Normalization of stable isotope data for carbonate minerals: Implementation of IUPAC guidelines

Carbonate minerals provide a rich source of geochemical information because their δ13C and δ¹⁸O values provide information about surface and subsurface Earth processes. However, a significant problem is that the same δ¹⁸O value is not reported for the identical carbonate sample when analyzed in different isotope laboratories in spite of the fact that the International Union of Pure and Applied Chemistry (IUPAC) has provided reporting guidelines for two decades. This issue arises because (1) the δ¹⁸O measurements are performed on CO₂ evolved by reaction of carbonates with phosphoric acid, (2) the acid-liberated CO₂ is isotopically fractionated (enriched in ¹⁸O) because it contains only two-thirds of the oxygen from the solid carbonate, (3) this oxygen isotopic fractionation factor is a function of mineralogy, temperature, concentration of the phosphoric acid, and δ¹⁸O value of water in the phosphoric acid, (4) researchers may use any one of an assortment of oxygen isotopic fractionation factors that have been published for various minerals at various reaction temperatures, and (5) it sometimes is not clear how one should calculate δ¹⁸OVPDB values on a scale normalized such that the δ¹⁸O value of SLAP reference water is -55.5% relative to VSMOW reference water. To enable researchers worldwide to publish the same δ¹⁸O value (within experimental uncertainty) for the same carbonate sample, we have re-evaluated reported acid fractionation factors for calcite at 25, 50, and 75 °C and propose a revised relation for the temperature dependence of oxygen isotopic acid fractionation factor, α_{CO2(ACID)-calcite,} of 1000lnaCO₂(ACID)-calcite = 3:48(10³/T) - 1:47 where T is temperature in kelvin. At 25 °C, α_{CO2(ACID)-calcite} = 1:01025, the most commonly accepted value for this quantity. We propose a normalization protocol in which (1) the internationally distributed carbonate isotopic reference materials NBS 18 and NBS 19 are interspersed among carbonate samples analyzed by treatment with phosphoric acid, (2) the δ¹⁸O values of the calcite reference materials and the carbonate samples are calculated, respectively, by using the aCO₂(ACID)-calcite relation above and oxygen-isotope acid fractionation factors appropriate for the sample mineralogy and reaction temperature, (3) the δ¹⁸O values of solid carbonate samples are determined on the VPDB scale (δ¹⁸OVPDB) with IUPAC-recommended scale expansion such that the δ¹⁸O of SLAP reference water is -55.5 & relative to VSMOW reference water by normalizing δ¹⁸O values of carbonate samples with 2014-IUPAC-recommended δ¹⁸O values of NBS 18 and NBS 19, and (4) δ¹⁸O values on the VPDB scale are converted to δ¹⁸O values on the VSMOW-SLAP scale by using IUPAC recommendations. To ease calculations in the protocol, a software application titled "Carbon and Oxygen Isotopic Normalization Tool for Carbonates" is available that relies upon IUPAC-recommended δ13C and δ¹⁸O values of carbonate isotopic reference materials.