Iodine-129 as an Oceanic Tracer

Abstract

The long-lived radionuclide ¹²⁹I (half-life: 15.7 × 10⁶ yr) is well-known as a useful environmental tracer. At present, the global ¹²⁹I in surface water is about 1-2 orders of magnitude higher than pre-1960 levels. In order to understand the spread of ¹²⁹I in the Pacific Ocean, ¹²⁹I time series from corals and seawater were reconstructed. Two iodine extraction methods were tested and compared to optimize the yield and precision of the measurements. A solvent extraction method was chosen over a resin column preparation technique giving that it provides higher signal and lower error. Coral ¹²⁹I time series records from the Western Pacific including Con Dao and Xisha Islands, in the South China Sea; Rabaul, Papua New Guinea, in the South Pacific; and Guam in the North Pacific were reconstructed and pathways and sources of ¹²⁹I from the 1950s onward were assessed. Nuclear weapons testing was the primary ¹²⁹I source to the ocean, notably from testing in the Marshall Islands and radiogenic iodine was carried primarily through surface ocean currents. A seawater ¹²⁹I monitoring program was instituted with seawater samples from Scripps Pier, La Jolla, California, USA a few months after the Great East Japan Earthquake, which involved the accidental release of substantial amounts of ¹²⁹I from the Fukushima Daichi nuclear power plant. These data were used to construct a 4-year record of surface water ¹²⁹I with monthly resolution. ¹²⁹I in Hanford site vicinity is also surveyed to understand the background value of the California Current System. It is clear that the California Current System is influenced by the Hanford site which can explain the higher ¹²⁹I/¹²⁷I background values observed along the west coast of the U.S. compared to other surface sites in the Pacific Ocean. Our result shows that the highly contaminated seawater from the Fukushima Daiichi Nuclear Power Plant accident has not yet seen at the Scripps Pier, La Jolla, California, USA four years of ocean transport.