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Project Progress

The study of “clumped isotopes” involves quantification of the preferential bonding between rare stable isotopes (e.g., ¹³C bound to ¹⁸O) in multiply substituted isotopologues as compared to stochastic (random) bonding predicted for that chemical compound or mineral lattice. Because clumped isotopes are defined by two or more isotopes of an element of relatively high mass bonded together, thermodynamic stability is further increased compared to singly substituted isotopologues. This stability is inherent to a material and is independent of the bulk isotope composition of the mineral, as well as the composition of a fluid from which the mineral formed. This later component sets clumped isotope thermometry apart from the traditional stable isotope approaches, where the fluid composition must be estimated to obtain a temperature. Essentially, clumped isotopes are an “internal thermometer”. Recent work intended to develop this thermodynamic property as a temperature record has revealed kinetic and vital effects that can produce inaccurate temperature signals. A number of experimental and field-based studies as well as theoretical calculations have been performed in an attempt to reliably calibrate the temperature dependence of ¹³C-¹⁸O bonding during mineral formation. Figure 1 illustrates the wide variability of such calibrations.

Project 2C has been designed to explore theories put forth to explain calibration discrepancies. Specifically, kinetic isotope effects are suspected to cause disequilibrium and prevent accurate temperature records and full ¹³C-¹⁸O bonding. These experiments were designed to systematically probe formation rate effects on ¹³C-¹⁸O bonding during calcite mineral lattice assembly from aqueous solutions. Calcite has been synthetically precipitated using a chemostat technique whereby solution conditions such as ion concentrations, pH, CO₂ partial pressure, and saturation index are held constant during mineral formation. This method is designed to maintain a constant growth rate and level of supersaturation as well as provide a precipitation mechanism that does not rely on homogenous nucleation, CO₂ diffusion, or CO₂ degassing. These conditions are necessary in order to maintain the constant precipitation rate desired during a given experiment.

Forty-six synthetic calcite precipitations were performed in 2014. Most calcite formed at 20°C. Reactant concentrations ranged from 6 mM to 10 mM for both Ca²⁺ and HCO_3-. Solution pH during precipitation ranged from 6.5 to 7 resulting in saturation indices between 1 and 2. Experiments lasted from three to eight days resulting in precipitation log rates ranging from approximately 2 to 2.5 μmol/m²/hr. Figure 2 is a plot of ¹³C-¹⁸O bonding (denoted Δ₄₇, which is the deviation of ¹³C¹⁸O¹⁶O species relative to random distribution) as a function of formation rate and shows values for experiments that have been analyzed multiple times. Δ₄₇ values determined for 20°C experiments averaged 0.639 ± 0.014‰ and do not correlate with precipitation rate over the range investigated. Oxygen isotope fractionation factors (α_{calcite-water}) were found to be slightly higher than those found previously, suggesting precipitation conditions approaching equilibrium. The preliminary results provide evidence that can be used to deconvolute and identify physiochemical conditions and processes that lead to disequilibrium in ¹³C-¹⁸O bonding during carbonate mineral formation.

Most experiments conducted at 10°C and 30°C have only been analyzed once for Δ₄₇. The ¹³C-¹⁸O bonding during these experiments follow the general trend of decreasing frequency with increasing formation temperature. Replicate analyses are. Additional work planned for this project includes the execution of further experiments and analyses that will expand the range of temperatures and/or precipitation rates explored.