The dissolved organic iodine species of the isotopic ratio of 129I/127I:: A novel tool for tracing terrestrial organic carbon in the estuarine surface waters of Galveston Bay, Texas
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Variations in 129I/127I ratios were used to trace terrestrial organic carbon (tDOC) across an estuary because (1) iodine is biophilic, up to 75% of total iodine in fresh and coastal marine waters partitions into organic iodine; (2) 129I/127I ratios in tDOC are greatly elevated over those from marine systems because atmospheric emissions of 129I from European nuclear fuel reprocessing facilities were mixed more quickly in the surface ocean, up to 500 m in a decade, than the terrestrial system, which mixed approximately 10 cm in 10 to 50 y; and (3) the oceanic contribution of 127I (50 to 65 ppb) to the ratio has a greater dilution effect than 127I from freshwater (0.5 to 40 ppb). Analytical techniques were developed for 129I/127I ratio determination in dissolved organic iodine (DOI) and the other iodine species, using dehydrohalogenation, anion chromatography, highperformance liquid chromatography, and accelerator mass spectrometry, to be applied to samples from Galveston Bay, Texas. Results indicate that 129I/127I ratios in DOI from terrestrial sources are elevated in the upper estuary up to salinity of about 20, similar to a behavior previously described for this estuary for stable isotopic signals for dissolved organic matter. 129I/127I ratios in the other iodine species, e.g., iodide and iodate, did not show this feature, indicating fast isotopic and chemical equilibration between the two isotopes among the different inorganic species in the estuary. These results thus provide proof of concept that 129I/127IDOI can serve as a tracer for tDOC in the coastal zone.
