129I/(127)I as a new environmental tracer or geochronometer for biogeochemical or hydrodynamic processes in the hydrosphere and geosphere: the central role of organo-iodine.

Iodine is a biophilic element, with several short-lived isotopes (e.g. (131)I, t(1/2)=8 days), one long-lived isotope, (129)I (t(1/2)=15.6 million years) and one stable isotope, (127)I. The inventory of (129)I in surface environments has been overwhelmed by anthropogenic releases over the past 50 years. Iodine and its isotopes are important for a number of reasons: (1) The largest fraction of the short-term and long-term dose from accidental releases and fallout from atomic bomb tests was from iodine isotopes. (2) (129)I is one of the two long-lived nuclides with highest mobility in stored radioactive waste. (3) (129)I could provide the scientific community with a new geochemical tracer and new geochronological applications in environmental science. (4) A better assessment of iodine deficiency disorders, mineralization in exploration geochemistry, and the transfer of volatile organic greenhouse-active and ozone-destroying iodine species from the oceans to the atmosphere is needed. One of the most promising future applications for the (129)I/(127)I ratio is not only as a new geochronometer, but also as a new source tracer for terrestrial organic matter with ages of 50 years or less. This is especially attractive, since radiocarbon can be, at times, an ambiguous chronometer for the 50-year time-scale, whereas (129)I concentrations during this time are overwhelming previous levels by orders of magnitude. Iodine is to a significant extent involved in the cycle of organic matter in all surface environments. Its biophilic nature is demonstrated by a relative enrichment of iodine in seaweed and dissolved macromolecular organic matter. Because of the close coupling of iodine and organic carbon cycles, our understanding of the underlying molecular mechanisms of the processes regulating iodination reactions in aquatic systems is still limited. The binding of iodine by organic matter has the potential to modify the transport, bioavailability and transfer of iodine isotopes to man. Equilibration times for (129)I in many reservoirs are likely long enough that (129)I could be used as a new source tracer for organic matter of terrestrial origin, and as a geochronometer. Current tracer applications of (129)I are limited by our knowledge of the effects of UV-radiation, microbial activity and geochemical redox conditions on organo-I compounds and overall iodine speciation. The biogeochemical behavior of iodine and its isotopes appears to be different in North America and European waters, possibly due to climatic, source and speciation differences.