130 



HISTORY OF THE OCEANS 





Fig. 1. Structural features of the Pacific Ocean (Menard and Fisher, 1958). 



marks the position of the geographic equator on the surface of the 

 sea, and the rain of organic remains which fall down on the ocean 

 floor below makes a permanent record of the position of the 

 equator at any given time. 



The nature of this record is indicated in the lower graph of 

 Fig. 2, a generalized meridional cross section through the top 5-m 

 layer of the sediments, at about 125° \V Long. Owing to the 

 strongly increased production of calcareous and siliceous plankton, 

 responsible for the bulk of the biogenous contribution to the 

 sediment, the sedimentary strata attain a maximum thickness 

 below the equator. It is obvious that we here have a means for 

 determining its position in the past, inasmuch as any displacement 

 of the geographic equator should be traceable as a corresponding 

 displacement of the thickness maximum of the strata. Provided 

 the position of the equator is determined in two places, preferably 

 about ninety degrees apart, the position of the poles is also fully 

 determined. Other parameters, which are still more useful for this 

 purpose because of their larger gradients toward the equator, are 

 the rates of accumulation of fossils, minerals, or chemical elements 



