46 BROECKER 



the average, less than 10% calcium carbonate (Fig. 7). To go from 90% to 10%, if 

 the rate of influx of the dilutant, clay, iron, and manganese oxides, and opal is 

 constant, requires a 100-fold change in the rate of calcium carbonate 

 accumulation. So the change represents a 99% dissolution of the carbonate out 

 of this sediment. It is quite interesting that one core shows no evidence of disso- 

 lution, and another, 50 m deeper, shows very strong evidence of dissolution, so 

 that this boundary is, at least in this part of the ocean, remarkably sharp. Above 

 the compensation level the carbonate content of the cores is amazingly uniform 

 with depth. In fact, in one core, 60% of the material is calcium carbonate, and 

 40% is iron and manganese oxide that almost certainly comes out of the crest of 

 the mid-ocean rise. So we have a core in which 40% of the material is related 

 to vulcanism and 60% is related to productivity at the surface spanning 

 250,000 years. There is no change in this ratio. The carbonate content goes from 

 58% to 62% and just bounces around within those limits. This again is very 

 interesting because it says that either there is coupling between vulcanism 

 and productivity, which most people would say is madness, or much more likely 

 neither of these processes has undergone an appreciable change in rate. 

 Apparently these two processes are going on at roughly the same ratio right up 

 to the present. 



We will next discuss a core that has been subjected to intense dissolution. If 

 we go deep enough in such a core, we come back to a high carbonate sediment. 

 During the course of its history, each area went from above the CaCOs to below 

 the CaCC>3 compensation level. The great oceanic plates that bear these 

 sediments are not only moving away from the ridges at a rate of a few 

 centimeters per year, they are also getting deeper. They go from 3000 m at their 

 point of origin to about 6000 m at the trench into which they disappear at a rate 

 of about 30 m/million years. At some point the plate drops through the 

 compensation level, and calcium carbonate accumulation ceases. 



Cores from below the compensation level show funny carbonate tails at their 

 tops. We first postulated that they represented kinetic lag; the carbonate 

 takes an average of 15,000 years to dissolve. This seemed true until we did 

 radiocarbon dating and found that the top point in the piston core and in the 

 trigger weight core give an age of 11,000 years. This may indicate that, during 

 the last glacial period, there was a high influx of carbonate that did not dissolve 

 at this point in the ocean. Probably these high carbonate peaks indicate that 

 during the height of the last glaciation the compensation level dropped a couple 

 of hundred meters or so and permitted carbonate to accumulate in regions of the 

 ocean floor where it is not accumulating today. So we see from the dissolution 

 phenomena that there was a change in compensation level corresponding to 

 times of glaciation. 



1 "X 



Now for the last method, the method based on C. To review what was said 

 before, there is a difference between I3 C content of surface Pacific carbon and 

 deep Pacific carbon today of about 2 per mil (Table 6). This is reflected by the 



