[A-CC27] Ice cores and paleoenvironmental modeling

[ACC27-P03] Effect of bubbles of SE-Dome ice core on Near-Infrared reflectance and dielectric anisotropy.

*Mai Matsumoto^1,2, Yoshinori Iizuka², Teruo Aoki³, Satoru Adachi⁴, Tomotaka Saruya³, Akihiro Hachikubo⁵, Kaoru Kawakami², Takeshi Saito², Sumito Matoba², Shuji Fujita^3,6 (1.Graduate School of Environmental Science, Hokkaido University, 2.Institute of Low Temperature Science, Hokkaido University, 3.National Institute of Polar Research, 4.Snow and Ice Research Center, NIED, 5.Kitami institute of Technology, 6.The Graduate University for Advanced Studies, SOKENDAI)

Near-Infrared Reflectance (hereafter NIR) and dielectric anisotropy are methods to elucidate the physical properties of ice cores. The NIR and dielectric anisotropy (difference in permittivity in two directions in ice core) of snow and ice are widely used methods, as they can elucidate the size and shape of snow particles and bubbles, and the degree of concentration of the c axis of ice crystals ^(1,2). The values of these physical quantities depend on the degree of metamorphosis at the surface snow and densification of the snowpack ⁽³⁾. A mechanism for how these physical quantities is preserved in close-off ice, where the O₂/N₂ ratio in bubbles can provide an indicator of summer snowpack metamorphosis, has been proposed by studies in low accumulation area in Antarctica ^(4,5). However, studies in high accumulation area are less well known. Two ice cores drilled in the high accumulation area (approximately 1 m w.e. a^-1) in the south-eastern part of Greenland are ice cores that provide seasonal resolution of accumulation rate and air temperature for the periods 1960–2014 (approximately 60 m long; hereafter SE-Dome I core) and 1799–2020 (approximately 250 m; hereafter SE-Dome II core) ⁽⁶⁾. These ice cores have a mean annual layer thickness of about 1 m after close-off, and it is possible to analyses thinner layers than seasonal resolution. In this study, we consider how close-off ice preserves of the near-surface metamorphosis by measuring NIR, dielectric anisotropy and investigating the mm scale bubbles of SE-Dome I and II cores. Seasonal variations in NIR and dielectric anisotropy were found in SE-Dome I and II cores. In the SE-Dome II core, NIR decreases in the spring/summer layer, with larger decreases after 1990 (0–50 m depth) and 1970–1950 (76–100 m depth), when the Arctic region is warmer ⁽⁷⁾. The dielectric anisotropy is smaller in the winter layer than in the spring layer after 1990, and the seasonal variation is small between 1950 and 1940. The depth in 1950 was already close-off, and the decrease in NIR seen in spring/summer is thought to be related to characteristics of air bubbles. Microscopic observations of bubbles in the ice of the 1950 sample show that the layers in which the NIR decreases in spring and summer have a rounder and smaller bubble shape than around the layer. Bubbles formed at the shallow firn are known to shrink more because they are affected more densification ⁽⁸⁾. In other words, the round, small bubbles are old and formed at the shallow firn, with a greater degree of snow metamorphosis near the surface. In SE-Dome ice cores in the high accumulation area, the NIR values are suggested to be an indicator of the degree of metamorphosis at the surface of the snowpack after close-off. The variation in dielectric anisotropy of these layers will also be reported on the presentation.

References
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(2)Fujita et al., 2014: doi:10.3189/2014JoG14J006.
(3)Stevens et al., 2023: doi:10.1017/jog.2023.87.
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(6)Kawakami et al., 2023: doi.org/10.1029/2023JD038874.
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