An optimal strategy for determining triple oxygen isotope ratios in natural water using a commercial cavity ring-down spectrometer

Abstract

Triple oxygen isotope ratios have been increasingly acknowledged as useful hydrological tracers, but measuring both 17O/16O and 18O/16O ratios with meaningful precision has been challenging due to the much lower natural abundance of 17O than 16O and even 18O. In this study, in line with the advancement of laser-based techniques and their growing applications, we demonstrate how to optimize the determination of 17O-excess (Δ17O = ln(δ17O+1) - 0.528 × ln(δ18O+1)) in natural water using a commercial wavelength-scanned cavity ring-down spectrometer (WSCRDS). We particularly focus on how to decide the injection numbers of samples and standard solutions utilized for normalization of sample results to the VSMOW-SLAP reference scale. With a measurement strategy aimed at an uncertainty better than 10 per meg (1σ), the Δ17O of Greenland Ice Sheet Precipitation (GISP) is determined to be 24±9 per meg (n = 104), in agreement with previous literature values. By applying this method to Antarctic glacial ice, it is shown to be useful in detecting the seasonality of Δ17O values in Antarctic precipitation. Our approach represents an underlying analytical method that provides guidelines for determining Δ17O from various types of natural waters.