12 GOLDBERG [CHAP. 1 



higher ionium/thorium ratios in southern Pacific and southern Atlantic surface 

 deposits as compared to their northern counterparts. 



A question pertinent to the marine geochemistry of thorium involves the 

 path of this element from run-off waters to the sea-floor precipitates. Previously, 

 it was proposed that the variations in the ionium/thorium ratio, as well as in 

 the lead isotopic ratios, in surface sediments were related to the direct input of 

 thorium and lead to the bottom waters from the continents (Goldberg, Patter- 

 son and Chow, 1958). However, it was difficult to reconcile the geographic 

 distributions of the ratios with the prevalent ideas of bottom-water circulation, 

 especially with respect to the rather well-defined differences in lead and thorium 

 isotopic ratios between northern and southern oceanic waters with the equator 

 acting as a boundary. 



Perhaps, as in the case of barium (see page 20) and 210 Pb (Rama, Koide and 

 Goldberg, 1961), 232Th is conveyed from shallow to deeper waters as a result 

 of biochemical and inorganic processes and is initially introduced to the open 

 ocean in surface waters. Such a path for thorium would not conflict with the 

 known travels of surface waters where the equatorial regions form a barrier to 

 the transfer of such waters from the Northern to the Southern Hemisphere. On 

 the other hand, near-bottom waters, which give the authigenic minerals their lead 

 and thorium isotopes, apparently travel from south to north (at least in the 

 Pacific) crossing the equator along the way. If river run-off were introduced 

 directly into deep-sea waters, it would be very difficult to account for the 

 observed differences in the isotopic ratios in the sediments. 



B. Degrees of Under saturation 



A second, but less satisfying measurement of the reactivities of elements in 

 sea-water can be gained on the basis of the degree of undersaturation, especially 

 in the case of metallic ions and dissolved gases. The upper limit of concentration 

 that a metal ion could attain would conceivably be regulated by the solubility 

 of its least soluble compound, as determined by the anionic species present, 

 if no other chemical reaction reduced its abundance. As a first approximation 

 it might be deduced that those elements with the highest degrees of 

 undersaturation would be the most reactive, while those near or at saturation 

 would be essentially inert in the marine environment. 



Krauskopf (1956) calculated the theoretical maximum concentrations that a 

 group of metal ions might reach on the basis of the compounds formed with the 

 major anions from sea-water which, from existing data in the literature, 

 would result in the lowest amount of metal ion in solution. His data, which 

 took into account the activity coefficients of the ions, are given in Table IV. 



He further initiated an experimental attack upon this problem. To sea- water 

 was added a solution of a given metal ion until a precipitate formed ; adequate 

 precautions were taken to maintain the pH of the sea-water relatively constant 

 (between 7.8 and 8.2, values characteristic of open ocean water). The amount 

 of the metal ion in solution was then determined, presumably an equilibrium 



