Reconstructing lead isotope exposure histories preserved in the growth layers of walrus teeth using the SHRIMP II ion microprobe

Development of a microprobe technique to determine Ph isotope ratios within the growth layers of mammal teeth could have widespread applications in Ph toxicology, Pb pollution tracing, and human and animal ecology. Here, the SHRIMP II ion microprobe is shown to possess sufficient sensitivity, accura cy, and precision to satisfactorily determine Ph isotope ratios in the cani ne tooth cementum of a walrus (Odobenus rosmarus rosmarus), with a sampling resolution of 130 mu m. The tooth layers were estimated to contain only 1- 3 mu g/g Pb. By combining multiple replicates (N=10-30) within each annual layer, the +/-1 SE uncertainty was typically +/-1% for Pb-206/Pb-207 and +/ -0.5% for Pb-208/Pb-207. Significant isotopic differences were found betwee n layers deposited at age 10 and ages 2, 27, and 30. This result, together with corroborative data on excised cementum fragments analyzed by thermal i onization mass spectrometry, indicates that the animal migrated into differ ent geological terrains several times during its life. There was no evidenc e of exchange between the Pb deposited in early growth layers and more rece nt ambient Pb.