The Influence of Kinetic Growth Factors on the Clumped Isotope Composition of Calcite

Abstract:

Clumped isotope paleothermometry is based on the association of ¹³C and ¹⁸O within carbonate minerals. Although the influence of temperature on equilibrium ¹³C–¹⁸O bond ordering has been studied, recent oxygen isotope studies of inorganic calcite demonstrate that calcite grown in laboratory experiments and in many natural settings does not form in equilibrium with water. It is therefore likely that the carbon and clumped isotope composition of these calcite crystals are not representative of true thermodynamic equilibrium. To isolate kinetic clumped isotope effects that arise at the mineral-solution interface, clumped isotopic equilibrium of DIC species must be maintained. This can be accomplished by dissolving the enzyme carbonic anhydrase (CA) into the solution, thereby reducing the time required for isotopic equilibration of DIC species by approximately two orders of magnitude between pH 7.7 and 9.3.

We conduct calcite growth experiments aimed specifically at measuring the pH-dependence of kinetic clumped isotope effects during non-equilibrium precipitation of calcite. We precipitated calcite from aqueous solution at a constant pH and controlled supersaturation over the pH range 7.7–9.3 in the presence of CA. For each experiment, a gas mixture of N₂ and CO₂ is bubbled through a beaker of solution without seed crystals. As CO₂ from the gas dissolves into solution, calcite crystals grow on the beaker walls. The pH of the solution is maintained by use of an autotitrator with NaOH as the titrant. We control the temperature, pH, the pCO₂ of the gas inflow, and the gas inflow rate, and monitor the total alkalinity, the pCO₂ of the gas outflow, and the amount of NaOH added. A constant crystal growth rate of ~1.6 mmol/m²/hr is maintained over all experiments. Results from these experiments are compared to predictions from a recently-developed isotopic ion-by-ion growth model of calcite. The model describes the rate, temperature and pH dependence of oxygen isotope uptake into calcite under non-equilibrium conditions. Adaptation of the model for clumped isotope uptake under non-equilibrium conditions requires knowledge of the clumped isotopic compositions of DIC species and any mass-dependent kinetic fractionation that arises during ion transport to or from the mineral surface.