Effects of micromilling on delta O-18 in biogenic aragonite

Extraction of small aliquots ( mu g- 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 delta O-18.