61 



to realize that the uppermost layers where most organisms live are, in this 

 sense, so extremely thin that even the thickness of a layer of ink printed on the 

 paper chart would exaggerate them. 



These superficial layers moving toward California, not being very thick, cannot 

 by themselves greatly dilute the substances that have entered them. Moreover, 

 there is a growing body of data indicating that certain important constituents 

 of global fallout do not pass rapidly through the moving layers so as to become 

 diluted by deeper water masses. Certain fallout constituents such as "'Cs and 

 °°Sr seem to remain many years in the upper moving layers before dispersing 

 downward. Because of this, these materials may be transported thousands of 

 miles away from the region where they entered the ocean. 



An extensive study of the continously changing patterns of fallout distributions 

 in upper layers of the open Pacific has been carried out at Scripps for more than 

 a decade. Most of this was accomplished by samplings made on "ships of oppor- 

 tunity" using special equipment developed for this program. Several thousand 

 measurements of fallout radiocesium, "'Cs, in the upper layers of the Pacific 

 have been made. 



This study of the progress of fallout in the upper layers of the open Pacific 

 has disclosed several large-scale features of this ocean that have been scarcely 

 suspected previously. Moreover, some of the patterns that have been observed 

 in the fnllout contaminations in the open ocean have led to better understanding 

 of the great variabilities in backgrounds that have been encountered during 

 investigations of coastal environments. However, in order to interpret even the 

 simpler fallout tagging patterns, it has been necessary to take into careful 

 consideration the charateristics of fallout inputs that have been arriving at 

 various zones of the earth. Global fallout has varied both with time and with 

 latitude. 



REPORTED FALLOUT RECORDS 



Figure 1 shows reported (1) trends of fallout deposits in terms of the °°Sr 

 constituent that is most commonly monitored. These data have been reduced to 

 a linear (rather than logarithmic) form that permits instant recognition of the 

 two large inputs to the ocean (peaking near 1958 and 1963). In the figure, there 

 has been drawn (from the 1964 input peak) three lines suggesting three rates 

 of attenuation that .should be taken into consideration whenever an earlier input 

 (say one during 1964) is to be evaluated in terms of what might remain at a 

 later date. That line showing a 30-year (half attenuation) regression rate 

 corresponds closely to the physical half lives of "'Cs and ™Sr, suggests what 

 might hai'pen if either of these nuclides fell upon dry land or accumulated in 

 a v^ell-stirred reservoir. The other two regression curves labeled Ti/2=15 years 

 and Ti/2=:10 years were derived from experimental data suggesting probable 

 retention rates in the upper layers of the N. Pacific of chemically-passive trace 

 materials. For example, after 19i)4 there apparently was so little "^Cs lost from 

 the upper layers in .some parts of the Pacific that the large inputs of 1958 and 

 19^3 still dominate the present concentrations of this fallout constituent. 



Figure 2 sketches two examples of water mass displacements that have been 

 di.sclosed by studies of fallout in shallow strata in northern oceanic regions of 

 the Pacific. The heavy lines indicate the major surface currents which bifur- 

 cate, one of which turns southward and forms the California Current. 



A fallout profile, measured near 1(>4°E, indicates exceptionally high fallout 

 concentrations between 400 to 600 meters. This is consistent with the well-known 

 oceanographic hypothesis (2) concerning the origin of intermediate water 

 masses from the downward sinking of surface waters at latitudes north of 40°N. 



ALso. an ob.served trend of surface fallout cesium concentrations has been 

 indicated graphically along the IH.l'W parallel. The significance of this surface 

 concentration trend will be discussed in connection with Figure 3. 



DISPLACEMENTS OF SURFACE WATER MASSES DELINEATED BY FALLOUT 



Figure 3 makes use of Pacific surface current vectors that have appeared in 

 one well-known chart (3). At the left hand, along 160°E. has been drawn a 

 simple graph depicting the trend of 137Cs concentrations as these vary (4) with 

 latitude. Tlie.se have been computed from fallout reports under the hypothetical 

 simplifying assumptions that (1) the integrated fallout at each latitxide has 

 mixed unifonnly to 100 meters depth, and (2) there has been no lateral motion. 

 It will be seen that a simple concentration maximum appears near 45°N. 



