The (129)Iodine bomb pulse recorded in Mississippi River Delta sediments: Results from isotopes of I, Pu, Cs, Pb, and C
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129I (t( 1/2 ) = 1.56 X 107 yr) has both natural as well as anthropogenic sources. Anthropogenic sources from nuclear reprocessing discharges and bomb test fallout have completely overwhelmed the natural signal on the surface of the earth in the last 50 years. However, the transfer functions in and out of environmental compartments are not well known due to temporal variations in the sources of 129I and to a lack of knowledge regarding the forms of iodine. From a vertical profile of 129I/127I ratios in sediments located in the Mississippi Delta region in approximately 60 meters water depth, the 129I input function to this region was reconstructed. Dates in the core were assigned based on the plutonium peak at 20 cm depth (assumed to have been deposited in 1963) and the excess 210Pb profile in the same depth interval, and below that, based on the steadily decreasing 240Pu/239Pu ratios from a ratio of 0.18 at 22 cm to 0.05 at 57 cm depth, the 1953 horizon. These low 240Pu/239Pu values are attributed to low yield, close-in, tropospherically transported bomb fallout produced from the Nevada test site in the early 1950s, which had a value of about 0.035, and strongly suggest a terrestrial source for Pu isotopes. 129I/127I ratios increased from 2 X 10-10 at 3 cm to 4 X 10-10 at 20 cm, and from there decreased monotonously to pre-anthropogenic values at 53 cm and below. 129I concentrations ranged from 8-13 X 106 atoms/g in the top 20 cm, and decreased to values of less than 1 X 106 atoms/g below 50 cm. Atom ratios of 129I/137Cs, decay corrected to 1962, the year of maximum radionuclide production, are about 0.3, very close to the production ratios of about 0.2 during atomic bomb tests. This evidence, combined with other observations, strongly suggests that 129I in Mississippi River Delta sediments originates from atomic bomb fallout eroded from soils of the Mississippi River drainage basin, with little alteration of the isotopic ratios during transport from watershed to coastal deposits. This is further corroborated by a close correspondence between 129I/127I ratios and other bomb fallout nuclides in this core. Based on these observations and on laboratory evidence, we propose a conceptual model which explains this correspondence and the low 129I/127I ratios found in our sediment core as caused by geochemical I-isotope fractionation processes during organic carbon leaching from erodible soils in the Mississippi River watershed. Differences in mobilities of the different chemical forms of 129I and 127I, as well as the variances in chemical forms of 129I from nuclear bomb fallout versus nuclear fuel reprocessing, are proposed to have created such a correspondence between I-isotope ratios and bomb fallout nuclides, without revealing recent inputs from nuclear fuel reprocessing releases to the northern hemisphere observed in watersheds of the USA and Europe. Copyright (C) 2000 Elsevier Science Ltd.
