Continuous-Flow Analysis of δ17O, δ18O, and δD of H2O on an Ice Core from the South Pole

Abstract

The δD and δ18O values of water are key measurements in polar ice-core research, owing to their strong and well-understood relationship with local temperature. Deuterium excess, d, the deviation from the average linear relationship between δD and δ18O, is also commonly used to provide information about the oceanic moisture sources where polar precipitation originates. Measurements of δ17O and “17O excess” (Δ17O) are also of interest because of their potential to provide information complementary to d. Such measurements are challenging because of the greater precision required, particularly for Δ17O. Here, high-precision measurements are reported for δ17O, δ18O, and δD on a new ice core from the South Pole, using a continuous-flow measurement system coupled to two cavity ring-down laser spectroscopy instruments. Replicate measurements show that at 0.5 cm resolution, external precision is ∼0.2‰ for δ17O and δ18O, and ∼1‰ for δD. For Δ17O, achieving external precision of <0.01‰ requires depth averages of ∼50 cm. The resulting ∼54,000-year record of the complete oxygen and hydrogen isotope ratios from the South Pole ice core is discussed. The time series of Δ17O variations from the South Pole shows significant millennial-scale variability, and is correlated with the logarithmic formulation of deuterium excess (dln), but not the traditional linear formulation (d).

Reconstruction of Temperature, Accumulation Rate, and Layer Thinning From an Ice Core at South Pole, Using a Statistical Inverse Method

“…obtained from replicate continuous‐flow measurements made on the South Pole ice core as reported in Steig et al. (2021). Results from the sensitivity tests (Section 5) using other firn models, and using independent δ 15 N constraints, yield the same result: slopes vary from 0.97 to greater than 1.2‰°C −1 .…”

“…(2017) and Steig et al. (2021). We measured δ 18 O and δ D for the entirety of the core and δ 17 O from a depth of 556 m through the bottom of the core.…”

“…We use discrete data sections of 250 years. We calculate the diffusion length, σ, for each section by fitting its power spectrum with a model of a diffused power spectrum and a two-component model of the measurement system noise: (Steig et al, 2021) from the South Pole ice core (SPC14), shown as discrete 100-year averages for clarity, on the SP19 ice timescale (Winski et al, 2019).…”

“…We take advantage of two timescales for SPC14, one for the ice (Winski et al., 2019) and one for the gas enclosed within it (Epifanio et al., 2020), to obtain an empirical measure of the gas‐age ice‐age difference (Δage). We also use high‐resolution measurements of δ 17 O, δ 18 O, and δ D of ice (Steig et al., 2021) to obtain water‐isotope diffusion lengths.…”

“…Figure 1. High-resolution δ 18 O record(Steig et al, 2021) from the South Pole ice core (SPC14), shown as discrete 100-year averages for clarity, on the SP19 ice timescale(Winski et al, 2019).…”