Dual clumped isotope (Δ47, Δ48) values for calcite grown at varying pH and carbonic anhydrase concentrations constrain calcite equilibrium and kinetic isotope effects
This study explores the isotopologues m/z 47 (¹³C¹⁸O¹⁶O, denoted Δ47) and m/z 48 (¹²C¹⁸O₂, denoted Δ48) in CO₂ derived from carbonate minerals, focusing on their temperature dependency, kinetic isotope effects, and distinct reaction pathways. By conducting experiments at four temperatures (5°C, 10°C, 15°C, 25°C) using the enzyme carbonic anhydrase (CA) at pH 8.3, we approached isotopic equilibrium in dissolved inorganic carbon and measured Δ47, Δ48, and δ¹⁸O. Our results were compared with data from Devils Hole cave calcite and existing temperature calibrations, yielding regression equations correlating Δ47 and Δ48 with temperature: Δ48 CDES90 = (0.429 ± 0.010) Δ47 CDES90 - (0.006 ± 0.006); r2 = 0.98; Δ47 I-CDES = (0.037 ± 0.001) × 106T-2 + (0.178 ± 0.009); r2 = 0.99; Δ48 CDES 90 = (0.015 ± 0.0005) × 106T-2 + (0.078 ± 0.006); r2 = 0.98. Further, calcite precipitated at varying temperatures and pH of 8.3 to 11 had kinetic enrichments of Δ47 and depletions of Δ48 and δ18O at pH≥ 9.5, with CA presence leading to shallower kinetic slopes and more efficient Δ48 catalysis. These findings support the kinetic effects from CO₂ hydration/hydroxylation, consistent with theoretical predictions. Additionally, methodological details for Δ48 measurements using the Nu-Perspective mass spectrometer are provided, including shot noise calculations, baseline corrections, and how nonlinearity evolves over time. We show these instruments do not have pressure baseline effects on m/z 48, which is a result of secondary electron suppression on the m/z 48 collector. Therefore, calibration data should be unbiased by these analytical effects.