文摘
Concentrations of the long-lived radioisotope 129I weremeasured in dated tree rings in order to determinewhether the distribution of this isotope reflects the historyof nuclear deposition. 129I concentrations and 129I/127Iratios were analyzed in tree rings and bark samples fromfour trees at West Valley, NY, and from one tree atRochester, NY. West Valley was the site of short-livednuclear fuel reprocessing activities (1966-1972), whileRochester, located 115 km to the northeast, provided aregional control site for the study. The selected trees reflectdifferent modes of fluid and nutrient transport in trees,with three species of ring-porous trees (elm, oak, and locust),one semidiffuse (cherry), and one diffuse-porous tree(maple). The results show that 129I levels in ring-poroustrees, in which xylem or hydrologic tissue is localized in theoutermost growth ring, are generally well correlatedwith the expected 129I deposition pattern for the region. Incontrast, tree rings of the more common semidiffuse todiffuse-porous wood, where xylem is disseminated throughoutthe trunk, show a less well developed 129I signal, probablydue to the transport of iodine ions across annual rings.Iodine concentrations in the tree rings range from 0.04 to2 mg/kg, 129I/127I ratios from 6 × 10-10 to 3.8 × 10-6.Tree bark and the outermost rings show significantly higher129I concentrations than the wood of the trunk. The 129I/127I ratios for bark are very similar to values obtained forsurface soil and water at the two localities, while inner ringshave ratios similar to those in deeper layers of the soil,reflecting different pathways for 129I uptake and the differencesin ambient 129I levels between the atmosphere and deepsoil. Although ring porous trees preserve the depositionalpattern of nuclear releases, rings older than or close tothe onset of the nuclear age have 129I/127I ratios significantlyabove the preanthropogenic level, suggesting that evenin these trees some redistribution of 129I occurs throughoutthe trunk. Our results indicate that growth rings from ring-porous wood are useful in time-series analyses ofregional 129I deposition, yielding reliable information onrelative changes in 129I concentrations but requiring cautionin the reconstruction of absolute ambient concentrationsduring any given time.