3.3. Strontium Isotope analysis

Due to availability and preservation we selected the third left mandibular molar from Chan Hol 3 for Sr-isotope analysis. Third molar enamel forms during adolescence, between 7 and 16 years of age (e.g. [34]).

We sampled 2 to 5 mg chips of tooth enamel using a 0.2 mm diamond-coated cutting disc to minimize material loss of the valuable sample. We carefully avoided dentine components in the samples, because dentine is known to be sensitive to diagenesis and, unlike enamel, easily takes up mobile geogenic strontium components from the surrounding rocks and sediments [35].

The enamel pieces were cleaned by repeated washing with ultrapure water. After drying the enamel grains were digested using nitric acid. Strontium was purified through wet-column extraction chemistry using an EiChrom SrResin^® column [36]. The protocol applied at the Institute of Earth Sciences, Heidelberg University was adopted after Kober et al. [37]. A 1 ml column was filled with SrResin (TRISKEM) and washed with 6 column volumes (CV) H₂O. Next the columns were loaded with 3 ml 7N HNO₃. The samples were dried and re-dissolved in 1 ml nitric acid added onto the columns. The columns were then washed with 6 CV 7 N HNO₃. To elute Sr from the resin the columns were rinsed with 3 CV H₂O. The samples were evaporated on a hot plate until a small barely visible drop remained at the bottom of the beaker. The column chemistry was repeated to further purify Sr from the sample matrix and the final Sr solution was evaporated to dryness. The sample was re-dissolved in a drop of concentrated HNO₃ and a drop of H₂O₂ to ensure dissolution of remains of the resin and it was then again evaporated to dryness. Finally, samples were dissolved in 10 μl 7N HNO₃ and were transferred onto a preheated rhenium filament. Isotopic measurements (10 sequences of each 10 measurements) were conducted on a thermal ionization mass spectrometer (Finnigan MAT-262) using a dynamic multi-collection method normalized to the Nier value of ⁸⁶Sr/⁸⁸Sr = 0.1194 using an exponential fractionation law. All isotopes were measured on Faraday cups with minimum ⁸⁶Sr intensities of 0.5 V. Each measurement was checked for ⁸⁵Rb. Isotope ratios were corrected for internal mass fractionation assuming a stable ⁸⁸Sr/⁸⁶Sr ratio of 8.375209. The NIST isotope standard SRM-987 was used for routine monitoring and correction of instrumental bias and to assess reproducibility. Replicate SRM-987 analysis of the ⁸⁷Sr/⁸⁶Sr ratio yields 0.710261 ± 0.000006 (2σ, N = 4).