Improving continuous-flow analysis of triple oxygen isotopes in ice cores: insights from replicate measurements

Abstract

Abstract. Stable water isotope measurements from polar ice cores provide high-resolution information about past hydrologic conditions and are therefore important for understanding earth's climate system. Routine high-resolution measurements of δ18O, δD, and deuterium excess are made by continuous-flow analysis (CFA) methods that include laser spectrometers. Cavity ring-down laser spectroscopy (CRDS) allows for simultaneous measurements of all stable water isotopes, including δ17O and 17O excess (Δ17O); however, the limitations of CFA methodologies for Δ17O are not well understood. Here, we describe a measurement methodology for all stable water isotopes that uses a CFA system coupled with a CRDS instrument. We make repeated measurements of an ice-core section using this method to explore the reproducibility of CFA–CRDS measurements for Δ17O. Our data demonstrate that the CFA–CRDS method can make high-precision measurements of Δ17O (< 5 per meg at averaging times > 3000 s). We show that the variations within our CFA ice-core measurements are well matched in magnitude and timing by the variations within the discrete CRDS measurements; we find that calibration offsets generate most of the variability among the replicate datasets. When these offsets are accounted for, the precision of CFA–CRDS ice-core data for Δ17O is as good as the precision of Δ17O for continuous reference water measurements. We demonstrate that this method can detect seasonal variability in Δ17O in Greenland ice, and our work suggests that the measurement resolution of CFA–CRDS is largely defined by the melt and measurement rate. We suggest that CFA–CRDS has the potential to increase measurement resolution of δ17O and Δ17O in ice cores, but also highlight the importance of developing calibration strategies with attention to Δ17O.

Advances in triple oxygen isotope analysis and applications for ice-core paleoclimate science

“…daily) data calibration against multiple reference waters, adopting a new calibration method that utilizes more reference water measurements than typical crDS strategies. Davidge et al (2022) demonstrated that cFA-crDS for ∆ 17 O performs as well as discrete methods by measuring replicate sections of an ice core from Greenland; annually resolved data from that study are provided in Figure 1c-d. they also found that, though small, the greatest source of uncertainty for ∆ 17 O by cFA-crDS is the calibration technique.…”

“…Emanuelsson et al 2015). recent work (Davidge et al 2022;Steig et al 2021) demonstrates that cFA-crDS for all stable water isotopes can greatly reduce the analysis time for ∆ 17 O and it can therefore improve the time resolution of ∆ 17 O measurements. cFA-crDS methods will be useful for improving the temporal and spatial resolution of ∆ 17 O to characterize the natural variability of meteoric ∆ 17 O. recent work demonstrates that cFA-crDS for ∆ 17 O can indeed improve the resolution of ∆ 17 O observations with high precision (<10 per meg), especially when cFA for ∆ 17 O is developed with specific attention to calibration strategies (Davidge et al 2022;Steig et al 2021).…”

“…recent work (Davidge et al 2022;Steig et al 2021) demonstrates that cFA-crDS for all stable water isotopes can greatly reduce the analysis time for ∆ 17 O and it can therefore improve the time resolution of ∆ 17 O measurements. cFA-crDS methods will be useful for improving the temporal and spatial resolution of ∆ 17 O to characterize the natural variability of meteoric ∆ 17 O. recent work demonstrates that cFA-crDS for ∆ 17 O can indeed improve the resolution of ∆ 17 O observations with high precision (<10 per meg), especially when cFA for ∆ 17 O is developed with specific attention to calibration strategies (Davidge et al 2022;Steig et al 2021). Steig et al (2021) measured the lower 1200 m of the South Pole ice core by cFA-crDS for all stable water isotopes, revealing significant millennial-scale variability in ∆ 17 O that is not observed in coarser records of ∆ 17 O from other ice-core sites, but that is coincident with other climatic events recorded by d and δ 18 O (Figs.…”

A versatile water vapor generation module for vapor isotope calibration and liquid isotope measurements

“…The system developed by Gkinis et al (2011) used a combination of a peristaltic pump and tuned back-pressure to push water through a silica capillary into a micro-Tee heated to 170 deg C. A similar system has also been deployed and used for calibration of water vapor isotope measurements (Benetti et al, 2017;Steen-Larsen et al, 2014). This system has also shown to be reliable for 17O-excess measurements in continuous-flow analysis of ice core samples (Davidge et al, 2022).…”