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Effects of micromilling on δ18O in biogenic aragonite

Extraction of small aliquots (μg‐mg) from aragonite samples such as corals, sclerosponges, bivalves, and otoliths is required for bulk oxygen isotope analyses. However, aragonite is a metastable polymorph of calcium carbonate that undergoes transformation to calcite, which is the more thermodynamically stable polymorph of CaCO3. This polymorph transformation occurs via heating and/or stress, and such conditions may occur during drilling. During this polymorph transition, exchange with atmospheric or organic C and O is possible. Therefore extracting aliquots during drilling without modification or contamination via isotope exchange is imperative for accurate isotopic analysis. Using a micromill, aliquots were taken from aragonite cod otoliths that had been grown in the laboratory under stable temperature conditions. X‐ray diffraction analysis showed that although 6% of the sample converts to calcite during drilling, there is no significant effect on δ18O.