129I: A New Tracer for Carbon Cycling

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129I: A New Tracer For Carbon Cycling Since the beginning of the nuclear age (circa 1945) human activities have greatly increased the environmental levels of the radio-iosotope of iodine (129I). Much of this radioactivity has collected in the surface layers of the ocean, where levels have been raised by at least a factor of 200 above the pre-nuclear activity levels. Iodine is incorporated into organic matter as it forms, so this increased 129I activity will accompany freshly formed organics and distinguish them from pre-nuclear. Except during primary production, the great majority of iodine in the biota is derived through the food chain, and not by direct uptake from seawater. Therefore past values of the ongoing build-up should be recorded in archived biological specimens. If, in a particular region, we can document the history of 129I concentration in the surface ocean waters, and in surface ocean biota that are the source of carbon (and iodine) for almost the entire food web below, then we may be able to identify the year-class of iodine present in all post-bomb marine organic matter in that region, and thereby iodine date the organic matter. This approach could then be extended to any other region where suitable archives exist. This work will establish past and present 129I values within the waters of the Gulf of Mexico. These levels are below detection by all commonly employed measurement techniques but are approximately 100,000 times the sensitivity limits when Accelerator Mass Spectrometry (AMS) is employed. The AMS group of the University of Rochester will make these measurements.