Rate of Deposition 



249 



similarity between contents of green por- 

 phyrin pigments in sediment at the top of 

 the piston core and at the top of a small open 

 gravity core used as a tripping weight for the 

 large piston core device. Since water content 

 and especially porphyrin content decrease 

 rapidly with depth in the sediment, failure to 

 core the topmost layers would have resulted 

 in great differences between the piston and 

 the simple gravity cores, which always con- 

 tain surface layers. As soon as the cores 

 were brought aboard ship, they were ex- 

 tracted from the core tubes, split, described, 

 and cut into about fifteen sections which 

 were sealed in glass jars. Measurements of 

 pH and Eh were made within a few hours. 

 On return to the laboratory, the bulk of the 

 bottommost section and commonly of the 

 surface section of each core was sent away 

 for carbon- 14 age determination. Sections 

 were chosen so as to avoid sandy turbidity 

 current layers. All sections were analyzed 

 in the laboratory for their percentage con- 

 tents of water, nitrogen, and calcium carbo- 

 nate (by gasometric analysis). 



Radiocarbon age determinations were 

 made in New Zealand by Dr. T. A. Rafter, 

 whose radiation-counting technique is based 

 on carbon dioxide referred to wood (Rafter, 

 1953, \955a, 1955Z); Fergusson, 1955). The 

 results of 37 age determinations are shown 

 in Table 19, along with measurements of the 

 contents of nitrogen and organic matter 

 (17 X nitrogen) near the surface (Orr, Emery, 

 and Grady, 1958) and composite calcium 

 carbonate for each core. Unfortunately, not 

 all the 37 age determinations could be used 

 for direct measurement of rate of deposition. 

 Three separate problems basic to the work 

 had to be investigated first, and half the 

 analyses were used for these problems. 



The first problem was that of whether the 

 ages should be based on carbon from organic 

 matter or carbon from calcium carbonate. 

 Pairs of organic carbon and carbonate ages 

 were determined for eleven core sections at 

 various depths in different basins. The re- 

 sults (Table 19) show close agreement be- 

 tween the two ages only for Santa Barbara, 

 San Clemente, and East Cortes Basins. In 

 Santa Monica, San Diego, Santa Catalina, 



and San Nicolas Basins the carbonate age 

 exceeds the organic carbon age by an aver- 

 age of 25 per cent. Thus, except for Santa 

 Barbara Basin, the greatest differences occur 

 in the nearshore basins. The differences are 

 beheved to result from erosion of old calcium 

 carbonate from shallow-water areas and re- 

 deposition of it on the basin floors. Rework- 

 ing can of course occur in any basin at any 

 depth, but it is probably most frequent in the 

 nearshore basins which have steep side 

 slopes and deeply indented submarine can- 

 yons. In addition, it wiU be recalled (Table 

 15, Figs. 188, 189) that the sediments of the 

 nearshore basins (again except for Santa 

 Barbara Basin) have a concentration of 

 calcium carbonate in the finer grain sizes, 

 which are similar to the sizes of the grains of 

 detrital sediment. This contrasts with a 

 concentration of calcium carbonate in the 

 coarser sizes of sediments in offshore basins 

 where it is mostly in the form of recent 

 foraminiferal tests. Thus the ages support 

 the grain-size data by indicating the presence 

 of more reworking of calcium carbonate in 

 nearshore basins than in offshore ones. 

 Similar reworking was inferred by Rubin 

 and Suess (1955) and Ericson, Broecker, 

 Kulp, and Wollin (1956) from the greater 

 age of fine-grained (<74 microns) than of 

 coarse-grained calcium carbonate in cores 

 from the Caribbean Sea. Santa Barbara 

 Basin is somewhat unusual because the 

 organic carbon near the surface is of greater 

 age than the carbonate. There, reworked 

 calcium carbonate may be minor because 

 the side slopes are gentle, no large sub- 

 marine canyons enter the basin, and the 

 shelf along the mainland to the north con- 

 tains little calcium carbonate. Conceivably 

 some reworked organic carbon is present in 

 the form of bits of tar carried by currents 

 from oil seeps near Point Conception; how- 

 ever, since the age discrepancy is not great, 

 it probably can be neglected in the problem 

 of rate of deposition. The organic carbon 

 dates appear to be far less subject to error 

 from reworking than the carbonate ones, so 

 all subsequent age determinations are based 

 on carbon from organic matter. 



The second problem is that of the zero 



