On the measurement of oxygen and carbon isotopic ratios of carbonates

Oxygen and carbon isotopic ratios of two working standards of our laboratory and three other standards were measured by McCrea's technique and the new McKINNEY type mass spectrometer in this Institute. The reproducibility of δ(13)C measurement is about ±0.2‰. Some systematic differences in δ(18)O values were observed among the three measurements in Apr.-May, July, and Aug., 1971. Because these systematic differences are supposed to be caused by the imperfection in the processes of decomposition of carbonates by phosphoric acid, the reproducibility of δ(18)O measurement may be ±O.2‰ in most cases. The calibrations of our working standard, CK-13, against the PDB standard were carried out on the assumption that δ(18)O and δ(13)C of CK-13 are -1.75‰ and +O.54‰ relative to PDB, respectively. The accuracy of these calibrations is about ±O.2‰ as shown by the interlaboratory comparison of some standards (Table 3). The δ(18)O values of the two working standards relative to PDB are independently recalculated relative to SMOW based on the two assumptions (Table 4). One of which is that a standard water MSA-2 is -8.20‰ relative to SMOW, and the other is that the δ(18)O of CK-13 is -1.75‰ relative to PDB. The results of both calculations agree within 0.4‰