SECT. 3] PELAGIC SEDIMENTS 711 



curve "Pleistocene minimum rate of dissolution (interglacial)", which pre- 

 supposes an increase under the equatorial productivity maximum due to the 

 effect of carbon dioxide produced by the respiration of bottom-living animals 

 that feed on the rain of organic detritus. 



The resultant rate of accumulation of calcium carbonate corresponds to the 

 ordinate difference between the two curves and constitutes the parameter 

 actually measured. At the intersection of the two curves near 9°N the carbonate 

 accumulation accordingly drops to zero. Siliceous clay sediments cover the 

 bottom of the ocean north of the present loci of this intersection, the carbonate 

 compensation line. 



Emiliani (1955) has used the calcium carbonate tests of both benthonic and 

 pelagic Foraminifera for paleotemperature measurements in this section. The 

 planktonic results for the j)resent sediment surface are indicated at the present- 

 time production curve in the middle diagram of Fig. 36. The benthonic tests 

 indicate a temperature of about 1°C, in good agreement with the directly 

 measured temperature of the bottom water. The planktonic tests display a 

 gradient, with temperature rising from the equator northward (14.8°C at the 

 equator, 15.3°C at 2°N and 15.6°C at 7°N). This trend agrees with the observed 

 temperature distribution in the surface layer shown in the top graph. i 



The lower diagram in Fig. 36 shows a generalized cross-section through the 

 bottom deposits. The ordinate for the present sediment surface is taken as 0. 

 The lower detached part of the graph, showing the stratification of the Middle 

 Tertiary sediments, lacks detail because of the paucity of observations. The 

 vertical scale is in this latter case relative, and reference to the zero level is not 

 available. 



The distribution of carbonate at the present sediment surface is a function 

 of the productivity distribution in the surface layer of the ocean with the 

 superimposed effect of dissolution. From high values below the Equatorial 

 Divergence, the concentration drops toward higher latitudes, and the present 

 northern carbonate compensation line is found near latitude 9°N. 



The profile further demonstrates the marked stratification of the calcareous 

 deposits. Below the surface layer, which is characterized by relatively low 

 carbonate concentrations, lies a stratum (2) subdivided into units 2.2, 2.3 and 

 2.4, high in carbonate and coinciding with the last glaciation in higher latitudes. 

 This stratum is preceded by another low carbonate unit (3) with a small 

 maximum, 3.2, between the minima 3.1 and 3.3.2 Below this is a high-carbonate 

 stratum (4) indicating extraordinarily high rates of accumulation of biogenous 

 carbonate and silica. Altogether about nine major carbonate maxima occur in 

 the Pleistocene part of the sequence. The underlying Pliocene sediments are 



1 For no known reason, the magnitudes of temperature derived from the oxygen 

 isotope ratio fall considerably below the temperature range in the top 50-m layer of the 

 ocean, where the meridional gradient occurs, and which is the main habitat of the Fora- 

 minifera. 



2 Not all the details of the stratification are included in this generalized figure, but they 

 can be seen in Figs. 38 and 39. 



