Triple Isotopic Composition of Oxygen in Water from Ice Cores

Significance of the Topic

The triple isotopic composition of oxygen in water offers an advanced proxy to reconstruct past climate and hydrological conditions. While traditional δ18O and δD measurements have long informed paleotemperature and moisture source studies, the derived parameters d-excess and especially 17O-excess provide enhanced sensitivity to relative humidity at evaporation, critical for interpreting ice core records.

Objectives and Overview of the Study

This application note presents the methodological basis, calibration strategy and practical application of high-precision measurements of δ18O, d-excess and 17O-excess in polar ice cores. The work aims to demonstrate the added value of 17O-excess for constraining source-region humidity, illustrated by seasonal snow analyses at the NEEM drilling site in Greenland.

Methodology

Water samples are converted to molecular oxygen via fluorination using cobalt trifluoride at 370 °C. The generated O2 is purified through serial liquid-nitrogen traps and transferred to a custom cold finger before analysis.

• Dual-inlet IRMS measurement of masses 32, 33 and 34 over two 16-cycle sequences per sample.
• Pressure tuning to 12 V on Delta V and 6 V on MAT 253 instruments.
• Calibration against VSMOW2 (δ18O = 0 ‰) and SLAP2 (δ18O ≈ –55.5 ‰) to correct raw 17O-excess values.
• Daily checks with internal water standards to ensure precision of ~5 ppm on 17O-excess.

Instrumentation Used

• Cobalt fluoride fluorination line for H2O to O2 conversion
• Thermo Scientific Delta V IRMS
• Thermo Scientific MAT 253 IRMS
• Cryogenic liquid-nitrogen traps and custom stainless-steel manifold

Main Results and Discussion

Seasonal δ18O cycles at NEEM closely follow local air temperature. D-excess peaks in autumn but remains influenced by temperature along the moisture transport path. In contrast, 17O-excess exhibits a clear anti-correlation with δ18O and a strong dependence on relative humidity in the tropical North Atlantic source region. Isotope-enabled modeling confirms a decrease of ~1 ppm in 17O-excess for each 1% increase in humidity at evaporation.

Benefits and Practical Applications

• Provides a more direct tracer of source-region humidity, reducing temperature bias inherent in d-excess.
• Enables robust model–data comparisons for climate and hydrological cycle studies.
• Supports standardization across laboratories through two-point SMOW/SLAP calibration of 17O-excess.

Future Trends and Applications

Expanding triple-isotope analyses across multiple ice cores will refine global humidity reconstructions. Integration into atmospheric general circulation models with explicit isotopic schemes can improve paleoclimate simulations. Further work on fractionation coefficients during vapor-solid transitions will enhance calibration and interpretation.

Conclusion

High-precision measurements of δ18O, d-excess and 17O-excess in ice cores represent a powerful toolkit for paleohumidity reconstruction. Rigorous calibration and standardized methodologies ensure consistency across instruments and laboratories, paving the way for improved understanding of past water cycle dynamics.

Reference

1. Landais A., Barkan E., Luz B. 2012. Deglaciation records of 17O-excess in East Antarctica. Climate of the Past 8(1):1–16.
2. Barkan E., Luz B. 2005. High-precision measurements of 17O/16O and 18O/16O in H2O. Rapid Communications in Mass Spectrometry 19:3737–3742.
3. Barkan E., Luz B. 2007. Diffusivity fractionations of H216O/H217O and H216O/H218O in air. Rapid Communications in Mass Spectrometry 21:2999–3005.
4. Meijer H.A.J., Li W.J. 1998. Use of electrolysis for accurate 17O and 18O isotope measurements in water. Isotopes in Environmental and Health Studies 34:349–369.
5. Jouzel J., Merlivat L., Lorius C. 1982. Deuterium excess in East Antarctic ice core suggests higher relative humidity during last glacial maximum. Nature 299:688–691.