PARENT SESSION
Poster Session # 14: Forest Ecology.
Wednesday, August 6 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.
Assessment of variations in heavy metal accumulation by Quercus alba: a dendrochemical-lead isotopic approach.
Mallison, Shannon*,1, Miller, Jerry2, Lechler, Paul3, 1 289 Jennings Dr., Franklin, NC2 Department of Geosciences and Natural Resource Management, Cullowhee, North Carolina3 Nevada Bureau of Mines and Geology, Reno, Nevada
ABSTRACT- Spatial trends in both total lead (Pb) and Pb isotopes in tree-ring series have been suggested as an effective means of assessing changes in both source and abundance of atmospheric lead pollution through time. In this investigation, total Pb, and selected Pb isotopic ratios were analyzed and compared for tree-ring series of Quercus alba collected at two heights (0.3 and 4.0 m) within a given tree (5 trees total), and at similar heights between trees (8 trees total) at a site in western North Carolina, USA. The Pb isotopic signature of the soil corresponded with the signature of Pb in the tree rings and, although not statistically significant, mean total Pb concentrations for the lower tree series consistently exceeded the upper series. These trends indicate that Pb accumulation primarily occurs through the roots for Quercus alba, and that some Pb is bound to woody materials during its upward migration. However, the variability of isotopic ratios within the tree rings was significantly greater than that of the soil, and 206Pb/208Pb values for the upper series slightly exceeded the lower series of the sampled trees. Assuming Pb fractionation does not occur, these data suggest that Pb from an additional source is accumulated through leaves or bark and bound in the upper portions of the tree. An increase in total Pb concentrations for most tree ring series was evident beginning in 1930; however, constant isotopic ratios indicate the source of the lead has not substantially changed through the time period sampled, 1840-1999. Invariable isotopic ratios are inconsistent with the conclusion that additional mid-20th century Pb found in these trees is from leaded gasoline. It may be due, however, to an increase in the availability of Pb associated with a shift in soil conditions, perhaps a decrease in pH related to acidic precipitation.
Key words: Pb, dendrochemistry, Pb isotopes, tree rings