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

• The mechanisms by which the isotopic composition of aragonite alters are investigated.
• As aragonite converts to calcite it can exchange C and O with available CO2 gas.
• During heating, the ∆47 value of aragonite alters more rapidly than mineralogy.
• Drilling biogenic aragonite can produce sufficient heat to facilitate this transition.

At the Earth's surface, aragonite, a polymorph of calcium carbonate, is thermodynamically unstable, and readily converts to calcite, a process accelerated by heating and mechanical stress. This study has investigated the behavior of the 13C/12C, 18O/16O ratios, and the clumped isotope temperature proxy (∆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 δ13C and δ18O values and percent transition from aragonite to calcite. The ∆47 value of the aragonite changed independently from the mineral transition at temperatures as low as 125 °C and was accompanied by a 0.5‰ decrease in the δ18O 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 ∆47. Even small degrees of conversion (~ 10%) can increase clumped isotope derived paleotemperature estimates by nearly 10 °C.