Triple oxygen isotopic fractionation in CO2-CaCO3-H2O system during CO2-H2O exchange, carbonate precipitation, and acid digestion

Abstract

The triple oxygen isotope composition of carbonate deposits provides a proxy for understanding changes in the past hydrological cycle. The reconstruction relies on knowing the temperature dependence of ¹⁷O and ¹⁸O fractionation factors, ¹⁷α and ¹⁸α, and θ = ln¹⁷α/ln¹⁸α, associated with the carbonate precipitation which can take place at various temperatures in nature. To determine these parameters, we synthesized carbonates at temperatures ranging from 10 °C to ∼50 °C from various substrate waters. The oxygen isotope composition of the waters was obtained using the CO₂-H₂O exchange method at 25 °C and the composition of the carbonate was derived from that of CO₂ generated by reacting the carbonate with phosphoric acid at 25 °C. In both cases, the δ¹⁷O and δ¹⁸O values of the CO₂ were measured. To convert the δ¹⁷O and δ¹⁸O values of the water equilibrated CO₂ to the corresponding values of the water phase, a prior knowledge of the ¹⁷O and ¹⁸O fractionations during the CO₂-H₂O exchange is necessary. Liang et al. (2023) previously determined these fractionations by equilibrating CO₂ with VSMOW2 (δ¹⁷O and δ¹⁸O = 0 ‰) at 25 °C and measuring the resulting isotope values. Building on this work, we present rescaled and refined fractionation values: ¹⁷α = 1.02135 ± 0.00001, ¹⁸α = 1.04122 ± 0.00002, and θ = 0.5230 ± 0.0001. With this knowledge, we calculated the values of θ relating the CO₂ derived from acid–reacted carbonates (precipitated at different temperatures) and the parent water (θ_CO2(carb)-w). The θ_CO2(carb)-w (mean ± SE) at 10 °C, 25 °C, and 35 °C are 0.5232 ± 0.0001, 0.5233 ± 0.0003, and 0.5229 ± 0.0002, respectively. We also did these experiments at 50 °C. However, the θ_CO2(carb)-w (0.5129 ± 0.0004) at 50 °C was likely affected by kinetic isotope fractionation. We also determined the acid digestion fractionations at various temperatures ranging from 0 °C to 110 °C. We did not observe significant temperature dependence in Δ′¹⁷O values of the acid digested CO₂ across the above temperature range, the triple oxygen isotope slope (relating δ’¹⁷O and δ’¹⁸O) is 0.5220 ± 0.0002.