Variations in the 129I/127I ratios of pre-world war II algae under long-term exposure to atmospheric iodine isotopes

The ¹²⁹I/¹²⁷I ratios of algae that were collected during the pre-World War II period are useful for reconstructing historical ¹²⁹I concentrations in seawater. We confirmed whether the primordial marine ¹²⁹I/¹²⁷I ratio in the environment during the global nuclear test era and the operation of reprocessing plants can be maintained in algae samples collected before World War II and stored in a museum over the long term. We determined the present contamination flux of ¹²⁹I in a storage room of a building located near the coast of the Sea of Japan and the source of ¹²⁹I contamination in old algae collected during the prewar era. The major sources of ¹²⁹I contamination in the other stored algal samples were atmospheric ¹²⁹I and atmospherically volatile ¹²⁹I derived from fresh algae collected during the 1960s. These samples were stored together in the museum storage room. The low ¹²⁹I/¹²⁷I ratios of the old algae, i.e., those collected before World War II, increased to levels greater than the original values. Finally, we could not effectively reconstruct the ¹²⁹I concentration in the marine environment after the nuclear testing era on the basis of the ¹²⁹I/¹²⁷I ratios of the algae samples collected before the war.