241pu AND 241 Am IN SEDIMENTS FROM COASTAL BASINS OFF CALIFORNIA AND MEXICO

In sediments deposited in the coastal basins off the western North American continent since the early 1960's, there is more 241 Am activity than one would predict if its presence was solely due to the decay of 241Pu that was produced during the testing of weapons in 1961-62 (taken as ~luly 1, 1962 for calculations). This excess can be accounted for by the decay of 241 Pu, if pre-1962 fallout (fallout from the pre-moratorium testing) has been continuously introduced into the recent sediments along with fallout from post-moratorium testing. This hypothesis is supported by the plutonium depth prof'des which do not reflect direct fallout from the atmosphere, as characterized by two peaks corresponding to the two major weapons testing periods, but continuously increasing plutonium and americium concentrations from 1954 to 1974. Thus, the nuclides may be coming from a large reservoir that has accumulated and mixed fallout since 1952. Such a concentration gradient for all of the nuclides requires their immobilization in the sediment after deposition.