The (129)Iodine bomb pulse recorded in Mississippi River Delta sediments: Results from isotopes of I, Pu, Cs, Pb, and C

I-129(t(1/2) = 1.56 X 10(7) yr) has both natural as well as anthropogenic s ources. 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 variat ions in the sources of I-129 and to a lack of knowledge regarding the forms of iodine. From a vertical profile of I-129/I-127 ratios in sediments loca ted in the Mississippi Delta region in approximately 60 meters water depth, the I-129 input function to this region was reconstructed. Dates in the co re were assigned based on the plutonium peak at 20 cm depth (assumed to hav e been deposited in 1963) and the excess (210)pb profile in the same depth interval, and below that, based on the steadily decreasing Pu-240/Pu-239 ra tios from a ratio of 0.18 at 22 cm to 0.05 at 57 cm depth, the 1953 horizon . These low Pu-240/Pu-239 values are attributed to low yield, close-in, tro pospherically transported bomb fallout produced from the Nevada test site i n the early 1950s, which had a value of about 0.035, and strongly suggest a terrestrial source for Pu isotopes. I-129/I-127 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. I-129 concentrations ranged from 8-13 X 10(6) atoms/g in the top 20 cm, and decreased to values of less than 1 x 10(6) atoms/g below 50 cm. Atom ratios of I-129/Cs-137, decay corrected to 1962, the year of maxi mum radionuclide production, are about 0.3, very close to the production ra tios of about 0.2 during atomic bomb tests. This evidence, combined with ot her observations, strongly suggests that I-129 in Mississippi River Delta s ediments originates from atomic bomb fallout eroded from soils of the Missi ssippi River drainage basin, with little alteration of the isotopic ratios during transport from watershed to coastal deposits. This is further corrob orated by a close correspondence between I-129/I-127 ratios and other bomb fallout nuclides in this core. Based on these observations and on laborator y evidence, we propose a conceptual model which explains this correspondenc e and the low I-129/I-127 ratios found in our sediment core as caused by ge ochemical I-isotope fractionation processes during organic carbon leaching from erodible soils in the Mississippi River watershed. Differences in mobi lities of the different chemical forms of I-129 and I-127, as well as the v ariances in chemical forms of I-129 from nuclear bomb fallout versus nuclea r fuel reprocessing, are proposed to have created such a correspondence bet ween I-isotope ratios and bomb fallout nuclides, without revealing recent i nputs from nuclear fuel reprocessing releases to the northern hemisphere ob served in watersheds of the USA and Europe. Copyright (C) 2000 Elsevier Sci ence Ltd.