10:45 AM - 12:15 PM
[ACC26-P06] Evolution of firn density and microstructure near the surface at multiple sites around Dome Fuji, Antarctica
Keywords:firn, density, microstructure, Dome Fuji, surface process, Antarctica
Density, microstructure, and their layering in polar firn are important factors controlling the densification rate over firn column and the depth range of bubble close-off in deep firn. Thus, it is essential to better understand the initial formation and development of layered density and microstructural variations in the near-surface firn. However, at low accumulation sites (< ~60 mm w.e. yr-1), there have only been a few continuous and high-resolution data of density and microstructure (e.g., Dome C (Calonne et al., 2017), Kohnen station (Moser et al., 2020)), partly because near-surface firn (~0–5 m) is often too fragile to collect and analyze in high quality. Here, we performed continuous and high-resolution (2.5–20-mm increment) analyses of density and microstructure on six firn cores with high recovery rates (few defects) collected around Dome Fuji (Table 1 and Figure 1). The properties measured include (1) density measured by a gamma-ray absorption method, (2) microwave permittivity ε (a proxy for density), (3) dielectric anisotropy Δε (a proxy for vertical elongation of ice and pores), and (4) near-infrared reflectance (a proxy for specific surface area).
We find large variation in density and the absence of overall density increase within ~0–2 m around Dome Fuji. According to semi-empirical densification models, small densification rate is expected from the small overburden pressure near the surface, but the modeled densification rate is still clearly larger than the data. We also find the developments of vertical elongation of ice and pores, and decrease of SSA within ~0–2 m where seasonal and diurnal temperature variations create large temperature gradients. From these ovservations, we suggest that temperature-gradient metamorphism is an important factor for the formation and development of firn microstructure and layering near the surface, and that part of sublimated water vapor in the firn may escape to the atmosphere to lose the firn mass (density decrease) near the surface. Furthermore, we find more developed vertical elongation and SSA decrease at low accumulation sites than at high accumulation sites, suggesting that the exposure time of a firn layer to the temperature gradients near the surface may determine the magnitude of near-surface metamorphism.
Reference:
Calonne, N. et al.: The layered evolution of fabric and microstructure of snow at Point Barnola, Central East Antarctica, Earth and Planetary Science Letters, 460, 293–301, 2017.
Moser, D. E. et al.: Microstructure of Snow and Its Link to Trace Elements and Isotopic Composition at Kohnen Station, Dronning Maud Land, Antarctica, Front. Earth Sci., 8, 23, 2020.
Oyabu, I. et al.: Temporal variations of surface mass balance over the last 5000 years around Dome Fuji, Dronning Maud Land, East Antarctica, Climate of the Past, 19, 293–321, 2023.
We find large variation in density and the absence of overall density increase within ~0–2 m around Dome Fuji. According to semi-empirical densification models, small densification rate is expected from the small overburden pressure near the surface, but the modeled densification rate is still clearly larger than the data. We also find the developments of vertical elongation of ice and pores, and decrease of SSA within ~0–2 m where seasonal and diurnal temperature variations create large temperature gradients. From these ovservations, we suggest that temperature-gradient metamorphism is an important factor for the formation and development of firn microstructure and layering near the surface, and that part of sublimated water vapor in the firn may escape to the atmosphere to lose the firn mass (density decrease) near the surface. Furthermore, we find more developed vertical elongation and SSA decrease at low accumulation sites than at high accumulation sites, suggesting that the exposure time of a firn layer to the temperature gradients near the surface may determine the magnitude of near-surface metamorphism.
Reference:
Calonne, N. et al.: The layered evolution of fabric and microstructure of snow at Point Barnola, Central East Antarctica, Earth and Planetary Science Letters, 460, 293–301, 2017.
Moser, D. E. et al.: Microstructure of Snow and Its Link to Trace Elements and Isotopic Composition at Kohnen Station, Dronning Maud Land, Antarctica, Front. Earth Sci., 8, 23, 2020.
Oyabu, I. et al.: Temporal variations of surface mass balance over the last 5000 years around Dome Fuji, Dronning Maud Land, East Antarctica, Climate of the Past, 19, 293–321, 2023.