CHEMISTRY IX THE OCEANS 587 



in the divalent state and are associated with manganese in marine 

 deposits. The residence times of these latter elements are about 

 500 times higher. This reactivity of manganese is apparently 

 related to the removal of this element from solution by oxidation 

 to the tetravalent state to form the ferromanganese deposits, a 

 subject we shall discuss subsequently. The sorption of the associ- 

 ated metals like zinc and nickel to the precipitated manganese 

 requires a relatively larger amount of manganese. This results in 

 longer residence times, compared to manganese, for these metals. 



Silicon and aluminum are among the elements with the shortest 

 calculated residence times. Both elements enter the oceans in both 

 solid and dissolved phases; the solid phases, such as quartz, 

 feldspars, and clay minerals, rapidly settle to the bottom. Iron 

 and titanium which also quickly pass through the hydrosphere, 

 prevail as solid phases during the major part of their residence. 

 The absolute values for the residence times of these elements are 

 somewhat tenuous as the assumption that such times are small in 

 comparison with the mixing times of oceanic water masses, say of 

 the order of hundreds to thousands of years, is invalid. It may be 

 worthwhile in the future, when values exist for the contribution of 

 solid and dissolved phases of these elements to the total concen- 

 trations, to calculate more accurately the residence times of the 

 components. 



The utility of such computations, the derivation of residence 

 times, arose recently in our laboratory in the study of thorium 

 isotopes as a dating tool for marine sediments. We were concerned 

 primarily with two isotopes, normally occurring thorium, Th"''", 

 and ionium, Th-'^'^, which is one of the daughter products of the 

 most abundant isotope of uranium, U'-''^^. The residence of thorium 

 can only be given in terms of an upper limit inasmuch as the 

 thorium concentration in sea water has never been determined. A 

 value of a life in sea water of less than one thousand years, probably 

 closer to one hundred years, is derived for thorium on the basis of 

 a maximum concentration in sea water of 10~^ g/liter. 



The extremely short residence time of thorium suggests the 

 possibility of ocean to ocean variations in its concentration. Those 

 bodies of water exposed to the introduction of large amounts of 

 continental runoff may very well be enriched in normally occurring 



