Fractionation of O<sub>2</sub>∕N<sub>2</sub> and Ar∕N<sub>2</sub> in the Antarctic ice sheet during bubble formation and bubble–clathrate hydrate transition from precise gas measurements of the Dome Fuji ice core

Abstract

Abstract. The variations of δO2/N2 and δAr/N2 in the Dome Fuji ice core were measured from 112 m (bubbly ice) to 2001 m (clathrate hydrate ice). Our method, combined with the low storage temperature of the samples (−50 ∘C), successfully excludes post-coring gas-loss fractionation signals from our data. From the bubbly ice to the middle of the bubble–clathrate transition zone (BCTZ) (112–800 m) and below the BCTZ (>1200 m), the δO2/N2 and δAr/N2 data exhibit orbital-scale variations similar to local summer insolation. The data in the lower BCTZ (800–1200 m) have large scatter, which may be caused by millimeter-scale inhomogeneity of air composition combined with finite sample lengths. The insolation signal originally recorded at the bubble close-off remains through the BCTZ, and the insolation signal may be reconstructed by analyzing long ice samples (more than 50 cm for the Dome Fuji core). In the clathrate hydrate zone, the scatter around the orbital-scale variability decreases with depth, indicating diffusive smoothing of δO2/N2 and δAr/N2. A simple gas diffusion model was used to reproduce the smoothing and thus constrain their permeation coefficients. The relationship between δAr/N2 and δO2/N2 is markedly different for the datasets representing bubble close-off (slope ∼ 0.5), bubble–clathrate hydrate transformation (∼1), and post-coring gas loss (∼0.2), suggesting that the contributions of the mass-independent and mass-dependent fractionation processes are different for those cases. The method and data presented here may be useful for improving the orbital dating of deep ice cores over the multiple glacial cycles and further studying non-insolation-driven signals (e.g., atmospheric composition) of these gases.

δ18O of O2 in a Tibetan Ice Core Constrains Its Chronology to the Holocene

“…In addition, we observe increasing Δ δ 18 O values with decreasing Δ δ Ar/N 2 and Δ δ O 2 /N 2 , while Δ δ 15 N is nearly unaffected as discussed above (Figures 2c–2d). The observed slope of Δ δ 18 O – Δ δ O 2 /N 2 is −0.0081, which is similar to the previously reported values of Siple Dome and Dome Fuji (Oyabu et al., 2021; Severinghaus et al., 2009). The correlation between Δ δ 18 O – Δ δ O 2 /N 2 and Δ δ 18 O – Δ δ Ar/N 2 for the “adjacent samples” provided strong evidence for post‐coring gas loss, which must be corrected before δ 18 O atm dating.…”

“…The fractionation of δ Ar/N 2 and δ O 2 /N 2 during this process is tightly correlated with a slope of ∼0.5 (Oyabu et al., 2021). In well‐preserved polar ice cores, the observed typical ranges of δ O 2 /N 2 and δ Ar/N 2 are −20 – 0‰ and −10‰ – 0‰, respectively (Figures 1a–1b, Bender, 2002; Oyabu et al., 2021). Therefore, δ O 2 /N 2 and δ Ar/N 2 values and their relative slopes could serve as indicators of core quality and extent of post depositional alteration.…”

“…In addition to low TAC, δ Ar/N 2 is another indicator for melting. Generally, δ Ar/N 2 values of ice core trapped air are close to zero or slightly lower than zero (Oyabu et al., 2021). Our data show high δ Ar/N 2,grav and large δ 18 O grav variability for samples with low TAC (Figures 1c–1d).…”

“…In general, smaller molecules, such as Ar or O 2 , and lighter isotopes, such as 16 O, escape faster from gas bubbles (Bender et al, 1995;Severinghaus et al, 2009;Suwa & Bender, 2008). Previous studies suggest a strong correlation between δAr/N 2 and δO 2 /N 2 during gas loss, with different slopes ranging from ∼0.2 to 0.5 (Bender et al, 1995;Oyabu et al, 2021;Severinghaus et al, 2009;Sowers et al, 1989). For oxygen isotopes, size-dependent fractionation would not lead to significant fractionations on 18 O/ 16 O, as they have identical diameters (Huber et al, 2006;Severinghaus & Battle, 2006).…”

Gas isotope thermometry in the South Pole and Dome Fuji ice cores provides evidence for seasonal rectification of ice core gas records

“…The transformation to clathrates occurs heterogeneously throughout the core, increasing the scatter in our δAr/N 2 grav measurements. Both the enrichment and increased scatter of elemental ratios in the BCTZ have been noted in many ice cores (Suwa and Bender, 2008a, b;Kobashi et al, 2008;Shackleton et al, 2019), but recent work appears to confirm that there is no appreciable isotope fractionation associated with clathration (Oyabu et al, 2021).…”