Isotopic behavior during the aragonite-calcite transition: Implications for sample preparation and proxy interpretation

At the Earth's surface, aragonite, a polymorph of calcium carbonate, is thermodynamically unstable, and readily converts to calcite, a process accelerated by heating andmechanical stress. This study has investigated the behavior of the C-13/C-12, O-18/O-16 ratios, and the clumped isotope temperature proxy (Delta(47)) during this mineral transition. Aragonite samples were partially or completely converted to calcite in an oven (normal atmospheric composition and pressure) or in a sealed tube in the presence of a vacuum or pure CO2 of varying isotopic compositions. Other samples were converted from aragonite to calcite using a drill. These experiments demonstrate a relationship between the delta C-13 and delta O-18 values and percent transition from aragonite to calcite. The Delta(47) value of the aragonite changed independently from the mineral transition at temperatures as low as 125 degrees C and was accompanied by a 0.5 parts per thousand decrease in the delta O-18 value. This change occurred irrespective of the presence of CO2 suggesting that water trapped in the mineral may facilitate some of this alteration. Results show that sampling techniques, such as drilling, produce sufficient heat to convert aragonite to calcite and can considerably alter Delta(47). Even small degrees of conversion (similar to 10%) can increase clumped isotope derived paleotemperature estimates by nearly 10 degrees C. (C) 2016 Elsevier B.V. All rights reserved.