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.
Highlights
Air trapped in polar ice sheets provides information on past climate and atmosphere
The variations of δO2/N2 and δAr/N2 within the ice sheet from bubbly ice to clathrate hydrate ice at Dome Fuji are reconstructed without post-coring gas-loss signals
Our δO2/N2 data from the clathrate hydrate ice zone agree with the previous data after the gas-loss correction (Kawamura et al, 2007), with much less scatter, demonstrating that the original air composition is preserved in the ice core stored at −50 ◦C
Summary
Air trapped in polar ice sheets provides information on past climate and atmosphere. During the bubble close-off process, relatively small gas molecules such as He, Ne, O2, and Ar are preferentially excluded from air bubbles to open pores (e.g., Bender et al, 1994a; Battle et al, 1996; Severinghaus and Battle, 2006; Huber et al, 2006). Deep ice cores from Antarctic inland show δO2/N2 of −5 ‰ to −10 ‰ relative to the atmosphere due to the close-off fractionation (Bender, 2002; Suwa and Bender, 2008a, b; Kawamura et al, 2007; Extier et al, 2018). Oyabu et al.: Fractionation of O2/N2 and Ar/N2 in the Antarctic ice sheet has been rarely reported, and it is less depleted than δO2/N2 (Sowers et al, 1989; Bender et al, 1995; Severinghaus et al, 2009)
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