SECT. 3] PELAGIC SEDIMENTS 715 



much larger than the experimental error of the absolute age determination. 

 This statement is based on two facts. First, accumulation measurements have 

 demonstrated that within the area in question and during the Pleistocene time- 

 intervals studied, the rate varies only little, and apparently randomly from one 

 station to another (Arrhenius, 1952, fig. 1.1.6.1). This approximate spatial 

 constancy demonstrates that the depositional area is far enough removed from 

 the source area of the settling particles for the suspension, from which sedi- 

 mentation takes place, to have assumed a uniform character. The time that the 

 particles have spent in suspension must consequently be long compared to the 

 half -life of the suspension, which is controlled by coagulation or other precipita- 

 tion mechanisms. If these reactions, as is likely, are of second or higher order, 

 even large variation in the rate of influx of suspensoids in the ocean will have a 

 negligible effect on the terminal precipitation rate (Arrhenius, 1954). Only 

 small variations with time of the rate of accumulation of inorganic components 

 would consequently be expected in an area where the conditions mentioned 

 above are met, and as long as the global ratio of chemical to mechanical 

 weathering is not markedly changed. 



Second, the Tertiary clay sediments in the Pacific, as compared with the 

 Quaternary, clearly indicate less dilution of the halmeic and biotic components 

 (such as manganese oxide minerals and skeletal phosphate) with terrigenous 

 material, and a variation of this dilution with time (Fig. 40). The lack of such 

 marked variations in the Quaternary part of the sequences confirms the assump- 

 tion of a relatively steady inorganic accumulation rate during this time, 

 particularly when averaged over at least one climatic cycle. This conclusion 

 pertains to only one specific region and is not valid in some other areas in- 

 vestigated (cf. section from East Pacific Rise in left part of fig. 1.1.5, Arrhenius, 

 1952; Broecker et al., 1958). Local deviations from the average conditions, 

 attributed to topographic control, are discussed below (Section 5). 



The time scale obtained for the Upper and Middle Pleistocene events is 

 indicated in Fig. 38, together with the recent protactinium/ionium date. This 

 absolute age determination confirms the extrapolated age within the limits of 

 experimental error (95,000 ± 13,000 years for stage 3.1 by protactinium/ionium, 

 as compared to 90,000 + 6000 years by radiocarbon extrapolation). 



The Lower Pleistocene and Upper Pliocene strata, which are conformably 

 represented in a few investigated sequences, are beyond the reach of present 

 nuclear age determination methods. In this stratigraphic range two events 

 characterize the Tertiary-Quaternary boundary: a distinct evolutionary change 

 in the coccolithophorids (Bramlette, unpublished) and a marked drop in 

 the surface temperature of the ocean, as indicated by the carbonate paleo- 

 temperature (Emiliani, 1955). These events have been recorded in two sequences, 

 one near the equator at an extrapolated age level of 1.4 million years (core Sw 

 62), the other near the north equatorial carbonate compensation surface (Fig. 

 36), at 1,7 million years (core Sw 58). Because the first area is smoother, its 

 value is more probable; the second area might contain redistributed sediment 

 from a nearby seamount which causes a rate of accumulation of inorganic 



