Is There a Bigger Place for 129I in the Oceanographer’s Transient Tracer Toolkit?

Thomas P Guilderson1, Scott J Tumey1, Thomas A Brown1 and Sabine Mecking2, (1)Lawrence Livermore National Laboratory, Center for Accelerator Mass Spectrometry, Livermore, CA, United States, (2)Applied Physics Laboratory, University of Washington, Seattle, Seattle, WA, United States
129I, a long lived radioisotope, is produced in the atmosphere via cosmic ray interactions and as a fission product of uranium. The low production of 129I conspires with a long oceanic residence time, where total inorganic iodine is nearly conservative, to yield a uniform natural 129I content (measured as 129I/127I ratio) in seawater of ≤1.5 x 10-12. Anthropogenic 129I has been produced and dispersed via atmospheric testing of nuclear weapons and via nuclear fuel reprocessing, with most of the reprocessing discharge entering the North Atlantic. Taking advantage of the point source releases from the reprocessing centers of Sellafield (UK) and La Hauge (France), 129I is used as tracer to elucidate water mass ventilation times and pathways of the Arctic and North Atlantic (e.g., Smith, 2016; Karcher, 2012; Smith, 2011).

The initial source function of anthropogenic 129I was likely very similar to that of atmospheric weapons produced tritium, leading to a similar distribution of 3H and 129I (Guilderson, 2014). Although potentially complicated in some regions due to point source input, we posit that the present-day distribution of total inorganic 129I in the Pacific Ocean, as a function of density, should reflect that of a conventional transient tracer. Results from the CLIVAR/GO-SHIP P02 cruise (along 30°N, 2013) near 150°W support this hypothesis. Although hydrogen (tritium) and iodine have distinctly different chemistry and exchange between the ocean and atmosphere, the 3H and 129I profiles are nearly identical. 3H and 129I are well mixed to a potential density of ~26.6, with a transient tracer front of decreasing 3H and 129I below this horizon. This front is independently constrained by CFC TTD ages to a surface outcrop time between 1941 and 1968 for a potential density of ~26.8, indicating that waters below and above the front likely originated at the surface before and after peak testing (1962-1963), respectively. We conclude that the dissipation of 3H and 129I as a function of depth (density) and time since atmospheric weapons testing, is similar. 129I with a significantly longer half-life and less complicated sample handling could be a complement to 3H.

Guilderson et al., 2014. Biogeosciences: Fukushima Special Issue; Karcher et al., 2012 J. Geophys Res.,; Smith et al., 2016, J. Geophys Res.; Smith et al., 2011, J. Geophys Res.