Rate of Deposition 



253 



Figure 206. Radiocarbon ages at several depths in sedi- 

 ments of Santa Barbara Basin. Most ages were deter- 

 mined by T. A. Rafter (Table 19) with depths corrected for 

 turbidity current silts. Curve for carbonate includes three 

 dates measured by Magnolia Petroleum Company on 

 simple gravity cores. Note the uniformity of rate of 

 deposition for the top 3 meters. No correction was made 

 for effects of compaction. 



certainty introduced by use of both fora- 

 miniferal and radiocarbon dating, however, 

 the following discussion on rates of deposi- 

 tion will be based only on radiocarbon 

 measurements. 



Computation 



Compaction of the normal basin muds at 

 depth and their interruption by occasional 

 rapidly deposited sand layers introduced 

 minor complications in the computations of 

 rate of deposition. The object was to de- 

 termine the total weight of all normally de- 

 posited (grain by grain) muds in a 1 sq cm 

 column between the sediment surface and 

 the depth of the bottommost radiocarbon 

 date. Use of weight per unit area (mg/sq 



cm/yr) avoids the uncertainties of compac- 

 tion inherent in the use of thickness of an- 

 nual layer as an expression of rate of 

 deposition. 



Examination of the field descriptions and 

 of grain-size analyses permitted recognition 

 of the turbidity current sands and silts and 

 their rejection from the following computa- 

 tions. The remaining sediments exhibit a 

 downward decrease of water content and, 

 by inspection of the curve of water content 

 versus depth, the average percentage water 

 content was determined for sections of core 

 ranging from 10 to 75 cm long. After apply- 

 ing a 0.25 per cent correction for sea salts 

 left in the sediment during drying at 110°C, 

 the percentage volume of solid grains in 

 each core section was computed, using an 

 average measured grain density of 2.65. 

 These figures are readily converted to abso- 

 lute volumes in 1 sq cm columns of the same 

 length as each core section. By totaling the 

 volumes of all sections between the sediment- 

 water interface and the depth of the deep- 

 est radiocarbon age determination, the total 

 volume of sediment was obtained. When this 

 was converted to total weight of solid grains 

 in a 1 sq cm column and divided by the dif- 

 ference between the deepest radiocarbon 

 age and the extrapolated age at the surface, 

 the weight of solid grains deposited on each 

 square centimeter per year was found. 



The main constituents of the total sedi- 

 ment are detrital grains, calcium carbonate, 

 and organic matter. Composite values of 

 calcium carbonate and surface values for 

 organic matter were measured for the same 

 cores used for age determinations (Table 19), 

 and these percentages multiplied by the 

 weight of total sediment deposited each year 

 yielded the annual weight per unit area of 

 calcium carbonate and of organic matter. 

 The remainder is the weight of normal detrital 

 sediments. It wiU be remembered that sand 

 layers were omitted from the computations; 

 these were then reintroduced and their 

 average rate of accumulation computed ac- 

 cording to the relative weights of total 

 normal sediment and of sandy layers. The 

 over-all average weight of sediment depos- 



