CHEMISTRY IX THE OCEANS 589 



contribution of land runoff waters to these oceanic areas. Table 

 III gives the areas of the oceans and the respective areas of land 

 that are drained into the oceans. The Pacific receives but one-sixth 

 of the amount of drainage waters that enter the Atlantic on an 

 areal basis, which apparently accounts for the higher values of 

 thorium, normalized to ionium, in the Pacific sediments. Similarly, 

 the well-known observation that more and larger rivers drain into 

 northern hemispheric marine areas is reflected in the higher 

 ionium/thorium ratios in southern Pacific deposits as compared 

 to their northern counterparts. Thus, the concentration of thorium 

 in the deeper parts of the ocean is seen to have an ocean to ocean 

 variation, explainable, on the basis of our simple thesis of the 

 residence time of thorium being small with respect to the times of 

 oceanic mixing, coupled with the yearly contributions of thorium 

 draining into the world's oceans. 



Let us now turn to a rather dramatic example of chemistry 

 occurring on the sea floor, the formation of the ferromanganese 

 minerals, the major components of the manganese nodules. These 

 minerals represent the most nearly unique and possibly the most 

 abundant, and for these reasons, the most studied of the marine 

 authigenic minerals. They exist in the form of nodular concretions 

 which range in size from millimeters to about a meter, coatings 

 about rocks and shells, and as components of the unconsolidated 

 sediments. The two principal metals, iron and manganese, which 

 occur as oxides or hydroxides, are normally present in similar 

 amounts, although, less generally, either of these two elements 

 can be dominant. They are found on all oceanic floors and normally 

 occur in localities where the total rate of accumulation of sediments 



Table III. Oceanic Areas and Complimentary Land Areas Draining into Them 

 (in thousands of square kilometers) (Lyman, 1958) 



Land Area 

 Ocean Area Drained Percentage 



