The inversion of aragonite to calcite during the sampling of skeletal archives: Implications for proxy interpretation

RATIONALE: Understanding changes in Earth's oceans and climate requires the reliable application of paleo-proxies. However, inconsistencies between individual delta O-18 records within biogenic specimens commonly have significant impacts on environmental reconstructions. This study addresses the stable isotope variability associated with sample milling from aragonitic organisms commonly used for paleoclimate studies. METHODS: Aragonite samples were hand-ground and milled from sclerosponge, coral, and mollusc specimens using a computerized micromill. An X-ray diffractometer was used to analyze sample mineralogy prior to measurement of delta O-18 and delta C-13 values via isotope ratio mass spectrometry of CO2 gases provided from the samples by a Kiel III device. Possible influences on the Sr/Ca ratios were assessed on a sclerosponge through paired elemental analysis by inductively coupled plasma optical emission spectrometry. RESULTS: Analyses revealed up to 36% inversion to calcite in milled samples that correlated with a decrease in the delta O-18 value of 0.02 parts per thousand per 1% inversion. Replicate sclerosponge transects yielded similar trends in delta C-13 values and Sr/Ca ratios, although the delta O-18 values showed irregular variations consistent with those measured for the inversion of individual organisms during milling. CONCLUSIONS: While the delta C-13 values and Sr/Ca ratios of milled samples were largely consistent, the delta O-18 values co-varied with the inversion of aragonite to calcite suggesting significant implications for the resulting temperature and salinity reconstructions from aragonitic archives. The effect appears to be density-driven and, given that the skeletal density tends to vary seasonally in organisms such as corals, would subsequently mask temperature-induced changes in skeletal delta O-18 values. Copyright (C) 2015 John Wiley & Sons, Ltd.