Other
Scientific paper
Mar 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000gecoa..64..989o&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 64, Issue 6, pp.989-996
Other
3
Scientific paper
129 I ( t 1/2 = 1.56 × 10 7 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 129 I and to a lack of knowledge regarding the forms of iodine. From a vertical profile of 129 I/ 127 I ratios in sediments located in the Mississippi Delta region in approximately 60 meters water depth, the 129 I 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 210 Pb profile in the same depth interval, and below that, based on the steadily decreasing 240 Pu/ 239 Pu ratios from a ratio of 0.18 at 22 cm to 0.05 at 57 cm depth, the 1953 horizon. These low 240 Pu/ 239 Pu 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. 129 I/ 127 I ratios increased from 2 × 10 -10 at 3 cm to 4 × 10 -10 at 20 cm, and from there decreased monotonously to pre-anthropogenic values at 53 cm and below. 129 I concentrations ranged from 8-13 × 10 6 atoms/g in the top 20 cm, and decreased to values of less than 1 × 10 6 atoms/g below 50 cm. Atom ratios of 129 I/ 137 Cs, 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 129 I 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 129 I/ 127 I 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 129 I/ 127 I 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 129 I and 127 I, as well as the variances in chemical forms of 129 I 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.
Moran Jean E.
Oktay S. D.
Santschi Peter H.
Sharma Pankaj
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