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) at high precision. 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 scatters, which may be caused by mm-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. In the clathrate hydrate zone, the scatters around the orbital-scale variability decrease 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/Ν2 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 contribution 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.

Discovery of argon in air-hydrate crystals in a deep ice core using scanning electron microscopy and energy-dispersive X-ray spectroscopy

“…Then, considering the fractionation effect, we expect that such an air-hydrate crystal would enclathrate more Ar in the crystal. Although the air components would be changed to the original one below BHTZ (Oyabu and others, in review, 2021), we chose higher possibility conditions that were likely to find Ar in the air-hydrate crystals.…”

“…For example, of the N 2 and O 2 components, the O 2 concentration becomes higher in the air-hydrate crystals than that in bubbles in the bubble-to-air hydrate transition zone (BHTZ), which has been identified by Raman spectroscopy as the change of the peak intensity ratio between O 2 and N 2 (Nakahara and others, 1988; Ikeda and others, 1999; Ikeda-Fukazawa and others, 2001). This bubble–air hydrate fractionation effect would be caused by the dissociation pressure of O 2 hydrate is lower than that of N 2 hydrate (about 6.5 MPa vs 9 MPa at −20 °C, Miller, 1969) and the permeation effect of O 2 in ice matrix is larger than that of N 2 (Salamatin and others, 2001; Ikeda-Fukazawa and others, 2005; Oyabu and others, in review, 2021).…”

“…Atmospheric components in ice cores are usually measured by extracting all air from the ice by either melting or by physically destroying the bulk ice core sample (100 g or less). Concerning Ar, detailed analyses of firn air and ice cores have found Ar subject to size-dependent and mass-dependent fractionation during the close-off and post-coring gas loss (Bender and others, 1995; Huber and others, 2006; Severinghaus and Battle, 2006; Severinghaus and others, 2009; Kobashi and others, 2015; Oyabu and others, in review, 2021). At depths below the bubble–air-hydrate transition zone, Ar is considered to be included in air-hydrate crystals, but has yet to be confirmed directly by microscopic analysis.…”

“…For example, the data on N 2 /O 2 fractionation in the transition zone obtained with Raman spectra measurements (Ikeda and others, 1999; Ikeda-Fukazawa and others, 2001) were used to constrain mathematical models describing the formation and growth of air-hydrate crystals in ice sheets and to estimate the permeation coefficients of N 2 and O 2 molecules in ice (Salamatin and others, 2001, 2003), later on these estimates of permeation coefficients were used to simulate the diffusive smoothing of the delta O 2 /N 2 orbital signal in old ice (e.g. Oyabu and others, in review, 2021). Knowledge of permeation coefficients (including that of Ar) is also important for assessing the selective gas loss from ice cores after coring (e.g.…”

Laser-induced sublimation extraction for centimeter-resolution multi-species greenhouse gas analysis on ice cores

“…The variability of 1-2 ppm within a 10 cm long ice sample is noticeably higher than in our standard-over-ice experiments, reflecting true variability in the concentration in the ice, as to be expected from the stochastic nature of bubble trapping at the firn-ice transition. The sample at 768.35 m depth is an outlier in this regard with a much higher intra-sample variability, likely due to the fact that this sample lies in the bubble-to-clathrate transition zone (Neff, 2014), where layered early clathratization occurs, and differently fast permeation rates between bubbly layers and clathrate layers for different gas species lead to the centimeter-scale variability in the gas composition (Lüthi et al, 2010;Oyabu et al, 2021).…”