Environ. Sci. Technol. 1999, 33, 1771-1775
Reconstructing Lead Isotope Exposure Histories Preserved in the Growth Layers of Walrus Teeth Using the SHRIMP II Ion Microprobe R . A . S T E R N , † P . M . O U T R I D G E , * ,‡ W. J. DAVIS,§ AND R. E. A. STEWART| J. C. Roddick Ion Microprobe Laboratory, Analytical Chemistry Laboratories, and Geochronology Laboratories, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada, and Department of Fisheries and Oceans, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada
Development of a microprobe technique to determine Pb isotope ratios within the growth layers of mammal teeth could have widespread applications in Pb toxicology, Pb pollution tracing, and human and animal ecology. Here, the SHRIMP II ion microprobe is shown to possess sufficient sensitivity, accuracy, and precision to satisfactorily determine Pb isotope ratios in the canine tooth cementum of a walrus (Odobenus rosmarus rosmarus), with a sampling resolution of 130 µm. The tooth layers were estimated to contain only 1-3 µg/g Pb. By combining multiple replicates (N ) 10-30) within each annual layer, the (1 SE uncertainty was typically (1% for 206Pb/207Pb and (0.5% for 208Pb/207Pb. Significant isotopic differences were found between layers deposited at age 10 and ages 2, 27, and 30. This result, together with corroborative data on excised cementum fragments analyzed by thermal ionization mass spectrometry, indicates that the animal migrated into different geological terrains several times during its life. There was no evidence of exchange between the Pb deposited in early growth layers and more recent ambient Pb.
Introduction Incremental growth layers are found in the hard structures of many types of animals, such as mollusc shells, vertebrate teeth and bones, fish otoliths, corals, etc. The layers are composed of calcium, phosphorus, and other major and minor elements that are assimilated from the ambient environment during the period of layer formation (1, 2). These structures thus potentially represent temporal archives of chemical information accumulated throughout the life of the animal. Elemental data retrieved from the growth layers using various microprobes (3, 4) have a number of possible uses, including the reconstruction of histories of pollution exposure and nutritional status. To date, no in situ microanalytical determinations of trace element isotope ratios have been reported from biological * Corresponding author e-mail:
[email protected]; phone: (613)992-5645; fax: (613)943-1286. † J. C. Roddick Ion Microprobe Laboratory, Geological Survey of Canada. ‡ Analytical Chemistry Laboratories, Geological Survey of Canada. § Geochronology Laboratories, Geological Survey of Canada. | Department of Fisheries and Oceans. 10.1021/es980807f CCC: $18.00 Published on Web 04/01/1999
1999 American Chemical Society
structures, although the development of a suitable technique would have diverse potential applications, especially with Pb and Sr isotopes. These isotopes are particularly powerful for tracing environmental exposure to their elements, as they may be related to specific geological terrains (5). For example, bulk analysis of teeth for Sr isotopes in humans (6) and Pb isotopes in Atlantic walrus (Odobenus rosmarus rosmarus) (7) identified putative immigrants from other geographical areas within “local” populations. A temporal reconstruction of isotopic patterns from tooth layers could allow the determination of important parameters such as age at migration, migration pathways, and the original source areas of migrants. With respect to Pb pollution, whole tooth or bone analyses of Pb isotopes may present a confounded impression of the source of Pb exposure if an individual was exposed to different sources over time (see ref 2), although bulk analysis of different sections of teeth can help partially resolve this variation (8). A more fine-scaled resolution of exposure history and greater certainty in the identification of Pb sources may be possible with the in situ analysis of Pb isotopes. In the teeth of most mammals, annual growth layers are generally
