SECT. 1] THE OCEANS AS A CHEMICAL SYSTEM 9 



2. Reactivities of the Elements 



If we consider the oceans as a dynamic chemical system, the reactions of 

 primary interest are those that influence or control the concentrations of 

 elements. The life processes have long been known to be responsible for the 

 most dramatic compositional changes and for the regulation of the abundances 

 of such elements as oxygen, phosphorus, silicon, nitrogen and carbon. An in- 

 sight into the important inorganic reactions has been gained through considera- 

 tions of the relative reactivities of the elements based on the average times 

 spent before removal to the sea floor or on the degree of saturation attained by 

 certain elements in sea-water. 



A. Residence Times 



Barth (1952) first proposed the concept of residence (passage) time of an 

 element in the marine environment by assuming an ocean in steady state in 

 which the amount of a given element introduced per unit time is compensated 

 by an equal amount deposited in the sediments. The residence time of an 

 element, t, can then be defined as the average time it remains in sea-water 

 before removal by some precipitation process. Thus, 



A 

 T ~ {dAjdt)' 



where A is the total amount of the element in the dissolved and particulate states 

 in the oceans and dAjdt is the amount of the element introduced or precipitating 

 per unit time. It is further assumed that there is a complete mixing of the 

 element in question within the world ocean in times that are short with respect 

 to the residence times. 



Barth made his computations on the basis of river influx of dissolved sub- 

 stances, neglecting the introduction of the suspended load as well as any solid 

 materials introduced into the oceans via the atmosphere. He used Clarke's 

 (1924) figure for the total amount of dissolved substances supplied by the rivers, 

 i.e. 2.73 x 10 15 g/year. The absolute concentrations of elements in rivers can not 

 be used directly to calculate dAjdt inasmuch as significant amounts of 

 materials are cycled from the sea via the atmosphere and rains to the rivers 

 and subsequently returned to the oceans. Conway (1943) has pointed out 

 that nearly all of the chlorine and about 53% of the sodium in rivers is of cyclic 

 origin. Barth took cognizance of Conway's data to obtain the net transfer of 

 material between the oceans and continents, and was able to derive the resi- 

 dence times of five elements (Table II). 



Goldberg and Arrhenius (1958) independently calculated the residence times 

 on the total rate of sedimentation in the marine environment, using a derived 

 figure of 2.5 x 10 15 g of weathered products and pyroclastics entering the ocean 

 per year. Assuming the composition of such materials to be the same as crustal 

 rocks, residence times for many of the elements whose sea-water concentrations 

 are available have been calculated (Table I). 



