Micromilling vs hand drilling in stable isotope analyses of incremental carbonates: The potential for δ13 C contamination by embedding resin.

RATIONALE: Embedding resins are commonly used to facilitate high-resolution sampling for stable isotope analysis but anomalous δ13 C values have been observed in some cases. Here we compare the results of microsampling strategies for hand-drilled versus resin-embedded micromilled samples from the same marine shells to assess whether resin contamination is implicated in δ13 C spikes. The comparison allows assessment of the relative benefits for spatial resolution, seasonal range for both δ18 O and δ13 C, and sample failure rates. METHODS: Hand-drilled samples were obtained from two bivalve shells (Spisula sachalinensis), corresponding to micromilled samples on the same shells where high δ13 C spikes were observed. All carbonate powders were analysed using a dual-inlet Isoprime mass spectrometer and Multiprep device. Results from both sample sets were compared statistically. RESULTS: No anomalous high δ13 C values and no failures due to insufficient gas were observed in the hand-drilled samples in contrast to the embedded micromilled sequences. Spatial resolution was reduced (~2.5×) in the former compared with the latter, resulting in a small reduction in the total range observed in the micromilled δ13 C and δ18 O values. Reduced sampling resolution between the two datasets was only significant for δ18 O. CONCLUSIONS: For S. sachalinensis (as with other similar bivalves), rapid growth mitigates the reduced sampling resolution of hand drilling and does not significantly impact observed isotopic range and seasonal patterning. Occurrence of anomalous δ13 C values were eliminated and failure rates due to insufficient sample size greatly reduced in the hand-drilled dataset. We can find no other explanation for the occurrence of δ13 C spikes than contamination by the embedding agent. We conclude that the logistical and interpretational benefits of careful hand drilling may be preferable to resin embedding for micromilling in marine shells, corals or speleothems where growth rate is rapid and the highest resolution is not required.