Rate of Deposition 



251 



age of the sediments, or the radiocarbon age 

 of the organic matter now being deposited 

 at the sediment-water interface. Six organic 

 carbon age determinations were made of 

 the surface samples (Table 19), but since a 

 length of core amounting to 10 to 15 cm 

 was needed in order to obtain enough carbon 

 for age measurement, the age obtained was 

 for a composite sample centered several 

 centimeters below the sediment-water inter- 

 face. Surface ages were computed by extra- 

 polation upward from this depth according 

 to the age of a sample near the bottom of a 

 core. The extrapolated ages of surface sed- 

 iments are as follows: 



Santa Barbara Basin 

 San Pedro Basin 

 Santa Catalina Basin 

 San Nicolas Basin 

 East Cortes Basin 

 West Cortes Basin 



2900 years 

 2200 years 

 1800 years 

 2400 years 

 2800 years 

 4200 years 



The age for West Cortes Basin is much 

 greater than those for the other basins, and 

 it will not be further considered except for 

 that particular basin. The remaining sur- 

 face ages average 2400 years. Radiocarbon 

 dates at various depths in eight suitable 

 cores from the deep-sea floor of the Atlantic 

 Ocean are given by Kulp, Feely, and Tryon 

 (1951), Suess (1954, 1956), Ruben and Suess 

 (1955), and Broecker and Kulp (1957). 

 Extrapolation to the sediment surface 

 yielded zero ages of 1000 to 5800 years, 

 with an average of 2900 years, not much 

 different from the average for the basin sed- 

 iments off southern California. 



The great age of the surface sediment in 

 the basins off southern California cannot be 

 ascribed to a lack of deposition during the 

 past 2400 years, for the surface sediments 

 are not different from those at depth. Great 

 radiocarbon ages for surface sediment have 

 sometimes been considered to result from 

 mixing of sediments by burrowing organ- 

 isms, but it is evident that mixing would 

 have to occur to depths of several meters to 

 make the surface sediments appear to be so 

 old as 2400 years. In Santa Barbara Basin 



the presence of many thin, probably annual, 

 laminae testifies to the absence of mixing, 

 as does the fact that benthonic animals have 

 not been found in samples and photographs 

 of this basin floor. Some other mechanism 

 is responsible for the great apparent age of 

 the surface sediment. Evidently the age is 

 in some way related to the nature of the 

 organic matter which falls onto the floor of 

 the basin. The time required for organic 

 matter to reach the bottom from the euphotic 

 zone must be a matter of not more than a 

 few months, certainly not 2400 years. 



Measurements of the radiocarbon ages of 

 six water samples from depths in the Atlantic 

 Ocean greater than 3000 meters yielded 

 values between 450 and 1950 years accord- 

 ing to Kulp (1953), but these values are now 

 considered erroneous (Ewing and Gerard, 

 1956). Recent measurements have shown a 

 complex pattern of apparent ages (Broecker, 

 Ewing, Gerard, and Heezen, in press) that 

 is related to the degree of equilibrium 

 reached by carbon dioxide between atmos- 

 phere and ocean when the water was last at 

 the surface. Although measurements off 

 southern California are lacking, analogy 

 with similar areas of the Atlantic Ocean in- 

 dicates that the radiocarbon age of the 

 water must be only a few hundred years at 

 most. Average surface water is considered 

 by Craig (1953, 1957) to have a radiocarbon 

 age of about 400 years on the basis of 

 equilibrium values of carbon 14 suggested 

 by C^VC^- ratios in surface water. In areas 

 of upwelling, of course, the age of the water 

 may be greater than that at the surface else- 

 where, but since the depth of upwelling is 

 only a few hundred meters, the increased 

 age should not be as great as the 2400 years 

 in question. Except for possible biological 

 fractionation of carbon 14 with respect to 

 carbon 12, the age of organic matter in 

 phytoplankton should be the same as the 

 age of the surface water in which it grows. 

 Additional fractionation may occur through 

 the activity of animals and bacteria which 

 repeatedly attack the original tissue of 

 phytoplankton during its fall through the 

 water column and subsequent exposure on 



