Micro-spatial variations of heavy metals in the teeth of walrus as determined by laser ablation ICP-MS: the potential for reconstructing a history of metal exposure.

This study explored the possibility of using laser ablation inductively-coupled plasma-mass spectroscopy to measure trace metals and other elements within the annual growth layers of the teeth of walrus harvested from the Canadian Arctic. Using sample ablation "footprints" of 125 microns diameter on transects across the exposed cross-sections of teeth, this technique detected Pb, Cu, Zn and Sr, but not Cd, in tooth cementum. The micro-spatial patterns of elements were consistent among different transects on the same tooth, and revealed subtle differences between animals of different ages. The youngest walrus in the sample (4 yr) contained higher concentrations of Pb and Cu than older animals in the growth layer deposited during the first year of life, while the oldest animal (33 yr) exhibited higher Pb and Zn than younger animals in the outer layer corresponding to the year 1988. The differences between animals and across annual layers may reflect both life history and metal exposure phenomena, including high amounts of metals transferred from mothers to pups in maternal milk. The ability to detect metals in a repeatable fashion within annual growth layers suggests that metal exposure histories accurate to within a year might be re-constructed for the life-times of long-lived animals, and that a series of such individual studies would allow exposure histories covering centuries to be quickly assembled. These data may suggest the most likely explanation for the currently high levels of some metals observed in certain Arctic marine mammals, i.e., natural phenomenon or anthropogenic contamination.