OCEANOGRAPHY AND FISHERIES 87 



been observed at depths of 2000-4000 meters ( 20 ) , and Swallow and Worthington(21 ) 

 have described a deep countercurrent underlying the Gulf Stream with speeds up to 18 

 cm/sec at depths of several thousand meters. These and other recent (unpublished) measure- 

 ments suggest that horizontal exchange at intermediate and greater depths is far more rapid 

 than had hitherto been realized. 



A technique complementary to these more popular methods for the study of near-bottom 

 deep-sea circulations involves the use of isotopic analyses of lead and thorium isotopes in 

 deep-sea sediments. The transfer of dissolved chemical species, characteristic of water mass 

 adjacent to the bottom, to one or more of the solid phases of the deposit results in a record in 

 the sediments of the travels of the bottom water. Goldberg, Chow, and Patterson (22, 23, and 

 24) have used two groups of isotopes, those of lead and thorium, to subdivide the bottom 

 waters of the Pacific into four domains presumed to reflect the points of origin of the lead and 

 thorium isotopes in the bottom waters, these species having been introduced as a result of 

 continental weathering. Four distinct regions in the Pacific appear, roughly classified as 

 West, Central, East, and South Pacific, all differing from the Atlantic which at present ap- 

 pears to be but one domain. The data so far establish that bottom waters in the Pacific are 

 incompletely mixed in times of the order of a million years or less. 



Recently, Koczy(25) has used the observed distribution of radium in the ocean to 

 evaluate the rate of mixing between deep and surface waters. Under the assumption that all 

 radium in ocean water originates from the sea floor, a simplified form of the Fickian diffusion 

 equation is used to compute deep vertical eddy diffusivity coefficients, which are found to be 

 about 8 cmVsec. In the layer of minimum eddy diffusion (700-1500 m), vertical transfer 

 of radium is due to advection, which is estimated at 0.7-2.0 m/yr. These results have been 

 used to compute the consequences of depositing large quantities of Sr'" on the sea floor. It is 

 shown that at the top of the deep layer, the maximum concentration of Sr"" is reached in 

 about 25 years when the concentration per cm' is 10""XQ, where Q is the total amount of 

 waste deposited on the sea floor. 



Application already has been made of tagging indicators for the study of sea-water 

 movements. A dye (fluoroscein) has been used to plot the dispersion of reactor effluent in 

 the Irish sea, and artificial radioisotopes have been used in several small-scale experiments 

 for following the movement of sediments and wastes. The problem of tagging water masses 

 in the open sea has been discussed in some detail by Folsom and Vine( 1 ). Because of the 

 immense size of the ocean, and because of the difficulty usually experienced in establishing 

 ships' positions accurately, small-scale tagging experiments are difficult to carry out, and 

 usually it is necessary to prepare for the detection of the tagging material after extreme dilu- 

 tion. The tagging material must never present a real human hazard — and frequently must 

 avoid even the appearance of being a hazard. For these reasons, considerable research effort 

 has centered around improving the techniques for detecting minute traces of dyes and artificial 

 nuclides at sea. 



Several institutions are doing work fundamental to the improvement of underwater 

 gamma-ray detectors. Large liquid scintillometers, plastic scintillometers, coincident gamma- 

 ray detectors, and portable pulse-height spectrometers are now under development for this 

 application. 



Information is being collected, compiled, and studied concerning the character and 

 magnitude of the gamma-ray background in the marine environments. 



It is apparent that several useful water-tagging studies could be done in deep water using 



