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

Sang Tae Kim, Tyler B. Coplen, Juske Horita

Research output: Contribution to journalArticle

53 Scopus citations

Abstract

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, αCO2(ACID)-calcite, of 1000lnaCO2(ACID)-calcite = 3:48(103/T) - 1:47 where T is temperature in kelvin. At 25 °C, αCO2(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.

Original languageEnglish
Pages (from-to)276-289
Number of pages14
JournalGeochimica et Cosmochimica Acta
Volume158
DOIs
StatePublished - 2015

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