Strontium isotope ratios in fossil teeth from South Africa: assessing laser ablation MC-ICP-MS analysis and the extent of diagenesis |
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Authors: | Sandi R. Copeland Matt Sponheimer Julia A. Lee-Thorp Petrus J. le Roux Darryl J. de Ruiter Michael P. Richards |
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Affiliation: | 1. Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany;2. Department of Anthropology, University of Colorado at Boulder, 233 UCB, Boulder, CO 80309, USA;3. Division of Archaeological, Geographical and Environmental Sciences, University of Bradford, Bradford BD7 1DP, UK;4. AEON EarthLAB, Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa;5. Department of Anthropology, Texas A&M University, College Station, TX 77843-4352, USA;6. Department of Anthropology, University of British Columbia, 6303 NW Marine Drive, Vancouver, BC, V6T 1Z1, Canada |
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Abstract: | This study tests the precision and accuracy of laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for measuring strontium isotope ratios (87Sr/86Sr) in fossil teeth from Swartkrans and Sterkfontein in South Africa, and explores the extent of diagenesis in these specimens. Compared to solution MC-ICP-MS or thermal ionization mass spectrometry (TIMS) methods, LA-MC-ICP-MS can be faster and less expensive, but has lower precision and accuracy. We found that 87Sr/86Sr ratios in fossil rodent teeth measured by LA-MC-ICP-MS were offset from solution values by a mean of 0.0005 ± 0.0010 (2σ). Because of the very wide spread of biologically available 87Sr/86Sr ratios (∼0.710 to ∼0.790) in the region, LA-MC-ICP-MS is more than sufficient for assessing the geographic origins of the fossil rodents. The concentration of strontium in modern and fossil enamel from the area is statistically indistinguishable, as is the range of 87Sr/86Sr ratios in modern rodents and fossil rodents from Swartkrans. Thus, there is little evidence of diagenetic Sr in the fossil enamel. In contrast, the fossil dentine may contain up to ∼50% diagenetic strontium, although many dentine 87Sr/86Sr ratios are still far removed from the background cave value of 0.729. We propose that LA-MC-ICP-MS will also be of use for investigating the geographic origins of other, non-rodent fossil teeth from the Sterkfontein and Swartkrans collections, and may prove of similar utility at other suitable sites. |
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Keywords: | Strontium isotopes Laser ablation LA-MC-ICP-MS Diagenesis South Africa Rodent |
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