Recent studies show that oxygen three isotope measurement (
16O,
17O, and
18O) of water provides additional information for investigating the hydrological cycle and paleoclimate. For determining the
18O/
16O value of water, a conventional CO
2-water equilibration method involves measurement of the ratios of CO
2 isotopologues which were equilibrated with water. However, this long-established technique was not intended to measure the
17O/
16O ratio, primarily because the historic ion correction scheme does not allow for possible deviations from a fixed (and mass-dependent) relationship between
17O/
16O and
18O/
16O isotope ratios. Here, we propose an improved method for obtaining the
17O/
16O isotope ratio of fresh water by the equilibration method and measurement of the 45/44 CO
2 ion abundance ratio. Equations which we formulated for
17O/
16O measurement have two features: first, instead of absolute isotope ratio (R), all equations are formulated in
δ values, measured by isotope ratio mass spectrometry. Second, we include two “assigned”
δ values of water standards in the equations, because the
δ18O are commonly measured against two working standards to normalize the span of the
δ scale. This approach clarifies that the contribution from
17O (
12C
16O
17O
+) to the molecular ion current at mass-to-charge ratio
m/z 45 signal depends not on the absolute
13C/
12C ratio, but on the relative
δ13C differences between the working standards and the sample. The pH value of water affects
δ17O estimation because
δ13C of CO
2 was changed in the water-CO
2 system. We reevaluated this effect using a set of equations, which explicitly includes CO
2 partial pressure effect on pH value. Our new estimation of pH effect is significantly smaller than previously reported value, but it does not alter the main conclusions in the previous study. The method was verified by
δ17O measurements of an international standard reference water (GISP) provided by the IAEA. We applied the method to investigate
17O-excess of the ice core drilled at the Dome Fuji station, Antarctica. A total of 1320 samples from a 130 m section around Marine Isotope Stage 9.3 (∼330,000 years before present) were measured. The error of a measurement for
δ17O is 0.175‰ and that of
17O-excess is 184 per meg. Although these analytical uncertainties hampered accurate estimation of the changes in
17O-excess, the averaged data indicate that
17O-excess around MIS 9.3 was higher than during the subsequent glacial period. This approach can be applied only to fresh water samples, and additional improvements will be needed to measure samples which contains significant amount of carbonate minerals.
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