LASER ABLATION GC/IRMS (d13C, d18O) AS A ROUTINE METHOD FOR ISOTOPIC ANALYSIS OF VERY SMALL FOSSIL AND MODERN TEETH

We have explored the use of laser ablation/gas chromogography/isotope ratio mass spectrometry (LA/GC/IRMS) methods (Cerling and Sharp 1996; Sharp and Cerling 1996) for in-situ isotopic analysis of tooth enamel from very small teeth. A key improvement is the use of several (3 to 8) low power (15 W) very short (85 ms) infrared laser pulses for each isotopic determination; this results in ablation pits that are ~0.3 mm in diameter and less than 0.05 mm deep. This depth is 1-2 orders of magnitude less than that reported by Cerling and Sharp (1996), and enables the analysis of teeth with very thin enamel. The 1s precision for d¹³C and d¹⁸O is typically better than 0.5‰, but this parameter is dependent on sample composition and heterogeneity, with very pristine and homogeneous samples giving higher precisions (up to 0.1‰). Carbon isotope ratios are typically accurate to within 1‰ of the number obtained by conventional acid digestion, and this can be improved by correcting relative to an enamel ‘standard' analyzed during the same run as unknowns. Oxygen isotope ratios of modern teeth are enriched by about 6.0±0.5‰ over the conventional acid-digestion carbonate number, reflecting a mixture of phosphate, carbonate, and hydroxide oxygen in the analyzed CO₂. Fossil teeth have more variable enrichments, indicating minor diagenesis of the primary mineral phases, or inclusion of oxygen from secondary mineral phases (e.g. oxyhydroxides). Because of the very small amount of CO₂ generated and analyzed (~30 nmole), background CO₂ coming from de-gassing of samples can be a major problem, and interaction between laser-generated CO₂ and active sample surfaces can fractionate the sample. These problems can be mitigated by pre-analysis flush-down or evacuation measures, limiting the number of samples in a single chamber, and use of injected CO₂ and enamel standards for monitoring the behavior of the system. We present results of the method applied to small teeth, including intra-tooth isotope profiles of evergrowing incisors.