). Except for these layers the muds 

 are nearly uniform throughout the length of 

 cores. The positions of cores taken up to 

 1950 that showed the layers were compiled 

 by Shepard (\95\b); subsequent work re- 

 vealed the sand layers in more than 150 new 

 cores, mostly from the San Pedro and Santa 

 Monica Basins. Sand layers have been 

 found now in cores from all except Santa 

 Barbara Basin where their place is taken by 

 gray silt layers. 



Some of the coarse-grained layers are 

 thick enough to provide good echoes from 

 below the mud-water interface. A traverse 

 across the San Diego Trough made by a 

 Scripps Institution of Oceanography ship 

 using a Precision Depth Recorder clearly 

 reveals the presence of two sound-reflecting 

 layers, doubtlessly sandy. Both layers are 

 covered by later sediments to a greater 

 depth nearshore than offshore (Table 16), as 

 a result of faster deposition of mud atop the 

 sand layers on the mainland side of the San 

 Diego Trough than farther offshore. Two 

 somewhat less distinct layers are also shown 

 in a sounding traverse across the north end 

 of San Clemente Basin at about the same 

 depths below the mud-water interface. 



Detailed analyses of cores from nearly all 

 basins (Emery and Rittenberg, 1952; Orr, 

 Emery, and Grady, 1958) reveal that each 

 sand or silt layer has a lower content of 

 water and of organic matter (expressed either 



