252 



Sediments 



the bottom before burial.* It is also possible 

 that selective oxidation of organic matter 

 takes place whereby components having 

 high C^VC'- ratios are preferentially lost 

 from the sediments, giving the remaining 

 organic matter an apparently greater age 

 than originally present. Since we really do 

 not know why the organic matter now being 

 deposited has so great a radiocarbon age, 

 the safest way of using the age information 

 for determining rate of deposition is prob- 

 ably by taking the lapsed time for accumu- 

 lation as the difference between the radio- 

 carbon ages at the bottoms and tops of the 

 cores. 



The third problem is the question of uni- 

 formity of rate of deposition throughout the 

 length of cores. A simple way of deter- 

 mining this is by measuring the ages at sev- 

 eral depths and learning how closely they 

 correspond to a straight-line function of 

 time against depth. The most complete 

 data for this problem come from Santa 

 Barbara Basin where eight ages are avail- 

 able, distributed between organic carbon 

 and calcium carbonate source materials. 

 The data (Fig. 206) show reasonably good 

 straight-line relationships for both organic 

 carbon and carbonate to a depth of about 

 3 meters. Below that depth the sediment 



* After this book had reached the galley proof stage, 

 some additional carbon- 14 age determinations (on organic 

 carbon) by Rafter became available. Those that bear on 

 the question of zero age of the sediment are: 



1. Sample 6094: Dried deep-sea fish, genus Myatoplms, 

 obtained in trawl at 640 meters in San Pedro Basin where 

 bottom depth is 840 meters (lat. 33°28', long. 118° 17'). 

 Carbon- 14 age is Modern (probably less than 200 years). 



2. Sample 6034: Santa Catahna Basin at same site as 

 sample 4704 (Table 19); living benthos, mostly worms 

 and echinoids. 1670 ± 150 years. 



3. Sample 6034: Sediment scraped from top centimeter 

 of large orange-peel bucket sample. 2100 ± 300 years. 



4. Sample 5993: Santa Monica Basin (lat. 33°50.7', 

 long. 119° 13.6'. 825 meters); sediment scraped from top 

 centimeter of large orange-peel bucket sample. 1230 ± 

 150 years. 



These ages suggest that the great increase in apparent 

 age from Modern of water and nekton to between 1700 

 and 4200 years for surface sediments is somehow pro- 

 duced during the growth of benthonic animals or shortly 

 afterward. 



appears to have been deposited more slowly 

 than afterward. Additional ages for each of 

 the other basins were desirable but could 

 not be obtained with the funds that were 

 available. 



In the outer basins, where sediments have 

 accumulated less rapidly than in Santa 

 Barbara Basin, there is of course a greater 

 chance of irregularity in past rates of depo- 

 sition. Numerous radiocarbon dates made 

 for a core of deep-sea sediments in the 

 Atlantic (Broecker, Turekian, and Heezen, 

 1958) revealed a decrease in rate of total 

 sedimentation and especially of the clay 

 fraction beginning about 11,000 years ago. 

 This date also corresponds to a change from 

 cold- to warm- water species of foraminifers 

 (Ericson and WoUin, \956b). According to 

 studies by Bandy (in press), the foraminifer 

 Globigerina pachy derma exhibits a change in 

 coiling direction from left to right in the 

 cores of Table 19. This change appears to 

 have occurred about 1 1 ,000 years ago (from 

 comparison with the radiocarbon dates). 

 Thus the depth of change in coiling direc- 

 tion might be taken as a dated depth. Com- 

 putation of the rates of deposition before 

 and after 11,000 years ago based on these 

 measurements suggests that the rate after 

 1 1,000 years ago was greater in some basins 

 and less in others than the over-all rate 

 based only on radiocarbon measurements at 

 the bottoms of the cores. f Owing to the un- 



fTwo additional carbon-14 ages from intermediate 

 depths in cores became available after the book had 

 reached galley proof stage. These are: 



1. Sample 4671: 89 to 102 cm. 15,200 ± 500 years. 



2. Sample 4704: 280 to 292 cm. 15,600 ± 600 years. 

 After correction for the 2800-year zero age at the sediment 

 surface, sample 4671 is 12,400 years, unaccountably far 

 different from the 4100 years interpolated from data of 

 Table 19. The age (corrected by 1800 years) for sample 

 4704 is 13.800 years, a smaller departure from the 10,800 

 years interpolated from data of Table 19. Both samples 

 were selected to determine whether a change from right- 

 hand to left-hand coiling direction of Globigerina pachy- 

 derma corresponds to a date 1 1,000 years ago. Differ- 

 ences of measured ages from those inferred from straight 

 interpolation and from foraminiferal counts indicate 

 that still more age measurements are needed for a com- 

 plete understanding of rates of deposition of sediments 

 in the basins. 



