Global distribution and long-term fate of anthropogenic 129I in marine and surface water reservoirs

Since the advent of the nuclear age in the mid-1940s, the mass of radioactive I-129 (t(1/2) = 15.7 Myr) circulating in the Earth's hydrosphere has increased nearly fortyfold from its natural background level of 140 kg. Nuclear fuel reprocessing has been by far the major contributor, responsible for releasing 5400 kg of I-129, primarily into the North Atlantic Ocean. Regional and global trends in the distribution of the I-129 inventory are elucidated from an examination of more than 600 determinations of I-129 in environmental samples from around the world. Because the major point sources are located in Europe and the United States, more than 99% of the present I-129 reservoir is distributed in the Northern Hemisphere, where both I-129 concentrations and I-129/I ratios in rivers, lakes, and shallow seawater are several orders of magnitude above the preanthropogenic background. Downwelling in the North Atlantic presently provides a major sink for marine I-129; however, marine upwelling along the margins of the Pacific will eventually return part of this anthropogenic input to the ocean surface, where it will find its way back into surface waters and the atmosphere. Iodine-129 has a long half-life (15.7 Myr), and consequently, there is also the possibility that climate change will influence the dynamics of iodine transfer in surface reservoirs. We model the effect of a collapse in thermohaline circulation and project a concentration increase of more than 3 orders of magnitude in shallow oceans over the 10,000 years that follow if nuclear reprocessing is to continue at the present rate.