Oxygen Isotope Phosphoric Acid Fractionation Factor and Uncertainty on the δ18O Measurements of Calcite

At present, the isotope ratio O-18/O-16 (in the text expressed as delta O-18) of carbonate (CO32-) is usually determined by isotope ratio mass spectrometry measuring the CO2 gas produced dissolving the CO32--bearing substance in concentrated H3PO4. As with any analytical data, the delta O-18 values of carbonate are also affected by uncertainty which must be considered mainly when data from different laboratories are compared. Usually, scientific papers report only repeatability and/or reproducibility of analytical results, which, have scarce significance for data comparison. With the aim of evaluating the overall uncertainty for new analytical data for low-Mg calcites, in this paper we reconsidered the delta O-18 data, which are reported in the literature. Two kinds of uncertainty must be taken into account: (1) that due to the calibration of the delta values using international standards (prediction uncertainty), (2) that due to small differences in the chemistry of low-Mg calcites. The two uncertainties must be added when comparing data produced in the same or different laboratories. We found that this overall uncertainty cannot be lower than about 0.2 parts per thousand. Thus, uncertainty lower than 0.2 parts per thousand, sometimes reported in the literature, is misleading.