Basins and Troughs 



223 



from most of the basins exhibit a decrease 

 of calcium carbonate with depth. If this 

 trend reflects changed conditions of deposi- 

 tion during the past, the date of change can 

 be estimated from a plot of calcium carbon- 

 ate against time instead of depth. Such a 

 plot allows for the diff'erent rates of deposi- 

 tion of sediments in different basins. As will 

 be discussed more completely in the section 

 on rate of deposition, radiocarbon dates at the 

 bottoms of cores from each basin and from 

 the tops of some cores have been made by 

 Dr. T. A. Rafter of New Zealand as part of 

 a University of Southern California project 

 financed by the National Science Founda- 

 tion. Knowing the date at which the sedi- 

 ment at the bottom of a reasonably uniform 

 core was deposited and the fact that similar 

 sediment still is being deposited, we can 

 make a first approximation of the date at 

 which any part of the core was deposited by 

 assuming a uniform weight of new sediment 

 added each year per unit area. Obviously 

 this date is subject to error if the rate of de- 

 position was not uniform throughout the 

 length of a core. Use of weight instead of 

 thickness corrects for the normal compaction 

 or decrease in water content with depth. Oc- 

 casional layers of sand deposited rapidly by 

 turbidity currents and containing abnormally 



high or low percentages of calcium carbonate 

 must be eliminated from consideration. 



The change of calcium carbonate concen- 

 tration with time is shown by Figure 191, 

 constructed as outhned in the previous para- 

 graph. Most of the cores exhibit a definite 

 decrease of calcium carbonate with depth 

 earlier than about 3000 years ago. Two 

 cores (Santa Barbara and Santa Monica) 

 show no change because sediments at their 

 bottoms were deposited only 2000 years ago. 

 One (San Pedro) shows no change, and two 

 (Santa Cruz, and west end of Santa Cata- 

 lina) show an increase with depth. The 

 curves, plus the data of Table 15, appear to 

 be sufficient to indicate that calcium car- 

 bonate exhibits a sharp decrease at depths 

 corresponding to a date 3000 years before 

 the present. 



Related studies of cores from the floor of 

 the deep sea have been made by various 

 workers. Hough ( 1 953) found an alternation 

 of layers of red clay and globigerina ooze 

 which he ascribed to alternations of glacial 

 and interglacial stages of the Pleistocene. 

 Arrhenius (1952), working with cores of the 

 Swedish Deep-Sea Expedition, believed that 

 the percentage of calcium carbonate in sedi- 

 ments of a large area of the deep-sea floor 

 is inversely related to the temperature for 



Figure 1 91 . Variation of cal- 

 cium carbonate in cores from 

 several basins during past time. 

 Data include corrections for 

 radiocarbon zero age, core 

 shortening, sand layers, and 

 variation in water content of 

 sediment with depth, as dis- 

 cussed in text. Calcium car- 

 bonate was measured from 

 volumes of carbon dioxide 

 produced by treatment of sedi- 

 ment with acid. Aged sections 

 are indicated by short horizon- 

 tal lines. Sample sites are given 

 in Figure 205 and Table 20. 



PERCENTAGE OF CaCO, 



20 25 30 35 ' 



