Evaluation of kinetic effects on clumped isotope fractionation (Δ47) during inorganic calcite precipitation

Considerable efforts have been made to calibrate the Δ47 paleothermometer, which derives from the quantity of 13C–18O bonds in carbon dioxide produced during acid digestion of carbonate minerals versus its expected stochastic abundance, in a range of materials. However the impacts of precipitation rate, ionic strength, and pH on carbonate Δ47 values are still unclear. Here we present a set of 75 measurements of Δ47 values from inorganic calcites grown under well-controlled experimental conditions, where we evaluate the impact on Δ47 values of precipitation rate (logR=1.8–4.4μmol/m2/h), pH (8.3–10.5; NBS pH scale), and ionic strength (I=35–832mM). With the data available and at the current instrumental resolution, our study does not resolve any clear effects of pH, ionic strength, growth rate effects on measured Δ47 when compared in magnitude to the effects on δ18O over most of the ranges of parameters sampled by our analyses. If these relationships exist, they must be smaller than our current ability to resolve them within our dataset. Under our experimental conditions, a Δ47-temperature equation, which is apparently insensitive to variation in pH, precipitation rate, and ionic strength over the range of variables sampled, can be written asΔ47=(0.0387±0.0072)×106/T2+(0.2532±0.0829)(r2=0.9998,p=0.009)where Δ47 values were reported on the absolute Δ47 reference frame after normalizing to conventional 25°C reaction temperature using an acid fractionation factor of −0.00141‰°C−1.