Triple oxygen isotope fractionation of carbonate during carbonate precipitation and acid digestion

Stable oxygen isotopic composition (δ¹⁸O) of CO₂ produced from carbonates in natural archives is a useful proxy for paleo precipitation and paleo temperature reconstruction. However, there exist multiple factors controlling the δ¹⁸O values, the applications of the δ¹⁸O alone for paleoclimate studies are thus limited. Anomaly in ¹⁷O in carbonates, expressed by Δ′¹⁷O=1000*ln(δ¹⁷O/1000+1)-λ*1000*ln(δ¹⁸O/1000+1) is another proxy to independently constrain aspects of climatic variables such as precipitation source variation and kinetic effects during carbonate precipitation. However, to use ¹⁷O anomaly for such studies, the triple oxygen isotope fractionation exponent (θ= lnα¹⁷/lnα¹⁸) must be known precisely. Knowledge of this parameter is central to emerging applications of carbonate triple oxygen isotopes to paleoclimate and paleo-hydrology studies. Though a number of theoretical and experimental studies have been carried out in the last few years, there remains no consensus on 𝛳 value for carbonate-water system, likely due to kinetic isotope fractionation during precipitation.

Here, we measured Δ′¹⁷O in synthetic carbonates as well as in the water from which the carbonates are precipitated to check how reliable the Δ′¹⁷O value of the parent water can be reconstructed from the carbonates or carbonate-digested CO₂. To determine θ_{carbonate_CO2-water} for precipitated carbonates, we synthesized carbonates in the laboratory at temperatures ranging from 10 ⸰C to 66 ⸰C using passive/active CO₂ degassing method. Triple oxygen isotope compositions of the water were determined using water-CO₂ equilibration followed by CO₂-O₂ exchange method and of the carbonate (CO₂ liberated by acid digestion) using CO₂-O₂ exchange method. We analyzed our isotope data for their possible kinetic isotope effect and determined the 𝛳_{carbonate_CO2-water} value for precipitated carbonates. We find that most of our synthetic carbonate samples did not attain the equilibrium. The 𝛳_{carbonate_CO2-water} increases as the disequilibrium effect increases. We determined the θ_{carbonate_CO2-water} from the samples precipitating in equilibrium. Furthermore, we do not find any differences in the 𝛳_{carbonate_CO2-water} value for carbonate precipitated in equilibrium at 25 ⸰C and 35 ⸰C. An important issue of using Δ′¹⁷O in carbonates is to resolve the 𝛳_acid for acid digestion which is resolved in the present study. Additionally, we determined the temperature dependent variation in 𝛳_acid and find no significant changes between 0 ⸰C and 70 ⸰C.