Study of the fallout of artificial iron-55. Application to the evaluation of the fallout of natural iron of stratospheric origin

The object of this work is to study the iron-55 fallout to establish the world balance of this radioelement. The utilisation of this tracer enables the calculation of the input of natural iron of stratospheric origin to the Earth and the input through the atmosphere of natural iron to the oceans. We have shown that: iron-55 is essentially produced in nuclear explosions according to the 56Fe(n,2n)55Fe reaction; the iron-55 fallout obeys a 'stratospheric pattern' with a decay period between 9 and 11 months; by surface unit and at the same latitude, this fallout is 3.5 times more important on the Eastern part of the Northern Atlantic than over France. By comparison with the strontium-90 fallout, we established the world balance of the iron-55 fallout in the 1962-1965 period. This balance amounts to approximately 50 mega-curies for the iron-55 introduced into the atmosphere; 25.3 mega-curies of which fall down on the Earth during this period. We estimated at 6 x 106 tons per year the input of natural iron of stratospheric origin to the Earth and at 1.2 x 107 tons per year the input through the atmosphere of natural iron to the oceans. This latter figure represents 13 to 38 per cent of the amount of iron incorporated annually in the pelagic sediments. (author)