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

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.